The prevalence of visual impairment in the UK
A review of the literature
Rosemary Tate, Liam Smeeth, Jennifer Evans, Astrid Fletcher
Dept of Epidemiology & Population Health
London School of Hygiene & Tropical Medicine
Chris Owen St George’s Hospital Medical School
Alicja Rudnicka Wolfson Institute of Preventive Medicine
Report commissioned by the Royal National Institute for the Blind
Acknowledgements
Funding for the review was provided by the Royal National Institute for the Blind
Advisory Committee
Liz Bates (Director Corporate Policy, Ashton, Leigh & Wigan Primary Care Trust)
Prof Ian Bruce (VOLPROF, City University)
Prof Alistair Fielder (Professor of Ophthalmology, Imperial College London and Western Eye
Hospital)
Dr Carol Lupton (Policy Research Programme, Department of Health
Dr Angela McCullagh (Research Director, The Pocklington Trust)
RNIB
Lesley-Anne Alexander (Chief Executive)
Nigel Charles (Research Development Manager)
Fazilet Hadi (Director of Policy)
Dr Adam Ockelford (Assistant Director, Education and Employment)
Many thanks also to Sue Keil at RNIB who provided extra information for Chapter 5. Special
thanks to Nigel Charles for commissioning this project and for invaluable support and advice.
Authors’ details
Rosemary Tate PhD, research fellow, London School of Hygiene & Tropical Medicine (now at
Institute of Child Health)
Liam Smeeth MRCP, Senior clinical lecturer in epidemiology, London School of Hygiene &
Tropical Medicine
Jennifer Evans PhD, lecturer in epidemiology, London School of Hygiene & Tropical Medicine
Astrid Fletcher PhD, Professor of epidemiology, London School of Hygiene & Tropical
Medicine
Chris Owen PhD, Senior Research Fellow in Epidemiology, St George's Hospital Medical
School
Alicja Rudnicka PhD, Lecturer in Epidemiology & Medical Statistics, Wolfson Institute of
Preventive Medicine
Contribution of authors
Rosemary Tate undertook the literature review and preparation of main tables. Liam Smeeth
reviewed and updated Chapter 5 on Children. Jenny Evans contributed the section on blindness
registrations and provided critical comments on the review. Chris Owen and Alicja Rudnicka
carried out the analyses and wrote the material for Chapter 6.
Astrid Fletcher took overall responsibility for the project and wrote the Summary, Chapter 1 and
Chapter 7.
Authors’ potential conflict of interest
Data on self reported visual difficulties and visual acuity measures reviewed in this report comes
from the MRC funded Trial of assessment and management of older people in the community
(Principal Investigator Astrid Fletcher), the associated causes of vision impairment study funded
by the The Pocklington Trust (with Jenny Evans and Richard Wormald) and the nested MRC
funded trial of screening for vision impairment within the main MRC Trial (with Liam Smeeth).
1
Summary
We identified three approaches to describing visual problems in the population.
1. Visual difficulties - based on self-report of difficulties with vision related functions
ranging from single item questions to disability scales. Most studies have used questions
on difficulty in “reading newsprint” as minimum criteria for difficulties with seeing.
2. Clinical measures, predominantly visual acuity. Distance visual acuity is the basis for
categorising vision impairment in the WHO International Classification of Diseases
(ICD). Most studies have measured distance acuity using an illuminated Snellen chart.
Visual acuity can be measured with usual aids if worn i.e. contact lenses or glasses
(“presenting vision”) or after full refraction for refractive error (“best corrected visual
acuity”). In surveys full refraction may not be available and studies may use pinhole
correction to try to remove some of the refractive error (pinhole corrected visual acuity).
Results may be presented as binocular vision or as vision in the better eye. The WHO
ICD classifications use best corrected visual acuity ie vision in the better eye after full
correction. Vision impairment is defined as a Snellen acuity <6/18 and blindness is
defined as visual acuity <3/60. Visual acuity <6/12 is not used internationally to define
impairment but is reported by some studies as it represents a reduction in visual acuity
sufficient to affect lifestyle e.g. it corresponds to the requirements for sight for the UK
driving licence. A recent report from the WHO Prevention of Blindness group
recommends that binocular presenting vision should also be reported in population based
surveys as this represents the actual impairment experienced by the individual in their
everyday life (whether due to underlying disease such as cataract or due to uncorrected
refractive error). Information on the causes of poor acuity provides important information
for policy and service provision. Among children, a commonly used criteria is visual loss
sufficiently bad to mean a child is identified as being in need of special educational or
social services.
3. Vision related “quality of life” describing the impact of vision problems on everyday
functioning and well-being using specially developed scales. This is a relatively new area
and to date there has been little experience in the use of such scales in population based
studies in the UK.
A review of the literature was undertaken for all UK population based studies and reports which
had included information on at least one of the above. Vision- related quality of life data were
available for one study only (from the authors of this report) and are presented in Chapter 6.
Findings
Visual difficulties
The estimates of visual difficulties (described as visual disability in some studies) derive
predominantly from large government surveys and are reviewed in Chapter 2. The results show,
when using similar survey methods and criteria for visual disability, (as in the OPCS surveys of
1985 and 1996/7) some consistency in the results in the older population: 22% of the over 75s in
private households reported difficulties with reading newsprint or worse and 26% when people
in communal establishments were included. There was less consistency in the results for the
younger age groups where the two surveys differed over 2 fold in their estimates for the
2
population aged 16-64 from 0.8% to 2.0 % mainly due to a difference in the selection criteria
into the survey. If the proportion of the younger population with visual difficulties is over
estimated this will substantially inflate the numbers with sight difficulties in the population since
the numbers of people in the population aged 16-64 is much greater than in the older age groups.
Thus the estimated numbers with sight difficulties in private households in England and Wales
based on the 1996/7 survey was estimated at nearly 2 million compared to 1.4 million from the
1985 survey.
There are doubts concerning the reliability of all estimates based on the criteria of “difficulties
with reading newsprint” or worse. The lack of reliability is based on findings from the
OPCS/RNIB 1998/1999 survey. The sample for this study included a re- survey of respondents
who had reported sight difficulties in the 1996/7 survey. A substantial proportion of respondents
who initially self reported with a sight problem denied on re-survey they had had a sight
problem. The main change in response was to the questions: “Difficulty in seeing a friend across
the road” and “Difficulty reading ordinary newsprint”. With more severe task difficulties the
level of misclassification was minimal. The estimates of visual disability in the OPCS/RNIB
1998/1999 study took account of this misreporting and were considerably lower than either of
the two previous surveys with a prevalence for difficulties with reading newsprint or worse of
14% for the over 75s and 1.1% for the 16-59 age group. However, estimates from the
OPCS/RNIB 1998/1999 study may be too low because they exclude some people who initially
had a problem and had subsequently received treatment.
Confidence in the results on is further limited by the lack of detailed information on response
rates in the government surveys. These surveys used a “sift” approach to screen out people with
no apparent disability with successive reductions in response with successive sifting. Although
the final response rates in the sample from which the estimates are derived are not clearly
presented they are of the order of 50% or less in most government surveys. The
representativeness of the surveyed population is therefore difficult to assess.
Visual acuity
The studies which have measured visual acuity in the population are reviewed in Chapter 3. The
overwhelming majority of studies have been in the older population. Because of differences
between studies in reporting results and criteria for defining visual impairment it was not
possible to pool the results to obtain a single estimate.
The prevalence of vision impairment varied according to the definitions for defining visual
impairment and whether refractive error was included in the estimates.
Prevalence of visual impairment
Studies using visual acuity measurements have used various criteria for definitions and cut points
of visual impairment. The results from two studies: National Diet and Nutrition Study (NDNS),
and MRC Assessment Trial that were nationally representative of the older population and use
uncorrected presenting VA and similar cut points are given in the table below (unpublished data
3
from NDNS provided by Dr van der Pols and unpublished data on VA <6/12 for MRC
Assessment Trial using same LogMar criteria as NDNS).
65-74
NDNS
community
sample1
VA <6/18
VA<6/12
MRC Assessment
Trial2
VA <6/18
VA<6/12
475
5.6
15.8
Age group
not
included
95% CI
75-84
95%CI
429
13.3
28.3
3.5, 7.6
12.6, 19.1
11500
8.5
18.7
85+
95% CI
10.1, 16.5
24.1, 32.6
222
31.7
54.0
25.5, 37.8
47.4, 60.6
7.1, 9.8
16.5, 20.9
3100
26.8
45.8
23.9, 29.7
42.2, 49.5
1
Best score (Glasgow Acuity Cards) of two eyes without pinhole correction
2
Binocular acuity without pinhole correction
These results show that the point prevalence estimates of vision impairment (VA <6/18) are : 6%
NDNS only) at ages 65-74, 8% to 13% at ages 75-84 and 27% to 32% at ages 85+. Estimates of
the prevalence of minor visual loss (visual acuity <6/12) are 16% (NDNS only) at ages 65-74,
between 19% to 28% at ages 75-84 and 46% to 54% at ages 85+.
Considering the upper and low bounds of the 95% confidence intervals the results show that the
estimates of vision impairment (VA <6/18) range from 4% to 8 % (NDNS only) at ages 65-74,
7% to 16% at ages 75-84 and 24% to 38% at ages 85+. Similarly the range of estimates of the
prevalence of minor visual loss (visual acuity <6/12) are from 13% to 19% (NDNS only) at ages
65-74, between 16% to 33% at ages 75-84 and 42% to 61% at ages 85+.
Estimates for the nursing home population from NDNS show high levels of pinhole corrected
vision impairment (VA <6/18): 12.1% at ages 65-74 (95% CI 9.7%, 23.2%), 30.0% of those
aged 75-84 (95% CI 20.5%, 39.5%) and 46.9% of those aged 85+ (95% CI 37.7%, 56.1%).
However as these are the only recent estimates available for the nursing home population we
recommend that they are viewed with some caution.
Two studies provided estimates of blindness using international criteria of VA < 3/60. In the
MRC assessment trial the estimates for blindness for the 75+ age group were 2.1 (95% CI 1.8,
2.4) and in a small study in London the estimates for blindness were very similar (1.9%)
although the confidence intervals were very wide (0.2, 6.6)
In Chapter 4 we reviewed the few UK studies which have provided some information on the
causes of vision impairment. Untreated refractive error and cataracts are the major remediable
causes of vision impairment in older people ranging from 50% to 70% as a proportion of visual
impairment. With increasing age the proportion of treatable vision impairment declines as age
related macular degeneration becomes more prevalent.
In Chapter 3 we reviewed an earlier report from the RNIB that raised concerns that a large
proportion of the visually impaired population were not on the Blind/partial Sight register. As the
4
results for visual impairment from that report were based on sight tests alone without excluding
possible remediable conditions (such as refractive error and cataracts) the registerable
component is likely to have been considerably overestimated (around two fold). We showed that,
based on other studies which have collected data on the causes of vision impairment, the
numbers likely to be registered because of permanent vision loss and the actual numbers
registered from Department of Health statistics were of a similar magnitude (and differed by
about 10%, possible range 0% to 20%). We conclude that, although registration rates may show
some under ascertainment in the older age groups, the evidence does not support substantial
under-reporting.
Chapter 5 summarises the literature relating to the prevalence of visual impairment and visual
disability in children in the UK. Most of the available data about visual loss in children comes
from registers or from surveys of providers of health care, social care or educational services to
children with visual loss. There are marked variations in the criteria used to define visual
disability and visual impairment in different areas of the UK and in the various surveys
performed. Caution is required in interpretation of any single estimate of the prevalence of visual
impairment or blindness among children in the UK. However, using a broad and pragmatic
definition of visual loss sufficiently poor as to mean a child is identified as being in need of
special educational or social services, the existing data suggest a prevalence of visual impairment
in the region of 10-20/10,000 children. For severe visual loss, the best estimate available
suggests a cumulative incidence of blindness or severe visual impairment by the age of 16 years
of 5.9/10,000 children. Notably, around half the children receiving support from visual
impairment services may have additional disabilities, and this proportion may be even higher for
children with severe visual loss.
Only one UK population based study reported vision related quality of life. Using the NEI-VFQ
25, 69% reported their vision was excellent or good and 31% reported their vision as fair or poor.
Around a half reported problems with near or distance activities. Visual acuity levels were
strongly associated with all sub-scales except ocular pain. The most important scales related to
visual acuity were general vision, near activities, and social functioning and, to a lesser extent,
dependency and colour vision. However visual acuity only explained about a fifth of the
variation between people in their reporting on these scales.
Most studies have been done in the older population and there is a scarcity of data in younger
adult age groups in the UK. A review of epidemiological studies performed in Western Europe,
North America and Australia covering the age group 20 to 59 years found the prevalence of
blindness was 0.08 and of visual acuity 6/24 to 6/48 was 0.07%. These figures agree well with
the prevalence of registrations in a similar age range and we conclude that registration data
provide reasonably accurate estimates of the prevalence of serious vision impairment in the
younger adult age groups. Information is lacking on less severe levels of visual acuity.
Conclusions and Recommendations
Policy
Estimates of the numbers of people in the population with sight problems are critically
dependent on the validity of the methods, definitions used and the quality of the studies (such as
response rates and representativeness of the population).
5
Estimates based on self report from government surveys are less reliable than estimates based on
visual acuity because of concerns about the validity of the questions and the poorer response
rates. In addition, these estimates are likely to be a substantial overestimate of the numbers since
they include people with minimal problems.
Estimates based on visual acuity measurements from the representative studies of the older
population are sufficiently consistent for general purposes of planning and estimation of the
proportion and numbers of people in the UK with vision impairment. However we caution that
these estimates are seen as approximations and not as “tablets of stone” as prevalence estimates
will vary from place to place and over time and because all estimates carry a range of uncertainty
as shown by the 95% confidence intervals.
Estimating the prevalence of vision impairment for future older populations requires assumptions
about temporal trends in the underlying conditions, and future levels of service provision and
uptake. No data are available on temporal trends in the incidence of refractive error, cataracts or
age related macular degeneration and such data would be prohibitively expensive to obtain. The
estimated increase in the proportion of people with age related macular degeneration eligible for
registration has been calculated to increase by 11% over a 11 year period to from 2000 to 2020
assuming no change in the prevalence of this condition or improved treatments
Studies in the older population have consistently shown that about 50% of visual impairment is
due to treatable conditions i.e. cataract and refractive error. Appropriate action e.g. media
campaigns and advocacy is required to ensure that eye care providers are aware of the current
unacceptably high levels of untreated conditions. Campaigns should also target older people and
their carers to raise awareness of poor vision in later life and the potential for improving visual
impairment through spectacle correction and cataract surgery.
For children, agreed criteria to define visual disability, visual impairment and the need for
specialist support are needed. The needs of children with visual disability who also have other
disabilities should also be a priority.
Recommendations for adoption of standards for measurement and reporting
We have shown the considerable variation in definitions and cut points used in the UK based
studies. We strongly recommend the adoption of standards for measurement and reporting which
have been recommended by international organisations (WHO and the International Council of
Ophthalmology) and are described in Chapter 7.
Vision related quality of life scales are a useful adjunct to clinical measurements, but should not
be used as a substitute for visual acuity. We recommend the use of validated questionnaires or
scales for measuring self reported vision problems or vision related quality of life. We emphasize
the need to thoroughly test all questions before use in surveys.
We encountered difficulties in understanding the methods and basis for calculations in some of
the reports we reviewed due to a lack of clarity and transparency in the reporting of data. We
recommend that reports which present novel research findings (but are not submitted for
publication in academic journals) should conform to the same standards of reporting as the
6
formal peer review process of academic journals (for example as published by the International
Council of Medical Editors). Further details are given in Chapter 7.
Recommendations for further research
Most studies have been done in the older population and there is a scarcity of data in younger
adult age groups in the UK. A review of epidemiological studies performed in Western Europe,
North America and Australia covering the age group 20 to 59 years found the prevalence of
blindness was 0.08 and of visual acuity 6/24 to 6/48 was 0.07%. These figures agree well with
the prevalence of registrations in a similar age range and we conclude that registration data
provide reasonably accurate estimates of the prevalence of serious vision impairment in the
younger adult age groups. Information is lacking on less severe levels of visual impairment.
Due to the lack of data on the prevalence of visual problems, visual impairment or specific types
of eye disease in ethnic groups in the UK population, studies in ethnic groups are required.
The reasons for higher levels of vision impairment in women and in more deprived social groups
requires further understanding, in particular the extent, if any, to which this reflects under
utilisation of services.
The reasons for high levels of treatable conditions in the older population require further
investigation. In particular information is required on public awareness of signs and symptoms of
treatable conditions, and barriers to service utilisation and access.
New strategies to identify the most cost effective way of screening the older population for
vision impairment are urgently needed.
7
Chapter 1
Introduction
8
Valid and reliable estimates of the levels of vision impairment in a population are important to a
number of agencies and professional groups, which include national policy makers, health and
social service providers, health professionals, public health departments and academic
institutions including epidemiologists and voluntary sector organizations. We emphasize validity
and reliability as key data requirements. Users of data on visual problems need to be reassured
that the measures are valid i.e. measuring vision problems relevant to the use to which the data
will be put. In this context we define reliable to mean that the estimates are obtained from well
conducted studies, measured without bias and with a high level of precision. Although user
perspectives may vary in emphasis and focus the critical information required relates to the
classic epidemiological questions: How much, Who, Where, When, and Why? All these
questions require a clear definition of What is the problem. We will discuss the What? question
at the end of this section although of course it normally would be the starting point in any study.
How much of a disease/condition is there in a population?
In quantifying how much of a condition there is in a population, different measures are used.
Prevalence is a measure of the number of people with a disease/ condition at a particular time
point in relation to the total number of people in the population. This is typically expressed in
terms of a proportion (usually %) and sometimes described as a prevalence rate. A prevalence
measure requires a time reference which relates to the point at which the estimates are made, e.g.
over a few days or months or annually. Often however the period is not clearly described. For
most surveys the prevalence is point prevalence.
Incidence is a measure that describes the number of new cases of the disease/condition that occur
over a specified time period in people who were disease/condition free at the start of the period.
Numbers of people in a population with the disease/condition at a particular time point are
estimated by applying prevalence rates, which are usually obtained from surveys, to national
population estimates.
These measures are all useful because they each provide a different perspective. Prevalence
rates describe the relative importance of a disease/condition in terms of how common it is. This
aids in prioritizing the diseases/conditions which makes the largest contributions to a
population’s ill health. Epidemiologists use prevalence data from a number of international
populations for comparative purposes as a first step in understanding the aetiology or risk factors
related to a disease/condition. Crucial to interpretation of these data are age standardisations to
ensure that the population data being compared are adjusted for any age differences as for most
diseases age is the major prognostic factor of morbidity. Incidence data provide information on
the natural history of a disease/condition; identifying people and the associated risk factors
which best predict the development of the disease/condition. The gap between prevalence and
incidence is also a measure of whether a disease/condition is chronic or curative. The actual
numbers in a population are vital for planning services and to voluntary sector organizations for
understanding the size of the client group on whose behalf they are campaigning.
95% confidence intervals. Although prevalence or incidence rates are a single number, the
estimates should be presented with their 95% confidence intervals. 95% confidence intervals
give the range i.e. an upper and lower boundary within which the true but unknown estimate will
lie. The 95% is a measure of our certainty about this range i.e. we are 95% certain.
Who has the disease/condition?
9
Prevalence measures are usually described by the major demographic characteristics of age
group and sex, because these data are easy to collect and because most disease/conditions vary
considerably across age groups. Sex differences may also be important suggesting either
difference in aetiology or in access to services. Other factors of relevance at the population level
may be socio-economic status and ethnic group both to identify whether there are different sub
populations for needs assessment, and access to services and to suggest possible factors in
aetiology.
There is an important distinction between the numbers in a population with a disease/ condition
and those sub groups in a population who have the highest proportion of a disease/condition. The
classic example is where the highest prevalence occurs in the older age groups but the actual
numbers in the population with the condition may be mainly contributed from younger age
groups with lower prevalence. This simply reflects the much lower proportions of the older age
groups in the population.
Where do people with the disease/condition live?
Geographical information such as regional, urban/rural and, for some conditions, living
circumstances (in the community or in communal establishments such as nursing and residential
homes) may help to estimate requirements for local service provision and, in the case of living
circumstances, where the main target population may be identified. As described above at an
international level these data are useful for suggesting possible true differences between
populations in the prevalence of risk factors
When?
Temporal data describe changes in the prevalence or incidence of a condition/disease usually
over a fairly extended time period for chronic conditions. Temporal data may be provided from
routinely collected national statistics (e.g. mortality, cancer incidence) or by comparing cross
sectional information such as from surveys collected at different years. There are a number of
methodological pitfalls in interpreting temporal data: definitions of disease/conditions may have
changed, the reported number of cases may have apparently increased as a result of greater
awareness or improved diagnostic procedures, better treatment may lead to reduced prevalence
but not incidence. Temporal changes in numbers only and not in rates are especially misleading
as there is no indication of the underlying population. Increases or decreases in numbers may
simply reflect changes in the population structure. Interpretation of Blindness and Partial Sight
registrations is such an example.
Why?
Understanding why some people in a population get a disease/condition compared to others who
do not is the main area for epidemiological research and outside the prime focus of this report.
What is the problem?
A number of terms are used to describe visual problems and we will discuss these in greater
detail. As an introduction to this area we note that the choice of terminology and of measurement
instruments is influenced by the purpose for which the information is required. We need to
distinguish between measures which relate to understanding of the underlying clinical
condition/disease and those which describe the impact of the condition/disease on the person’s
function and everyday life. The objective of clinical measures of vision is to describe the level of
visual loss and the reasons for this. Since at the population level in the UK most vision problems
10
are due to refractive error, untreated cataracts, age-related macular degeneration, diabetic
retinopathy and glaucoma, a service provider perspective would include the need for specific
data that assists in planning for services and treatments that include: optometry, cataract surgery
and other clinical treatments, low vision aids, and for some conditions, frequent hospital visits
for monitoring.
Most importantly, estimates of the prevalence of visual impairment in a population will vary
over time and between populations according to the proportion of untreated but remediable
vision problems. Thus, measures of vision loss alone are a first step to describing the population
burden but are not informative unless we understand what the problem is due to. Although we
would expect a high correlation between functional difficulties described by a person and a
clinical measure, a person’s functional problems will be also influenced by a number of both
personal and situational factors. Clinicians will also use patients’ perspectives as an adjunct to
both understanding at an individual level of the impact of the problem on a person, and at a
group level to evaluate the effectives of clinical treatments and services.
Measurement of Vision and Visual Problems
A recent report to the International Council of Ophthalmology [1]
(www.icoph.org/pdf/visualstandards.pdf ) provides an excellent summary of different
approaches to measure and categorise vision and vision problems. From the perspective of this
report and the available literature we will concentrate on the following measures since they have
been used the most frequently in UK based population studies.
1. Vision difficulties - based on self-report of difficulties with vision related functions
ranging from single item questions to disability scales
2. Clinical measures, predominantly visual acuity.
3. Vision related “quality of life” describing the impact of vision problems on everyday
functioning and well-being (few studies at present).
1. Vision difficulties
1.1 OPCS disability questions
In most of the UK national surveys carried out by the government survey services (Office of
National Statistics, previously Office of Population Census and Surveys OPCS) which are
described in this report a number of questions related to difficulties with seeing were used (Table
1.1.). All the questions on “seeing” included the adjunct “wearing glasses or contact lenses “so
that disability was based on usual vision with corrective aids (though not usually magnifying or
other low vision aids). These questions on seeing were part of a set of scales from four linked
surveys that were carried out to establish the prevalence and type of disability in the UK. The
concept of disability was heavily influenced by the WHO classification of International
Classification of Impairments, Disabilities and Handicaps (ICIDH) [2]which drew distinctions
between impairments (“ loss or abnormality of psychological, physiological or anatomical
structure or function”), disability (“ restriction or lack of ability (resulting from an impairment)
to perform an activity in the manner or within the range considered normal for a human being”)
and handicap (“ a disadvantage for a given individual, resulting from an impairment or
disability that limits or prevents the fulfilment of a role depending on age, sex and social and
cultural factors for that individual). The ICIDH classification of disability was adopted for the
OPCS surveys; separate scales were developed for 10 disability areas (later expanded to 13
areas) including seeing. Within each of the areas of disability the severity of specific difficulties
11
was ranked on a 10 point scale using panels drawn from OPCS staff, professional experts in
disability and disabled people and their carers. Further work involved the use of a common 15
point disability scale to which the individual disability specific items were mapped. Finally an
overall disability score was produced based on the three highest severity scores from the separate
areas.
None of the disability scales underwent the process known as validation that would nowadays be
expected as part of the process of development of questionnaires. In particular the sensitivity and
specificity of the questions and overall seeing scale against other measures of visual problems
(such as visual acuity) were not established. This was largely because the concept of disability
was considered as independent of external “gold standards”. However a potential problem in the
lack of validation could arise when some of the questions were used as a preliminary screen for
people with disabilities, known as “the sift questionnaire”. In several of the OPCS studies a selfcompleted questionnaire was used as a quick and easy method of identifying people with
disabilities for a further more detailed assessment (Table 1.1). Two specific vision questions
from the full seeing scale were included in the sift questionnaire (S9: “difficulty reading ordinary
newspaper print” and S10 “difficulty recognising a friend across the road”). However a decision
was made that people who responded only to S10 and not to any other question were treated as
the non-disabled. People answering positively only to S9 or worse were included.
Since the prevalence estimates in some surveys are derived from people who have been “sifted
in”, it is important that the sift questions have high sensitivity (i.e. people with vision problems
are not missed). Specificity is also critical because estimates may be biased by false positives.
Sift questions can also be used to measure prevalence based only on the two sift questions asked.
Many of the estimates presented in the report are based on the combined prevalence of S9 and/or
S8. In later surveys, different questions from the Seeing difficulty scale were used e.g. in the
Health Survey for England [3, 4] prevalence was based on a positive answer to S8 “Cannot see
well enough to recognise a friend across a road” with S4 “Cannot see well enough to recognise a
friend who is an arms length away” being asked for a positive response to S8.
1.2 Other questions used in surveys.
The English Longitudinal Study on Ageing (ELSA) [5] moves away from the “difficulty” or
“cannot” style of questions in the OPCS surveys to asking people to rate their vision, overall and
for two specific examples. Participants were asked to rate their eyesight, using glasses or
corrective lenses, as excellent, very good, good, fair or poor. They were also asked to rate how
good their eyesight was for seeing things at a distance and for seeing things close up.
2. Clinical measures
A number of clinical measures exist of which the most commonly used and internationally
accepted is visual acuity. Other clinical measures include visual fields, colour vision, contrast
sensitivity, light/dark adaptation and motion perception. Visual acuity and, to a lesser extent,
visual fields are measures that are practical to use in population based studies. Other clinical
measures described above are used mainly in the clinical setting for diagnosis and for clinical
research. The current consensus is that these other clinical measures are not yet suitable for use
in the population[6]. Further work is required to refine the use of such measures and
demonstrate their added value, over that of visual acuity, in characterisation of visual loss at the
population level.
Visual acuity
12
Visual acuity is the basis for categorising vision impairment in the WHO International
Classification of Diseases (ICD10) [7] (Table 1.2) but the definitions used may not necessarily
fit with the goals of a particular survey. Alternative definitions have been proposed by the
International Council of Ophthalmology[1] which include minor categories of visual loss.
Distance visual acuity is the most usual measurement but additionally some surveys have also
included near acuity i.e. close up vision.
Measurement of distance visual acuity.
A detailed explanation of the letter charts and scoring methods used in population surveys is
given in the report to the ICO [1]. The most commonly used measure has been the Snellen chart,
preferably illuminated, tested at a distance of six metres. Most people will be familiar with this
method of testing as it is routinely used in high street optometrists. The Snellen chart has been in
use for over a hundred years [8]. Snellen designed a chart for screening and for use with
refraction based on special characters called “optotypes”. He defined the reference standard as
the ability to recognize letters that are 5 min of arc high. However the chart is not fine enough at
lower levels of visual acuity and, for this reason, a chart with a logarithmic progression of letter
sizes was first proposed by Bailey and Lovie [9]and adopted in the ETDRS[10] (Early Treatment
Diabetic Retinopathy Study). The EDTRS chart and others similar [11] which provide equal
steps at all levels have come to be preferred in clinical use and are also increasingly
recommended as the preferred method in population surveys [1, 6]. These charts are commonly
known as LogMAR (Log of the Minimum Angle of Resolution) charts. Near acuity is commonly
measured by the distance at which newsprint can be read, usually 40 cm. Although some surveys
use cards with text (to simulate news or book print) another method is to use LogMAR or
Snellen cards.
From the point of view of classification there are a number of important features related to the
measurements of distance acuity.
1. Visual acuity can be measured either unaided or with glasses /contact lenses if worn. This
latter measurement is usually described as “presenting “or “walk about” vision and is regarded as
the measurement that best reflects the everyday vision of the person.
2. Vision can be measured in both eyes (separately) or together (binocular). Binocular vision is
also regarded as better reflecting the everyday vision than the uniocular vision. For many
definitions, including the WHO definitions and the legal requirements for registration, the vision
in the better eye is the criterion by which the person’s visual acuity is categorized.
3. “Best” corrected visual acuity is the visual acuity after using the best available methods for
correction for refractive error. In population surveys full refraction may not be available and a
simple (though imperfect) method to try to correct for refractive error is to use a pinhole and remeasure visual acuity with the person looking through the pinhole.
In the WHO categories the visual acuity definitions are based on best correction in the better eye
(Table 1.2) Vision impairment is defined as a Snellen acuity <6/18 and blindness is defined as
visual acuity <3/60. Visual acuity <6/12 is not used internationally to define impairment but is
reported by some studies as it represents a reduction in visual acuity sufficient to affect lifestyle
e.g. it corresponds to the requirements for sight for driving licence in the UK and in other
countries.
Obtaining high quality measurements of visual acuity in population based surveys requires a
written protocol on how to administer the tests, attention to adequate lighting and necessary
distances between the patient and the charts, standard instructions on amount of prompting and
encouragement to be given to ensure that participants have the best chance to attempt the letters.
13
Training and quality control of fieldworkers in population based surveys is absolutely essential
to ensure reliable results.
Visual fields
Visual field loss is more complex to characterise than visual acuity. However visual field loss
can occur independently of visual acuity. A recent report on standards for characterization of
vision less and visual functioning concluded that more work needed to be done to develop good
methods for testing visual fields in population surveys[6]. Automated perimetery is the
recommended method for field surveys but requires more expertise and care than measurement
of visual acuity. The ICO report [1]notes that the most appropriate visual fields depends on the
question: central fields are the most important for identification of the underlying cause of vision
loss, such as glaucoma while peripheral fields are a more relevant measure of the consequences
of the underlying condition. A Visual Field score has been developed which provides a single
summary score based on assigning points for every point seen on a visual field grid. This score
has been correlated with orientation and mobility performance scores. There are to date no data
on the use of the Visual Field scores in the UK population. Such data that are available on visual
fields are in the context of estimating the prevalence of glaucoma and include other criteria such
as cup: disc ratio, and intra-ocular pressure.
Blindness and partial Sight Registration in the UK
The number of blind people in Britain has been recorded since 1851. Initially, this consisted of a
declaration of blindness on Census returns. These were discontinued after 1911. In 1920, the
Blind Persons Act created statutory benefits to the blind and its implementation led to a register
of blind persons. Initially, all that was required for registration was a certificate from any
medical practitioner that the patient was blind. From the mid-1930’s, certificates were only
accepted on designated forms (BD8) signed by ophthalmologists. The National Assistance Act in
1948 set up the current system of registration, with local authorities required to establish
registers of people with disabilities, including those blind or partially sighted, and to administer
the statutory services to which the visually disabled are entitled.
Any person seeking registration as blind or partially sighted is required to see an ophthalmologist
who certifies that the person is eligible for registration. A person is certified as blind if they are
“so blind that they cannot do any work for which eyesight is essential”. This is recommended to
be a best binocular acuity of less than 3/60 Snellen, or a much contracted field of vision. A
person can be certified as partially sighted if they are “substantially and permanently
handicapped by defective vision cause or by congenital defect or illness or injury”. It is
suggested that this corresponds to an acuity of 6/60 or less or gross field defects (or 6/24 or less
in certain cases).
Since November 2003, the system has changed in England only. This change was initiated by
concerns, probably unfounded, that only a minority of people eligible for registration were taking
up registration. The BD8 form has been superseded by a “certificate of vision impairment
(CVI)”. There are now two categories of vision impairment: “sight impaired or partially sighted”
and “severely sight impaired or blind”. Mechanisms have also been put into place to ensure that
visually impaired people are referred promptly from the high street optician and hospital eye
service for support.
Data on number of people on the registers and new registrations
These data are collected separately in England, Wales, Scotland and Northern Ireland. Local
Authority Social Services departments complete forms SSDA 902 in England and Wales and
14
SWB in Scotland and return these to relevant government agencies (Department of Health,
Scottish Executive and Welsh assembly) whose job it is to analyse and publish these data.
Data from Scotland is most easily available being online and up to date (published to year end
March 2003 (http://www.scotland.gov.uk, accessed September 14th 2004). Data for England are
available from the Department of Health publications office [12].
Cause of visual loss
Data on the cause of visual loss has been analysed periodically[13-16]. Since 1990, part 5 of the
BD8 form has been sent to the Office of National Statistics (ONS) and coded according to ICD9.
The last time period for which national data were analysed for England and Wales was 1990/91
[13]. At that time the main cause of blindness was “degeneration of the macula and posterior
pole” (48.5%) followed by glaucoma (11.7%), diabetic retinopathy (3.4%), optic atrophy (3.4%),
cataract (3.3%) and other conditions (20.0%). For partial sight the main causes of visual loss
were similar. Some authors have reported cause of visual loss at local area level [17-19].
Current arrangements for the analysis of cause of visual loss at the national level have not been
publicised.
Strengths of the Registration System
Data on number of people on the blind and partially sighted registers and number of new
registrations is national, routinely collected data reflecting the prevalence and incidence of
severe vision loss in the population. It is available over long periods of time and, in theory, could
be used for monitoring temporal trends in causes of severe visual loss that are not amenable to
treatment [20].
The registers potentially provide the opportunity for monitoring the incidence of important
causes of preventable or “avoidable” visual loss, such as diabetic retinopathy[21] . However, that
would depend on having regular analyses of data on cause of visual loss at the regional level.
Weaknesses
Registration is voluntary and the legal definitions are vague. This means that there is uncertainty
as to the coverage of registration. One small study at the population level indicated that the blind
register was probably fairly complete, the partial sight register less so [22]. Other studies at the
hospital level have indicated that a proportion of visually impaired people attending the hospital
eye service are not registered[23, 24]. There might also be considerable time delay between
onset of severe vision loss and registration.
Chapter Summary
In this chapter we have reviewed the importance of obtaining information on the levels of visual
problems in a population, different approaches to the methods of measuring visual problems
including self reported, visual acuity measurements and blindness and partial sight registrations.
In the following chapters we will review the available literature using different methods for
categorising vision:
Chapter 2: Vision difficulties in adults
Chapter 3: Visual acuity estimates in adults
Chapter 4: Causes of vision impairment in adults
15
Chapter 5: Vision impairment in children
Chapter 6: Vision related quality of life
Chapter 7: Conclusions and Recommendations
The literature was ascertained (i) from a PubMed search using the key words ``prevalence"
``visual" and ``blindness" (ii) by asking the advisory group members for relevant publications
and (iii) searching the bibliography of those most recently published.
16
Table 1.1 Seeing Severity Scales and categories as described in Martin et al 1988 [25]
Scale
Description
Score
S1
Cannot tell by the light where the windows are
12
S2
Cannot see the shapes of the furniture in the room
11
S3
Cannot see well enough to recognise a friend if close to his face
10
S4
Cannot see well enough to recognise a friend who is an arm’s length away 8
S5
Cannot see well enough to read a newspaper headline
5.5
S6
Cannot see well enough to read a large print book
5
S7
Cannot see well enough to recognise a friend across a room
4.5
S8
Cannot see well enough to recognise a friend across a road
1.5
S9
Has difficulty reading ordinary newspaper print
0.5
S10*
Has difficulty recognising a friend across the road
*Not included as a criteria for sift but in the original scale
17
Table 1.2 Visual acuity scoring systems, WHO ICD classifications of blindness and low vision (ICD10) [7]and ICO recommendations for
categorisation of visual loss[1]
WHO ICD 10
Based on VA in better eye
after best correction for
refractive error
International Council of Ophthalmology
Ranges of vision loss2 defined by
decimal notation
Normal vision >= 0.8
Not defined by WHO
Mild vision loss < 0.8 and >= 0.3
2
Moderate
vision
Moderate vision loss < 0.3 and >= 0.125
Low vision impairment
< 6/18 –
6/60
2
Severe vision loss < 0.125 and >= 0.05
Severe
vision
impairment
<6-60Profound vision loss < 0.05 and >= 0.02
3/60
Blindness
< 3/60
Near-total vision loss (near blindness) <
0.02 and >= NLP
Total vision loss (total blindness NLP
Decimal
notation
Snellen
(UK
notation)
Snellen
(USA
notation)
1.0
0.8
0.5
0.32
0.25
0.16
6/6
6/7.5
6/121
6/18
6/24
6/36
20/20
20/25
20/40
20/63
20/80
20/125
LogMAR
ETDRS or
Bailey
Lovie
0
0.1
0.3
0.5
0.6
0.8
0.125
6/48
20/160
0.9
0.1
6/60
20/200
1.0
3/60
20/400
1.3
<0.02
1/60 or
less
less
1.8-1.9
0.0
NLP3
NLP
2.0
0.05
1
<6/12 approximates to driving licence threshold
2
These definitions of have been proposed as alternatives to the terminology of Low Vision which may be confused with people eligible for Low Vision services
3
No Light Perception
Chapter 2
Vision Difficulties in Adults
Included in this section are all studies which have used self reported responses to questions on
problems with vision related tasks (vision difficulties) or to self assessment of vision (as
described in the previous chapter) (Table 2.1). Some reports use the term “visual disability” to
describe the prevalence of “seeing difficulties”.
The Prevalence of Disability among Adults (Martin et al.1988) [25]
Summary
The OPCS disability survey of adults in private household and communal establishments in
Great Britain was carried out in 1985. Questionnaires were sent to 100,000 private households.
28,415 adults, aged 16 and over, were identified as disabled of whom 18,000 were selected for
interview and 14,308 (79%) were interviewed. 3,775 people in communal establishments were
interviewed, either personally or by proxy. The criteria for “visual disability” were “has
difficulty reading ordinary newspaper print” and/or “cannot see well enough to recognise a
friend across the road”. The prevalence of visual disability for adults aged 16 and over in private
households was estimated to be 3.2% increasing to 3.8% when residents of communal
establishments were included.
Methods
Private Households
A screening (“sift”) questionnaire was sent out to a random sample of 100,000 households
identified from the Postcode Address File (PAF). The PAF divides Great Britain into about
8,500 postal sections. The OPCS frame combined these into some 800 areas each containing a
minimum of 500 addresses. The first stage was to sample 500 of these areas with probability
proportional to size. The sectors were stratified by region, by a metropolitan/non metropolitan
split and then systematically ordered by the proportion of people above retirement age. The
second stage was to select 200 addresses from each of the selected areas producing a total of
100,000 addresses. In 400 of the areas (covering 80,000 addresses) the screening was carried out
by post. In the remaining 100 (urban) areas (and also addresses that had been identified as having
multiple households) interviewers delivered the screening questionnaire personally.
The questionnaire contained detailed questions on health problems and disabilities. Those related
to seeing were “difficulty recognising a friend across the road even if glasses or contact lenses
are worn” and “difficulty reading ordinary newspaper print even if glasses or contact lenses are
worn”. People with disabilities were then followed up and interviewed.
The response rate to the screening questionnaire was 80%. There were 28,415 adults who
answered “yes” to at least one question relating to long-term health problem or disability. 50%
of those over 65 and all those aged under 65 were selected for interview (n = 18,000). Of these,
14,308 (79%) were interviewed. Of these 11,035 were identified as disabled from the interviews.
2534 of these were categorised as visually disabled because they answered ``yes" to the question
“has difficulty reading ordinary newspaper print” or “cannot see well enough to recognise a
friend across the road”.
In the derivation of the estimates of the number of disabled adults in the total population the
numbers are weighted to account for under-sampling of 60+ and also non-response.
2
Communal Establishments
A list of institutions held by the Vital Statistics Branch of the Population Statistics division of the
OPCS (approximately 20,000 entries) and an equivalent list from the General Registrar's Office
for Scotland was used as the sampling frame. Hospitals, homes and hostels were included.
Educational establishments, places of detention and military establishments were excluded.
1,408 institutions were contacted. 892 (63% of institutions) were found to be eligible, i.e. that
they had four or more permanent residents and were willing to co-operate. One third of the 892
institutions were randomly rejected leaving a sample of 595 institutions. One in four residents
were selected for interview in institutions where the number of permanent residents was 80 or
less, and one in twelve where the number was 81 or more. Selection was made by the interviewer
or institution administrator. Interviews were obtained from 3775 permanent residents from 595
institutions. In some cases interviews were split between a member of staff and the resident and
in other cases the administrator was interviewed on behalf of all the subjects, even if they were
competent to answer. It was assumed that all permanent residents in establishment had some
degree of disability. Weights were used to produce national estimates, to allow for those who
could not be interviewed, the institutions that did not reply, the sampling within the selected
establishments and for sampling and non-response of establishments.
Results
Figures 2.1 and 2.2 show the stages in the sampling and selection process that led to the
estimates for visually disabled adults in private households and communal establishments
respectively. Table 2.2 shows the estimated prevalence rates for visually disabled adults aged 16
and over, by age and gender. Of the 10,561 interviewed, 2534 were classified as visually
disabled. The overall prevalence rate of adults, aged 16+ in private households in GB with visual
disabilities, was estimated to be 3.2% and the estimated number of visually disabled adults in
private households in GB to be 1,384,000. Prevalence estimates for those in communal
establishments were not reported, only those for the total population i.e. private plus communal.
The estimates were based on the assumption that there are no disabled people in the excluded
types of establishment. The prevalence rate when residential homes were included increased to
3.8%. The estimated percentage prevalence by age group for adults in all establishments was 0.8
for the 16-59 age group, 5.6 for 60-74 and 26.2 for those ages 75 and over.
3
Figure 2.1: Flow chart showing the numbers of participants at each stage of the OPCS 1985
survey of adults in private households
4
Figure 2.2: Flow chart showing the numbers of participants at each stage of the OPCS 1985
communal establishments’ survey
.
Comment
The survey was a large random sample of the total population of Great Britain. A specially
devised questionnaire to measure disability based on successive “sifting” was used which
allowed in-depth interviews to be carried out on those screened into the survey as positives. This
approach is cost effective because large numbers of people with no disability do not need to be
interviewed in depth. The disability scales and sift questions are described in detail in the report.
The selection process of the 1 in 4 or 1 in 12 residents in communal establishments was not
random. Some residents were interviewed personally whereas in some homes the administrator
provided all the answers, even if the residents were capable of answering. No figures were given
for the number found to be disabled in communal establishments, and thus it was not possible to
calculate the number of visually disabled in the sample.
Although the estimates are based on large numbers, 95% confidence intervals were not provided.
5
Disability in Great Britain: Results From the Office for National Statistics (ONS) 1996/97
Disability Follow-Up to the Family Resources Survey (Grundy et al.1999)[26]
Summary
This report is based on interviews obtained from a subset of those interviewed in the 1996/7
Family Resources Survey. It provided estimates of adults (16+) with seeing disabilities in private
homes. It used the same criteria for disability as in the 1985 survey, but the questions on the
``sift" questionnaire for overall disability were different and included questions on long standing
illness or disability. Overall 23% of the 5589 disabled adults included in the survey were
classified as having a “seeing” disability. The estimated weighted prevalence of visual disability
in adults aged 16+ in private households in Great Britain was 4.5%. The total number of visually
disabled adults in Great Britain was estimated to be 1,973,860.
Methods
The sample for this survey was a sub-sample of people interviewed for the Family Resources
Survey (FRS) between July 1986 and March 1997. The FRS includes questions on limiting long
standing illness, on health problems limiting ability to work and on benefit receipt. FRS is
conducted on a continuous basis with some 25,000 households. 47,000 adults are interviewed
each year using computer assisted personal interviews. Those included in the disability follow-up
survey fulfilled at least one of the following criteria:
 75 years of age or older
 long-standing illness or disability
 Restricted in type or amount of work
 receipt of War Disablement Pension, Disability Working Allowance, Severe Disablement
Allowance, Attendance Allowance, Mobility or Care component of Disability Living
Allowance, Incapacity Benefit or Industrial Injury Disablement Benefit
 Awaiting a claim for Disability Living Allowance, Incapacity Benefit or Industrial Injury
Disablement Award
 Receiving certain types of pensions with increments.
The criteria for being classified as visually disabled was the same as in the OPCS disability study
described previously[25] i.e. at least S9 in Table 1.1. Base figures from the original FRS survey
were used to produce the prevalence estimates. Figure 2.3 shows an outline of the sampling
procedure. Weighting procedures were used to allow for the probability of being selected for the
FRS, response to the FRS and response to the Disability survey among FRS respondents. It is not
clear how, or if, weighting was carried out to allow for the over-sampling of people aged over
75.
6
Figure 2.3: Flow chart showing the sampling procedures and numbers included at each stage of
the OPCS 1996/7 FRS follow-up private household survey
Results
The number of participants at each stage is shown in Figure 2.3. The response rate at the first
stage was 80% and to the second stage was 85% giving an overall response rate of 68%.
Prevalence estimates by age and sex are shown in Table 2.2. The total estimated number of
disabled adults aged 16+ in GB was 8,582,000. Of these 1,973,860 were estimated to be visually
disabled. The estimated prevalence of visual disability in adults 16+ in private households in GB
was 4.5% and for age 75+ was 22.6%.
Comment
This was a large survey, for which the sample was drawn from the GB population. Because of
the way in which the sample were selected ie as a sub-sample of those assessed as disabled (or
aged 75+) it is not easy to find the exact numbers of people interviewed at each stage, as they
appear to differ between tables and text. The report is somewhat opaque and base figures are not
provided in the tables that give data on visual disability. 95% confidence intervals were not
provided. The report is concerned with all types of disability and contains very little information
about people with sight problems.
7
Health Survey for England. Disability among older people (DOH 2000 and 2001) [3, 4]
Summary
The 2000 report from the Health Survey for England focused on the health of older people aged
65+ and included a sample from care homes in the UK. The prevalence of S8 (cannot recognize
a friend across the road) in 2,493 people in care homes (75% response) was 27%. Data on those
in private households was combined with data from the 2001 survey of those aged 16 and over
resulting in 1,677 adults aged 65 and over (response 75%) from the 2000 survey and 15,647
(67% response). The prevalence of S8 in the 16+ population was 2.6% and 9.8% in the over 75
population.
Methods
The Health Survey for England is a series of annual surveys about the health of people in
England. The Health Survey was first proposed by the Department of Health in 1990 to improve
information on morbidity in the population. The survey was carried out in 1991-1993 by the
Office for Population Censuses and Surveys (OPCS), now the Office for National Statistics
(ONS). From 1994 onwards the survey has been carried out by the Joint Survey Unit of the
National Centre of Social Research and the Department of Epidemiology and Public Health at
University College London. The main focus of the 2000 survey was on the health of older people
aged 65 and over living in private households and care homes. The questions relating to sight
disability were: “can you see well enough to recognise a friend at a distance of four metres
(across the road)” (S8) and, “if no, can you see well enough to recognise a friend at a distance of
one metre (at arms length)?” (S4)
Care Homes
In the 2000 survey a stratified random sample was selected from a large database of all care
homes in the UK. In homes with six or fewer residents aged 65 and over all residents were
interviewed. If there were seven or more such residents six were randomly selected for
interview. Interviews were carried out with 2,493 residents comprising 75% of those selected in
these homes. Of these 1,273 were carried out via proxy information.
Private Households
A stratified random sample was selected from the PAF. Interviews were obtained with 1,677
adults aged 65 and over. Interviews were carried out at 75% of the private households at the
selected addresses and interviews were obtained with 95% of adults aged 65 and over living in
these households. For the 2001 survey a stratified random sample was selected from the PAF,
giving a total selected sample of 13,680 addresses aged 16 and over. Interviews were obtained
with 15,647 adults aged 16 and over. The response rate to interview was 67% (71% among
women and 63% among men). The 2001 sample was combined with the equivalent sample of the
population resident in private households in Health Survey for England 2000 to improve the
precision of estimates. The same questions were asked as in the 2000 survey.
Results
Care Homes
The estimated prevalence of sight disability in care homes (as defined by answering a positive
response to S8 by age and gender) is shown in Table 2.2. The prevalence rates were 20% for
8
males aged 65-79 and 27% for those over 80 and for females 17% for those aged 65-79 and 31%
for those aged over 80. The report also showed the reported causes of disability but only a very
small percentage ( 3%) of those visually impaired gave the cause.
Results for private Households
The estimated prevalence for all ages 16+ was 2.6%. The estimated prevalence by age and
gender were males 16-64, 1.2%, 65-74, 3%, 75-84 6%, females 16-64, 1.6%, 65-74 5%, 75+
11% with an estimated prevalence of 9.8% for both sexes age 75+.
Comment
The results were based on two large surveys of the population and a separate component for care
homes. No 95% confidence intervals were provided.
1998/99 Survey of the Needs and Lifestyles of Visually Impaired Adults. (RNIB/ONS
2000)[27]
Summary
This survey was commissioned by RNIB to provide a picture of the needs and lifestyles of
visually impaired people in Great Britain and Northern Ireland. The random component of the
sample was obtained from the FRS/disability follow-up survey, three Omnibus surveys and
several other smaller surveys. 1076 people aged 16 and over were interviewed (response less
than 30%).The prevalence in those aged 16+ of at least S9 (difficulty reading ordinary
newsprint) was 2.8% and 14.4% in those aged 75+.
Methods
The survey estimated prevalence based on (i) the sift questions (ii) results of sight tests. In this
chapter we consider only the results for the sift questions and present the results for the sight
tests in Chapter 3. The Great Britain sample of visually impaired adults was drawn from a
number of sources: three surveys which included questions to identify respondents with sight
problems: the Disability follow-up survey to the 1996 Family Resources Survey (FRS), the
1997-98 Labour Force Survey (LFS), and the ONS Omnibus surveys for April, June and July
1998. Local Authority registers of blind or partially sighted people and a “boost” of ethnic
minority respondents from a focused enumeration within 50 districts found at the 1991 Census to
contain an ethnic minority population density of 40 % or higher. Questions were added to the
Omnibus to identify respondents with sight problems. Interviews were sought from people
answering “yes” to S9 or worse in the FRS/disability follow-up survey (n=1166) and the
OMNIBUS surveys of April June and July 1998 (n=817). The Omnibus surveys were used due
to a low response and ineligibility rate for the FRS sample (see below). The interview covered a
wide variety of topics including: causes of visual impairment; daily living skills; communication
(use of Braille, tapes, computers); mobility; shopping; use and awareness of Social Services.
Weighting procedures were used to allow for non-response and to adjust for the non-random
sampling in the calculation of prevalence estimates.
9
Results
Interviews were achieved with only 640 (32.3%) of the 1,983 subjects originally classified as
visually impaired in the FRS/disability follow-up survey and the three omnibus surveys. Figure
2.5 shows a breakdown of the response rates at each stage for these two components of the
sample. The main reason for low response was ineligibility of many participants who had
originally been classified as visually disabled on first sift, but when the (same) questions were
repeated for the RNIB, participants claimed they had never had a seeing problem or their sight
had improved (see table 2.3). Approximately 30% from the FRS survey fell into this category
and 55% of those from the OMNIBUS surveys. Of the 117 people from the FRS who said their
sight had improved the main reasons were cataract removed (36%), new/better glasses (36%).
7.3% of those in the FRS study who said they had never had a problem who would have been
classified as Blind (S1 to S5), and 47.9% as Partially Sighted (S6 to S8) based on their original
responses. Similar information was not provided for the Omnibus Survey. The authors of the
report were unable to establish the reason for the poor reliability of the vision questions and
concluded that (i) the sift questions may be imprecise because they use the words “have
difficulty” which is open to wide interpretation by respondents and (ii) the misreporting error
seemed to have been greatest in the Omnibus survey when people were asked questions on
vision and other disabilities as part of a general purpose questionnaire.
Table 2.3
People at least S9 on previous
FRS ONS survey
Omnibus survey Both sources
survey
N=1166
N=926
N= 2092
Refusals/non contact
170 (14.6%)
112 (12.1%)
282 (13.5%)
Sight improved
117* (10.0%)
23 (2.5%)
140 (6.7%)
Never had a problem
234 ** (20.1%)
487 (52.6%)
721 (34.5%)
Other
93 (8.0%)
7 (0.8%)
100 (4.8%)
Prevalence rates by age group were estimated for the sift questions (Table 2.2). On the sift
questions, people were categorised as partially sighted for S6-S8, Blind at least S1-S5, and PS+
(sight problem but better than partially sighted) for S10. The prevalence estimates included
people who were sift positive at the re survey plus those whose sight problems had improved.
Comment
The poor response rate is a cause for concern. This was the first report to suggest poor reliability
of the S9 question.
10
Figure 2.5: Flow chart showing the numbers of participants at each stage of the
sampling of the random components of the RNIB 1998/99 survey of adults in
private households
11
MRC Trial of Assessment and management of older people in the community (Smeeth et al
2002 and unpublished data provided by AF) [28]
Methods
The MRC Trial of the Assessment and Management of Older People in the Community is a
cluster randomised trial comparing different methods of multidimensional screening in people
aged 75 years and over [29]. One hundred and six general practices from the UK Medical
Research Council General Practice Research Framework were recruited to the trial. The practices
were recruited from England, Wales and Scotland and were stratified to provide a representative
sample of the mortality experience (Standardised Mortality Ratio) and deprivation (Jarman
Score) of general practices within Britain. In each practice all patients aged 75 years and over
were invited to take part excluding those in long term care or with a terminal disease. The trial
consisted of two arms. In the “universal” arm, all participants received a brief health
questionnaire followed by a more detailed assessment by a practice nurse. In the “targeted” arm,
all participants received a brief questionnaire but only those who “triggered” on predefined
responses received a detailed assessment. Practices were also randomised to one of three
methods of administering the brief questionnaire: postal, lay interviewer, practice nurse. The
brief questionnaire included a single question on sight. : Do you have difficulty in seeing
newsprint, even if you are wearing glasses? Participants were given a choice of three responses:
No difficulty, A little difficulty and a lot of difficulty. The visual acuity tests in the detailed nurse
assessment are described in Chapter 3.
Results
Of 42,278 eligible patients, 32,990 people responded to the brief questionnaire, an overall
response rate of 78.0%. Men were more likely to respond than women (80.5% versus 76.7%,
P<0.001) and the sex difference persisted after controlling for age. Responders were slightly
younger than non-responders (median age of responders 80.3 years, non-responders 81.0 years,
P<0.001). Response rates were slightly higher with the postal questionnaire (84%) compared to
Lay (74%) or Nurse (76%). The proportions with missing responses to the vision question were
very low: Postal (2%), Lay (0.7%), Nurse (0.9%) The percentages in each response category are
shown according to the method of administration (Figure 2.6) with relatively small differences
other than a slightly higher response for postal administration for “a lot of difficulty”.
Overall 67.6% reported “no difficulty”, 22.8% reported “a little difficulty” and 9.6% reported “a
lot of difficulty”. The proportions reporting a lot of difficulty were higher in women (11.2%)
than in men (7.5%) and rose steeply with age 9.8% in those aged 75-84 compared with 19.5% in
the over 85 age group (Table 2.2). For any difficulty (“a little” combined with “a lot” the
proportions overall are 32% and again higher in women than in men and rising with age.
12
Comment
The data provide good estimates of the prevalence of the response to S9 in the older population
being derived from a large study with high response rates and drawn to be representative of
patients in general practice in Britain. An advantage over other surveys which have used the S9
question was the use of a categorical rather than binary response scale to distinguish those with
minor difficulties (“a little”) from those with major difficulties (“a lot”). 95% confidence
intervals are provided.
The limitations of the study are that it does not include people in nursing home care although
people in residential or sheltered housing are included.
Figure 2.6
100%
8
80%
24
10
11
19
24
60%
40%
68
71
65
A lot of
difficulty
A little
difficulty
No difficulty
20%
0%
Nurse = 10,358
Lay = 9,757
Postal = 12,857
13
English Longitudinal Study of Ageing (ELSA) [5]
The ELSA sample was drawn from households previously responding to the Health Survey for
England (HSE) during 1998, 1999 and 2001. Individuals aged over 50 were eligible for
interview. 12,000 people took part (response rate of 67%). The true response rate is lower than
this (at around 45%) since the HSE response rates were about 70%. The topic areas covered by
the ELSA questionnaire included: individual and household characteristics; physical, cognitive,
mental and psychological health; social participation and social support; housing, work,
pensions, income and assets; and expectations for the future.
The vision questions used were
“How good is your eyesight for seeing things at a distance, like recognising a friend across the
street”;
“How good is your eyesight for seeing things close up, like reading ordinary newspaper print”.
The choice of response scales was: excellent, very good, good, fair or, poor. The questions
included (using glasses or corrective lens as usual)
The first results from the 2002 data collection were reported in December 2003. Results were
provided only for the responses to “reporting fair or poor eyesight” (Table 2.2). The results show
16% of the over 50 age group reporting fair or poor eyesight with a steep increase with age. The
proportions are higher in women than men across the age range.
Comment
ELSA is a nationally representative sample of people in private households with the opportunity
to obtain longitudinal data. Overall response rates to successive sampling are less than 50%. It is
difficult to understand what is being measured by the question on rating eye sight. Unlike the
questions used in the other surveys, which are task oriented, the question is general and may be
influenced by the person’s expectations of their eyesight and other personal characteristics.
14
Chapter summary
The estimates of the prevalence of visual difficulties in the different studies are summarised in
Tables 2.2 and 2.4. As S8“cannot recognise a friend across the road” is a worst severity than the
question S9 “difficulty reading newspaper”, the estimates from the surveys that have used at
least S9 are considered together. The figures show some consistency for the older age groups
with most studies showing a prevalence in the over 75s of around 25% to 30% for reading
difficulties or worse. The exception is the second RNIB/ONS 2000 survey which reports a lower
prevalence for the over 75s age group. The overall prevalence in the adult population aged 16+
varies from 2.6 to 4.8%. For more serious difficulties (as in the MRC assessment trial or the
Health Survey for England) the prevalence in the over 75s is around 10%.
The difference between the first and second OPCS disability surveys (i.e. 1985, and 1996/7) are
more difficult to explain. The second survey produced a much higher predicted number of people
with overall disability in the community than the first one (5,600,000 vs. 8,582,200). The authors
of the second study state that it is difficult to explain the apparent increase but suggest it is due to
different sift criteria for overall disability. The prevalence of visual disability in the overall adult
population (16+ age group) is also higher (3.2% vs. 4.5%). However, the differences are
primarily in the younger age groups; the prevalence at ages 16-64 in males is 0.8% in the first
survey and 2.2% in the second survey while for women at ages 16-59 the prevalences are 0.9%
in the first survey and 2.0% in the second. In the oldest age groups the prevalences were 22.5
and 22.6 respectively in the two surveys. Another factor could be that the reliability of the
responses in the second survey was poor. As described earlier in the chapter, information from
the second RNIB survey showed that 30% of the original FRS/disability follow-up participants
who had originally said that they had a seeing problem, subsequently stated that they had never
had a problem, despite being asked the same questions on both occasions. No information is
available on the reliability of the questions in the first ONS survey.
Estimates of the proportion or numbers in the population with self reported visual difficulties are
based on the criteria of difficulties reading newsprint or worse. They do not give information on
the severity of the problem and use a criterion which includes minor visual problems. Only one
study (the MRC trial of assessment in the elderly) used a categorical scale for severity of reading
newsprint. 32% of the over 75s age group reported any problem with reading newsprint but only
9.8% reported a lot of difficulty. The large Health Survey for England used the question “could
not recognise a friend at 40 metres” to categorise sight difficulties with 9.8% of the over 75s
reporting positively to this question.
Confidence in the results is further limited by the lack of detailed information on response rates
especially in the government surveys. Response rates are difficult to estimate for many of the
studies that have used different samples and successive sifting. For example the second
RNIB/ONS 2000 Needs survey includes people from previous surveys (FRS and Omnibus) plus
top up samples from a number of other sources including non random groups from local
authority registers. The response rate from the FRS sample was 86% but the FRS sample itself
had a response rate of 68% (cumulative response rate was less than 60%). Most studies have not
provided the final response rates in the sample from whom the estimates are derived.
15
Table 2.1 Summary of studies considered in this Chapter
Survey
Age
Number
surveyed
Response
ONS Disability survey 1988
16+
189,479
64%
ONS/FRS Disability survey 1999
16+
33,897
68%
ONS /RNIB 2nd Needs Survey 2000
16+
11,673
59%
Health Survey for England 2001
16+
17,324
67%
MRC Assessment trial 2002
75+
33,326
77%
English Longitudinal Study of Ageing 2003
50+
11,384
50%
16
Table 2.2: Prevalence of reported visual difficulties among adults in Great Britain.
Survey
Population
OPCS 1985
Disability Survey
(Martin et al.1988)
[25]
Private
Household
Original
Sample
Size
185,688
Criteria for
prevalence estimates
Difficulty reading
newsprint and/or
cannot recognise
friend across road
Sex
Age
Prevalence
%
M1
16-64
65-74
75+
16-59
60-74
75+
16+
16-59
60-74
75+
16-59
60-74
75+
16+
16-64
65-74
75+
All
16+
16-59
60-74
75+
All
16+
16+
75+
16-59
0.8
5.7
19.4
0.9
5.2
25.4
3.2
0.8
5.2
22.5
0.9
5.6
26.2
3.8
2.2
6.4
17
3.5
60-74
4.00
75+
16+
14.4
2.82
65-79
80+
65-79
80+
65+
16-64
65-74
75+
16-64
65-74
75+
16+
75+
20
27
17
31
27
1.2
3
8
1.6
5
11
2.6
9.8
75-84
85+
5.8 (5.1, 6.6)
15.4 (13.7, 17.2)
F
All
All
1996/7
Disability Survey:
FRS follow-up
(Grundy 1999)
[26]
Private
households and
communal
establishments
Private
households
189,479
Difficulty reading
newsprint and/or
cannot recognise
friend across road
33,897
All
M
Difficulty reading
newsprint and/or cannot
recognise friend across
road
F
All
RNIB/ONS 2000
[27]
DOH Health
Survey for
England
2000
[3]
Private
households and
communal
establishments
Care
Homes
All
11,673
2,493
17,324
DOH Health
Survey for
England
2000/ & 2001
[3, 4]
At least S9 on sift
questions on re-survey
Cannot recognise
friend at 4 metres
(across the road)
Cannot recognise
friend at 4 metres
(across the road)
M
F
All
M
F
All
MRC trial of
assessment of older
1
106 general
practices
33,000
people
Reporting “ a lot of
M
Figures for men and women for the 1985 OPCS survey are obtained from [Grundy et al.1999]
17
2.0
6.7
26.3
5.3
4.5
22.6
1.14
people
[28]
(additional results
provided by AF)
representative
of mortality
and
deprivation in
the UK
aged 75+
excluding
those in
nursing
homes
difficulty” in reading
newsprint
F
All
Reporting “any
difficulty in reading
newsprint”
M
F
All
English
Longitudinal Study
of Ageing
[5]
Random
sample from 3
separate years
of HSE
11,384
aged 50+
Reporting fair or poor
eyesight in response to
question How good is
your eyesight?
M
F
All
Table 2.4 Prevalence of self reported visual difficulties in different studies
16+
Difficulty reading newsprint
OPCS 1985 Disability Survey[25]
FRS follow-up survey 1999[26]
RNIB/ONS 2000 [27]
MRC assessment trial [28]
Any difficulty
A lot of difficulty
Cannot recognise friend across
road
DOH HSE 200 and 2001 [3, 4]
75+
3.2
4.5
2.8
22.5
22.6
14.4
32.4
9.6
2.6
Fair /Poor eyesight
ELSA 2003 [5]
9.8
27.9
18
75-84
85+
75-84
85+
75+
75-84
85+
75-84
85+
75-84
85+
75+
50-64
65-74
75+
50-64
65-74
75+
50-64
65-74
75+
8.0 (7.2, 8.7)
21.2 (19.3, 23.0)
7.1 (6.5, 7.8)
19.5 (17.9, 21.0)
9.8 (9.0, 10.6)
23.4 (21.5, 25.3)
43.2 (40.5, 45.9)
29.9 (28.1. 31.7)
50.9 (48.5, 53.3)
27.4 (25.6, 29.2)
48.6 (46.5, 50.7)
32.0 (30.2, 33.7 )
11.2
13.2
24.0
12.6
16.4
30.3
11.9
14.7
27.9
Chapter 3
Population-based surveys using visual acuity measurements
19
The methods employed for each survey are shown in Table 3.1 and the key results are presented
in Table 3.2. Since most of the relevant material is presented in these tables, only a short
summary of each survey is provided in the following text. The exception to this is the 1991
report by Bruce et al which is a sub-survey of the OPCS 1988 disability survey ([25]). A full
description of the study methods and results is presented in the text.
Vision and visual-acuity in an elderly population (Lavery et al.1988) [30]
Summary
Visual acuity data were obtained from 529 people (response rate 78%) from a large general
practice in Melton Mowbray. 18% of men and 30.2 % of women aged 76 and over were found to
have a best corrected visual acuity in the better eye of 6/18 or worse.
Methods
The data were obtained from an earlier eye disease study carried out in 1982 [31]. All those
registered in a general practice in Melton Mowbray (the only general practice in the town) who
were 75 and over on December 31 1980 and alive on April 1st 1982 were included in the sample.
529 (out of the 677 eligible) subjects were examined (78.1%), 55 at home or in hospital, the rest
were examined in a hospital outpatients department. Acuity was measured using a standard 6
metre illuminated Snellen chart, both with and without spectacles with full refraction by an
optometrist. The prevalence estimates given are for best corrected acuity in the better eye.
Results
2.6% of the men and 4.4% of the women were found to have visual acuity (best corrected) of
less than 6/36 (3.8% both sexes). 15.4% of the men and 25.8% of the women had visual acuity
between 6/18 and 6/36 (22.4%, both). 18% of men and 30.2 % of women aged 76 and over were
found to have a best corrected visual acuity in the better eye of 6/18 or worse. 88.9% of males
and 88.6% of females had near vision of N8 of better and 94.9% males and 94.0% females had
N10 or better.
Comment
The measures of visual acuity were carried out by an optometrist and likely therefore to be
accurate. Near acuity was measured. The response rate was high. The main limitations are: the
results are based on a small sample size; 95% CIs were not provided; the population surveyed
was from a small market town and it is difficult to extrapolate to other urban or rural areas.
Visual problems in the elderly population and implications for services (Wormald et
al.1992) [32]
Summary
207 subjects aged 65+ (response 72%) from an inner London general practice were examined.
The prevalence of blindness was found to be 1% by WHO criteria (VA <3/60) and 3.9% by the
US criteria (VA < 6/60). The prevalence of vision impairment (VA <6/18) was 7.7%. 75% of
visual impairment was due to cataract.
20
Methods
The study was a random sample from an inner London general practice. 207 subjects aged 65+
(response rate of 72%) were examined in a day centre for the elderly. Subjects underwent a full
eye exam by an ophthalmologist. Binocular distance visual acuity was measured using Snellen
charts and best monocular 3 m Sonksen Silver acuity in each eye, with spectacles if normally
worn. Near vision was recorded as being able to able to read N6 with normal reading correction.
Confidence intervals were calculated by exact methods for small proportions and normal
approximation to binomial distribution for larger ones.
Results
The prevalence of blindness was 1% by WHO criteria (<3/60 in better eye) and 3.9% by US
criteria (better eye < 6/60). The prevalence of low vision by WHO criteria (best acuity <6/18)
was 7.7%. The tests for binocular near vision showed that that 88.9% of males and 88.6% of
females had near vision of N8 or better and 94.9 males and 94.0 females had near vision of N10
or better. The proportion of acuity problems due to eye diseases was recorded (see Chapter 4).
Comment
The measures of visual acuity were carried out by an ophthalmologist. Near acuity was
measured. The response rate was reasonable. The main limitations are: small sample size and
hence lack of precision; the participants were from a single inner city general practice.
Prevalence of serious eye disease and visual impairment in a north London population:
population based, cross sectional study (Reidy et al.1998) [33]
Summary
1547 participants (84% response) were recruited from 17 randomly sampled general practices
from north west London. 30% had presenting visual acuity of <6/12 in both eyes and a further
23.6% in one eye only.
Methods
Cross sectional survey using two stage cluster random sampling. The sample frame was 17 GP
practices (1840 people) in 6 electoral wards in north west London. The wards were chosen after
consultation with district and regional health authorities. 1,547 people, (response 84%) aged 65
and over, were examined and visual acuity measured using a LogMAR chart. Glasses were used
if worn. Each eye was tested with and without a pinhole. Full refraction was also carried out. .
Visual impairment was defined as a visual acuity of less than 6/12. Calculation of the population
prevalence estimates and their 95% confidence intervals took into account the two stage cluster
random sampling design.
Results
30.2 % (448) of the sample aged 65 or older were found to be visually impaired (<6/12) in both
eyes. Of these, the visual acuity in the worst affected eye was <6/12 to 6/18 in 19%, <6/18 to
6/60 in 61%, and <6/60 in the remaining 21%.
21
Comment
The study had high response rates and a moderate sample size. Confidence intervals were
presented. The eye examination was conducted by ophthalmologists. The report was mainly
concerned with the prevalence of eye conditions and there is very little information by levels of
visual acuity or by age and gender specific rates. No data are presented on visual acuity levels in
the better eye for comparative purposes with other data.
Visual acuity measurements in a national sample of British elderly people (van der Pols et
al.2000) [34]
Summary
1,362 people aged 65 and older from private homes and nursing homes (response rate 75% in the
private homes and 94% in nursing homes) participating in the National Diet and Nutrition
Survey (NDNS) were measured by a nurse for visual acuity. 14.3% (9.3% from private homes
and 35.6% in nursing homes) had pinhole corrected visual acuity <6/18.
Methods
A random cross-sectional survey of people aged 65+ was carried out for the National Diet and
Nutrition Survey. Participants were given eye tests using Glasgow Acuity Cards (LogMAR) at 3
metres at their place of residence by a nurse who had been trained in this method. The tests were
carried out (i) without spectacles (ii) with spectacles and (iii) spectacles and pinhole occluder.
Prevalence rates are based on pinhole corrected visual acuity in the better eye.
Results
14.4% of those interviewed had visual impairment (pinhole corrected<6/18). The prevalence of
visual impairment increased significantly with age (65-74 years 3.1%; 75-84 years 11.6%; 85+
years 35.5%. Impaired vision was more common in subjects living in a nursing home, odds ratio
adjusted for age 2.59 (95% CI 2.23 to 2.96) and in women, odds ratio adjusted for age 1.55 (95%
CI 1.21 to 1.89). 132 (9.7%) subjects had previously undergone cataract surgery and another 157
(11.5%) had been told that they currently had cataract.
Comment
The study population was a representative sample covering the whole population of Great Britain
and of a moderate sample size. This is the only survey to give figures for those in
residential/nursing homes. The measurements were done by a nurse in the participant’s home
and the conditions under which the test were done (e.g. poor lighting, inadequate distance) may
have affected the results. The accuracy of nurses to carry out the measurements, compared to eye
specialists (optometrists or ophthalmologists), is not known.
22
Prevalence of visual impairment in people aged 75 years and older in Britain: results from
the MRC trial of assessment and management of older people in the community (Evans et
al.2002) [35]
Summary
14,600 subjects aged 75 and over (response 71%) from 53 general practices across Britain were
examined for visual acuity. 12.4% overall (95% CI 10.8 to 13.9) were visually impaired
(binocular VA < 6/18); 10.3% (95% CI 8.7 to 11.8) were categorised as having low vision
(binocular acuity <6/18 to 3/60) and 2.1% (95% CI 1.8 to 2.4) were blind (binocular acuity
<3/60).
Methods
The design of the MRC Trial was described in Chapter 2. The data used in this Chapter relate to
the results of the visual acuity testing reported for the universal assessment arm of the trial which
took place in 53 practices. Visual acuity was measured at 3 metres with a Glasgow Acuity Chart.
Binocular vision was measured first, followed by vision in the right and left eyes. All vision
measurements were conducted with usual spectacle correction. People with visual acuity of 0.5
or more in either eye (equivalent to less than 6/18 Snellen acuity) were retested with a pinhole
occluder. Prevalence rates are presented for four levels of visual acuity also visual acuity of less
than 6/12 and less than 6/18, along with 95% confidence intervals adjusted for cluster design.
Results
Out of 21,241 eligible people, 15,126 (71%) had a detailed assessment. Of these 699 did not
have a vision test: 173 were registered blind/partially sighted and in 526 (3.5%) there was no
information on vision (these people were older, more likely to be women and more likely to
require a proxy for the interview). People who were registered blind or partially sighted and who
did not have a visual acuity test were included in the estimates for visual impairment.
Using the criterion of binocular presenting vision, 12.4% overall (1803) were visually impaired:
1501 (10.3%) were categorised as having low vision (<6/18- 3/60) and 302 (2.1%) were blind
(<3/60). The prevalence was higher in women than men and rose steeply with age. At ages 7579, 6.2% of the cohort was visually impaired with 36.9% at age 90+. At ages 75-79, 0.6% of the
study population were blind, with 6.9% at age 90+. After pinhole correction, the prevalence of
VA < 6/18 in the better eye was 10.3%. However there were problems with the use of the
pinhole in the study: only 62% of those with VA 6/18 in either eye completed the pinhole test
satisfactorily and therefore the pinhole corrected estimates are likely to be an overestimate since
some correctable refractive error was not removed. If visual impairment had been defined as
visual acuity of less than 6/12 the age-specific prevalence estimates would have increased by
60% with 19.9% of study participants with a binocular acuity of less than 6/12.
Comment
Results are based on a large sample size covering many areas of Britain and the participants are
from general practices which were recruited to be representative of the mortality and deprivation
of the UK. The response rate is moderately high. The large sample size allowed for presentation
of data in the oldest age groups i.e. 80-84, 85-89 and 90+. Measurements were carried out by
nurses and the reliability of nurses compared to eye specialists (optometrists or
ophthalmologists) is not known. Full refraction was not possible and difficulty with the pinhole
23
means that a proportion of people with refractive errors may have been underestimated. The
study sample excluded those in long stay hospital or nursing homes.
What is the prevalence of visual impairment in the general and diabetic populations: are
there ethnic and gender differences? (Hayward et al.2002) [36]
Summary
This was a population-based cross-sectional observational study investigating the prevalence of
visual impairment and ethnic and gender differences in the general and diabetic population in
Leicestershire of all ages (0+). Using capture mark recapture methods, the prevalence of
blindness and partial sight in the general and diabetic populations was estimated to be 0.52% and
1.49% respectively.
Methods
Subjects were classified as blind or partially sighted if (i) already registered (ii) visual acuity in
appropriate criteria and/or registration was reported by hospital staff (iii) participants were
unregistered but answered “yes” to questions S9 or S8 (as described in table 1.3). Visual acuity
tests for category (iii) are mentioned, but not described. Capture Mark recapture (CMR) methods
were used to estimate prevalence. Data on visual impairment in the population to be studied was
obtained from the Royal Leicestershire Rutland and Wycliffe Society for the Blind (RLRWSB).
Secondary recapture, used to correct for under-ascertainment of those eligible for registrations,
was from four sources: (i) Mass media inviting people who had, or knew others, with poor vision
to volunteer information (ii) information on patients known to be blind or partially sighted
provided by consultants and nurses on hospital wards (iii) hospital admissions data on anyone
with ICD10 code for blindness or low vision and (iv) information from general practitioners. A
census day was set (November 5th 1997) with computer cross-linkage to Leicestershire Health
authority data ensured patients were alive and resident in Leicestershire at that date. Estimates of
diabetes, ethnicity and gender prevalence were calculated using the 1991 census.
Results
According to the register and 1990 census figures, 0.52% (4,477) of the population of
Leicestershire was registered as blind or partially sighted (all ages). The prevalence of blindness
and partial sight in the diabetic population was 1.49% .Data was also presented for Asians
compared to Whites. In the white population there were significantly more females with visual
loss than males, but the reverse was true in the Indo-Asian population. The estimated prevalence
for Asian males was more than double that for Asian women. Approximately 30% of those
eligible for registration were not registered as blind or partially sighted (based on sift criteria).
Comment
This was the only survey to use CMR and registration figures. The method for classifying
unregistered subjects as blind or partially sighted for mass media recapture group was unclear.
There are very few (41) Asians included in the study and the prevalence estimates for Asians
will therefore be unreliable. No age-related estimates of blindness and partial sight were
provided. Using the sift questions as an estimate of registerable blindness and partial sight is
inappropriate as the estimates may include those with treatable conditions.
24
Blind and partially sighted adults in Britain: The RNIB survey (Bruce et al 1991) [37]
Summary
This report was an investigation of blind and partially sighted people and their needs by the
RNIB. The survey was based on interviews with 595 people, two thirds of whom were
respondents to the 1985 OPCS survey[25] and one third whose names were obtained from the
local authority partial or blind sight registers. Prevalence estimates were calculated based on 338
people aged 25 + with sight tests and who were interviewed. These results were applied to the
adult population 16+ and expressed as those eligible to be registered as blind or partially sighted.
These figures were 0.03% aged 16-59, 2.3% aged 60 to 74, 15% aged 75 and over. Overall
response rates were probably lower than 50% as people were lost at successive stages.
Methods
The sampling procedure used to select the interviewees is depicted in Figure 3.1 along with the
response rates (where given). The initial sample was 2,359 respondents aged 16+ from the 1985
OPCS disability survey that had responded positively to the questions “has difficulty to read
ordinary newspaper print” and/or “cannot see well enough to recognise a friend across the road”
Of these, all aged under 60 and 50% of those aged over 60 were offered a home sight test . The
response rates to the sight test were not given. The sight test was measured using a standard
Snellen chart (scaled down for use at 10 feet). Spectacles, if worn, were used. For near vision the
test type used was the standard ‘N’ form. The criterion for inclusion in the RNIB survey was
visual acuity no better than 6/24 or no better than N14 for near vision in the better eye. People
aged 16-24 were not eligible for the sight test as this age group was being used by OPCS for
another study.
1,248 people aged 25 + fulfilled these criteria. Further selection of the sample took place to
identify for invitation (i) all those who said that they were registered, (ii) all those reporting they
were not registered but with “lower residual vision levels “(not defined but assumed to be <6/36)
(iii) all aged under 60 and half of those aged 60 and over who had visual acuity equal or better
than 6/36. OPCS initially approached those selected for an “in principle” agreement to take part
(the numbers refused are not given but there is a statement that “a handful refused). 524 people
were invited for the needs survey of whom 338 took part (response rate of 65%).
Because of concern over low numbers, especially in the under 60s age group, a “top up sample”
was recruited. 67 people from the OPCS group “at registerable vision levels” were included;
69% of these participated. In addition nine local authorities were approached and asked to
provide names of people on their registers. No information provided on how this was done and
whether the numbers represented 100% of those on the registers or whether this was a selected
sample. 312 “agreed to be interviewed” of whom 211 participated (68% response rate)
This mixed sample i.e. 384 from the OPCS survey plus 211 from the local authority registers
formed the 595 people.
Estimating prevalence and numbers
The population prevalence for private households by age groups were calculated by using, as the
denominator, the actual numbers in these age groups in the full OPCS sample applying the
relevant weighting to the >60 group who were successively under sampled. The numerator was
the 384 people (described above) above who had a sight test in the OPCS sample and who
25
fulfilled the visual acuity distance or near vision criterion described above. These people were
distributed by age group: 25-59 (133); 60-74 (84); 75+ (167).
These prevalence estimates were then applied to the population of Great Britain to provide
estimated numbers of people in private households with registerable visual impairment. Since the
age group 16-25 were not given sight tests and hence not in the needs survey, the estimated
prevalence rates for the age group 16-59 were based on the assumption that people aged 16-25
have the same prevalence of visual impairment as those 25-59.
The estimated number of those in residential institutions was calculated by analysing the
responses to the OPCS questionnaires by residence in communal establishments and assuming
that all those in severity stage 3-10 for seeing difficulties were visually disabled, i.e. S6 and
worse which is those that cannot read large print and/or cannot see well enough to recognise a
friend across a room. It was unclear as to why people in the communal sample of the OPCS
survey were not offered sight tests.
Results
The report estimated there were 757,000 visually “impaired” adults in private households aged
16+ in Great Britain eligible for registration. The estimate for all adults 16+ (including those in
residential institutions) was 959,000. The age-related prevalence estimates, for people in private
households classified as eligible for registration as blind or partially sighted, were: 0.3% for
ages 16-59, 2.3% for ages 60-64 and 15.2% for 75+. These estimates are based on those who
have a binocular visual acuity of no better than 6/24 and/or near vision of no better than N14.
Gender related figures are not provided in this report. The estimates for those aged 16-59 are
based on the sample of those aged 25-59, based on the assumption that the prevalence for 16-25
is the same as that of 25-59. They also found that the responses to the sift questions were related
to the Snellen scores but that there was considerable deviation to a one-to-one relationship.
Another interesting finding was that not all of those reporting that they were registered blind had
acuity levels of 3/60. Some had acuity measurements of better than 6/60. Conversely, some of
those reporting that they were registered as partially sighted had acuity of 3/60 or worse.
At interview additional information was collected on a number of aspects of services, mobility,
activities of daily living leisure and employment and self report of cause of sight problem. The
main cause of sight problems reported by a third of respondents was cataract (15% ages 25-59,
35% 60-74, and 35% aged 75+); 8% overall (30% of 25-59, 12% aged 60-74 and 3% of 75+)
reported vision problem from birth while the respective figures for accident were 8%, 7% and
3%.
26
Figure 3.1 Sampling procedures
100,000 households selected
Response rate 81%
185,688 on sift forms
28,415 aged 16+ with disability
18,000 selected for interview
Response rate 79%
14,308 interviewed
2,534 with seeing difficulties
All < 60 + 50% 60+
Invited for home sight test of near and distance vision
1,248 aged 25+ with VA <=6/24 or <=N14
Response rate ?
Not given
Selected
1. All who reported they were registered
2. All reported they were unregistered with VA <6/36
3. 50% of those aged 60+ with VA >= 6/36
Numbers not given
OPCS wrote to those selected asking if they would agree to be
approached for re-interview
Response rate ?
Not given “handful refused”
524 people invited; 338 interviewed
Ages 25-59 110
60-74 75
75+ 153
TOP UP SAMPLE
67 aged 25+ from OPCS sample with “registerable” vision levels
(all ages) of whom 46 interviewed
plus
Non random selection of nine local authorities (LAs)
LAs wrote to people registered as blind or partially sighted
(number not given) aged 16+
312 agreed to be interviewed
Response rate 65%
Ages 25-59 69%
60-74 66%
75+ 61%
Response rate
69% of original OPCS sample
plus 68% of LA sample
211 actually interviewed
Combined number in OPCS and LA sample interviewed = 595
27
Comment
The RNIB OPCS sample was derived from sight test results of people who were a sample of
those who reported a seeing disability. As Figure 2.3 shows the identification of these people
assumes that the sift process and seeing disability questions have high sensitivity so that people
with visual impairment were not missed at the successive stages. Additionally, one third of the
interviews were obtained from a non-randomly sampled population and since these people were
aggregated with the OPCS sample for almost all analyses, it is not possible to examine the results
separately for the national sample. However the estimates of prevalence and numbers were
carried out separately. Although at each stage of sample selection the response rates are
reasonable (varying from 65% for the needs survey to 81% for the first step in the OPCS
disability study), the cumulative response rate is much lower (at least less than 41.5%). It is
likely to be lower than this since, at various stages, response rates are not provided e.g. the
proportions that refused a sight test. There is little information on the results of the sight tests in
the full sample that underwent these.
1998/99 Survey of the Needs and Lifestyles of Visually Impaired Adults. (RNIB/ONS 2000)
[27]
The methods for this survey were described in detail in the previous chapter. Interviewers also
administered a sight test (using a Snellen light box scaled down for use at 10 feet) in their homes.
For near vision the standard ‘N’ form was used. Glasses, if used, were worn. The report does not
present details of the results of the sight test. A variable was derived which defined visual
impairment according to the following: registered as blind or partially sighted, or not registered
but with vision < 6/24, or with VA in the range 6/24 – 6/12, or better than 6/12 with field loss or
failed reading test. The reading test was defined as failed if one word or more was wrong on the
smallest (6pt) line of the grey card. In the calculations of prevalence we excluded those with
vision better than 6/12 but with field loss as it was not clear how this information on field loss
was obtained. The results show lower prevalences than obtained on other studies.
28
Chapter summary
This chapter includes data from six studies and two OPCS/RNIB reports. Only two studies (the
MRC Assessment trial [35]and the NDNS study [34]) and the two OPCS/RNIB reports [27, 37]
were based on samples covering the whole of Britain. The others are from selected regions, two
in London [32, 33]and two in Leicestershire[30, 36] .
The MRC Assessment Trial is much the largest with 14,600 people aged 75 and over examined.
The North London Study [33]and the NDNS [34] examined 1547 and 1362 people respectively
(aged 65+). Lavery et al [30] examined 529 people (aged 75+), and Wormald et al [32]examined
207 (aged 75+). The number included in the secondary recapture in the survey by Hayward et al
[36]was 499 although it appears that only 26 of these were given visual acuity measurements.
The survey by Hayward was the only survey to include people of all ages, and the only one to
include people aged under 65. Unfortunately they do not give a breakdown of the estimates by
age or sex.
In order to compare the rates in the different studies, a summary of the age-specific prevalence
rates for each survey is shown in Table 3.3 together with the 95% confidence intervals. If
confidence intervals were not provided, these were calculated using the binomial method with no
adjustment for clustering (since the clustering information was not available). For the two RNIB
surveys it was not possible to calculate confidence intervals since the base population was not
given. Caution must be used in comparing results from different studies as it is not possible to
age standardise across the studies and the relative proportions of the very elderly (with the
highest levels of impairment) may be very different across the studies. Carrying out a formal
pooling of the studies in the UK was attempted but was not possible because the age specific
data (numbers examined and numbers with different acuity levels) could only be provided by
two studies.
Table 3.3 shows that there appears to be reasonable agreement for the over 75 age group between
three of the studies that presented estimates for <6/18, i.e. NDNS (15.2%), MRC Assessment
Trial (12.4%) and Wormald et al (14.2%) but that of Lavery et al is much higher at 26.2%.
However the measurements are all of different parameters: NDNS reported pinhole corrected,
the MRC Assessment Trial binocular presenting and Wormald et al and Lavery et al are best
corrected. The MRC Trial estimate is lower than in the other studies and even lower when
pinhole corrected acuity is considered. On the other hand, the 95% confidence interval shows
that the estimates from the trial are the most precise and the upper bound of 13.9 excludes the
point estimates of both Wormald et al and for the NDNS. The confidence interval for the
Wormald et al study is very wide. The Lavery et al estimate may reflect a genuine difference due
to the population in Melton Mowbray or it may reflect a higher level of vision impairment in an
elderly population measured nearly 20 years before the more recent surveys, for example if there
were more untreated cataracts 20 years ago the prevalence of vision impairment would be
higher. Age differences will also play a part. For example, of those aged 75+, proportionately
more were aged over 85 years in the NDNS study (34%) compared to the MRC Trial (21%) and
Lavery et al (21%). The age specific rates for 75-84 and aged 85+ which we have calculated
from Table 3.2 are very similar for the MRC Trial (8.6 %) and the NDNS (7.7%) (pinhole
corrected) and 26.5 and 29.7 respectively in the 85+ group.
29
The North London Study estimates are also very high which again may again be peculiar to the
North London population. The prevalence estimates for the two ONS/RNIB studies are very
similar for the age groups 16-59 (0.3 vs. 0.31) and 60-74 (2.3 vs. 2.05) but are completely
different for those aged over 75 (15.2 vs. 5.4). As discussed above, the lower prevalence in the
2000 RNIB survey may reflect the exclusion of those identified on the sift questions whose sight
problems had been resolved (mainly cataract and new glasses). It is therefore (as we will discuss
in the next chapter) closer to the criterion for registerable impairment. If the 1990 ONS/RNIB
estimates are taken as binocular presenting vision (without any criteria for registration) they are
close to the estimates for the over 75s age group reported by the MRC Trial and NDNS.
30
Table 3.1 Population based surveys using visual acuity measurements - methods
Name
North
London
Study
Locatio
n and
date of
survey
North
London
1995-6
Reidy et al.
1998
[33]
NDNS
Van Der
Pols et al.
2000
[34]
Great
Britain
1994 1995
Aims
Inclusion
/Exclusion
Criteria
Design and methods
Visual
measurement
Sample size
Response
rates
Number
examined
Demographi
c
To estimate
prevalence
of main
serious eye
disorders
Age 65+
Cross sectional survey
using two stage cluster
random sampling.
Sample frame 17 GP
practices in 6 electoral
wards. Wards chosen after
consultation with district
and regional health
authorities.
Accounted for clustering
effect in estimation of
standard errors.
Cross-sectional survey.
80 postcode sectors
selected with probability
according to size.
Individuals randomly
selected with probabilities
required to produce
suitable numbers for
statistical comparisons in
each sex and age group.
LogMAR at 6
metres by
ophthalmic nurse.
Glasses if worn
were not removed.
1,840 from 7
GP practices
84%
1547 from
7 GP
practices
Age and sex
distribution
similar to
that of
sample
population.
94.3%
white
2,060
75% of
those in
private
household
s and 94%
in nursing
homes
1362
Mean Age
78.2 (sd 8.0)
To measure
visual acuity
in elderly
Registered at
GP practices
Age 65+
Private
households
and nursing
homes (no
geriatric
hospitals)
Not mentally
impaired
One person per
household
3 per nursing
home
Pinhole test
Glasgow acuity
Cards (LogMAR)
at 3 metres at their
home by a nurse
who had been
trained in this
method.
1.without
spectacles.
2. with spectacles
3. spectacles and
pinhole occluder
Prevalence figures
are based on best
visual acuity score
from 1,2, or 3
22.6% of
those
measured
live in
institutions
99.2%
white
49.2 female
MRC
Assessment
Trial
Evans et al.
2002
[35]
Great
Britain
1994 1999
To estimate
Prevalence
of visual
impairment
Age 75+ .
Not in long
stay hospitals
or nursing
homes or
terminally ill
Cluster randomised cross
sectional survey (using
data from MRC cluster
randomised trial).
Sample frame 106 GP
practices stratified by
SMR and Jarman score.
Glasgow acuity
Cards (logMAR) at
3 metres by trained
practice nurse.
Glasses if worn
were not removed.
21,241
from 53 GP
practices
71%
4477 on
blind and PS
registers.
499 of the
720 from
secondary
sources
deemed
eligible
(of whom
340 were
already
registered)
14,600
from 53
GP
practices
75+. Median
age 80.3
Pinhole test.
Accounted for clustering
effect in estimation of
standard errors.
Hayward et
al. 2002
Leicest
ershire
1997
[36]
Wormald et
al. 1992
[32]
Inner
London
(date
not
To estimate
prevalence
of visual
impairment
in general
and diabetic
population.
Investigatio
n of ethnic
and gender
differences
To estimate
prevalence
of visual
disability
All ages
(including
children)
Included if
1.already
registered or
certified as B
or Ps.
2. visual status
and
registration
reported by
hospital staff
3. unregistered
but answered
“yes” to RNIB
questions.
Age 65+
Population-based crosssectional observational
study.
Population derived from
1991 census.
229 counted as
visually impaired
on basis of selfreported
registration.
Unclear. Mention
Snellen charts and
`E’ charts for the
mass media source,
but do not explain
how performed.
Capture Mark Recapture
methods used to estimates
of prevalence.
Cross-sectional random
sample survey.
Confidence intervals
Complete ocular
examination.
Binocular visual
acuity – Snellen,
32
720 from
secondary
recapture
sources
288 from
one practice
72%
Details on
age
distribution
shown in
table 3.1
207
51% aged
75+
given)
Lavery et al
1988
[30]
Melton
Mowbr
ay
1982
and common
eye disease
among
elderly
people on
inner
London
To present
visual acuity
data for the
Eye study.
Aged 75 and
over on
December 31
1980 and
surviving on
April 1st 1982
calculated by exact
methods for small
proportions and normal
approximation to
binomial distribution for
larger ones.
and Uniocular 3
m Sonksen Silver
visual acuity with
spectacles normally
worn. Near Vision.
Cross-sectional study of
all those registered in the
single general practice in
Melton Mowbray.
Full refraction by
an optometrist
Acuity
measurements with
and without
spectacles.
33
677
78.1%
529
Age/sex
distribution
shown in a
chart.
Table 3.2 Population based surveys using visual acuity measurements - results.
Study
Key Results
Presenting visual acuity <6/12
North London Study
[33]
Aged 65+
Number of
cases
One eye
Binocular
367
448
MRC Assessment
Trial
[35]
%Prevalence
(95% CI)
23.6
30.2
20.9 to 26.3
24.8 to 35.5
All ages
75+
Total
Number
14 600
All visual impairment.
Presenting Binocular
acuity <6/18
%
95% CI
Prevalence
12.4
10.8 to 13.9
Men
5620
9.1
Women
8980
Men and
women
75-79
Low vision. Presenting
Binocular acuity <6/18 to 3/60
Blindness.
Presenting Binocular
acuity < 3/60
%
95%
Prevalence
CI
2.1
1.8 to
2.4
1.7
1.3 to
2.0
2.3
1.9 to
2.8
Presenting Binocular Acuity
<6/12
% Prevalence
95% CI
10.3
8.7 to 11.8
7.9 to 10.4
7.5
6.2 to 8.7
14.4
12.6 to 16.2
12.1
10.2 to 13.9
6898
6.2
5.1 to 7.3
5.6
4.5 to 6.6
0.6
80-84
4602
11.9
9.9 to 13.8
9.6
7.6 to 11.5
2.3
85-89
2319
23.4
20.5 to 26.4
19.2
16.2 to 22.1
4.3
90+
781
36.9
32.5 to 41.3
30
25.8 to 34.1
6.9
Men
34
0.4 to
0.8
1.8 to
2.8
3.4 to
5.2
4.8 to
9.0
% Prevalence
95% CI
19.9
17.8 to 22.0
15.2
13.5 to 16.9
22.8
20.3 to 25.3
10.8
9.1 to 12.6
20.0
17.6 to 22.4
35.3
31.7 to 38.8
53.1
48.3 to 57.9
75-79
2961
4.8
3.6 to 5.9
4.2
3.1 to 5.2
0.6
80-84
1695
10
8.4 to 11.7
7.7
6.0 to 9.4
2.3
85-89
782
19.2
15.5 to 22.9
16
12.4 to 19.6
3.2
90+
182
28.6
21.6 to 35.5
22
15.6 to 28.4
6.6
75-79
3937
7.2
6.0 to 8.5
6.6
5.4 to 7.8
0.6
80-84
2907
12.9
10.4 to 15.5
10.6
8.1 to 13.1
2.3
85-89
1537
25.6
22.3 to 28.9
20.8
17.5 to 24.0
4.8
90+
599
39.4
34.5 to 44.3
32.4
27.3 to 37.4
7.0
0.3 to
0.9
1.5 to
3.1
2.0 to
4.4
3.1 to
10.1
8.9
7.1 to 10.7
16.3
14.3 to 18.4
30.2
25.9 to 34.5
42.3
34.5 to 50.1
0.3 to
1.0
1.6 to
3.0
3.6 to
6.0
4.7 to
9.3
12.3
10.4 to 14.2
22.1
19.0 to 25.2
37.9
33.9 to 41.8
56.4
51.0 to 61.9
Women
NDNS
[34]
Pinhole-corrected visual acuity <6/18
Age in years
All
65-74
75-84
85+
All subjects
Community
Institution
All subjects
Community
Institution
All subjects
Community
Institution
All subjects
Community
Institution
No measured
1362
1126
236
508
475
33
519
429
90
335
222
113
No. of cases
195
111
84
16
12
4
60
33
27
119
66
53
% Prevalence
14.3
9.9
35.6
3.1
2.5
12.1
11.6
7.7
30.0
35.5
29.7
46.9
Gender
M
Number measured
690
No. cases
72
10.4
Age in years
All
35
65-74
75-84
85+
F
M
F
M
F
M
F
Age
All (65+)
Hayward et al.
[36]
All subjects
Community
Institution
65-74
All subjects
Community
Institution
75-84
All subjects
Community
Institution
85+
All subjects
Community
Institution
Population group age range 0 +
White
Asian
White Female
White Male
Asian Female
Asian Male
Wormald et al.
[32]
672
272
236
283
236
135
200
123
5
11
28
32
39
80
Pinhole corrected visual acuity <6/12
18.3
1.8
4.7
9.9
13.6
28.9
40.0
Number measured
Number of cases
Estimated % prevalence
1362
386
28.3
1126
252
22.4
236
134
56.8
508
50
9.8
475
41
8.6
33
9
27.3
519
134
25.8
429
87
20.3
90
47
52.2
335
202
60.3
222
124
55.9
113
78
69.0
Age standardised prevalence %
95% CI
Blindness and partial sight (registerable)
0.49
0.48 to 0.503
1.21
1.10 to 1.32
0.51
0.49, to 0.52
0.46
0.44 to 0.48
0.77
0.67 to 0.89
1.88
1.64 to 2.12
Best corrected VA <3/60
Age
Gender
Number measured
36
% Prevalence
95% CI
65-74
75+
All (65+)
Both
Men
Women
Both
Men
Women
Both
Men
Women
101
38
63
106
28
78
207
66
141
0
0
0
1.9
0
2.6
1.0
0
1.41
Gender
Number measured
% Prevalence
95% CI
Both
101
0.99
0.03 to 5.4
Men
38
0
Women
63
1.59
0.04 to 8.5
Both
Men
106
28
6.6
3.57
2.7 to 13.1
0.09 to 18.4
Women
Both
Men
Women
78
207
66
141
7.7
3.9
1.5
5.0
2.9 to 16
1.7 to 7.5
4.3 to 8.2
2.0 to 10.0
0.2 to 6.6
0.3 to 8.9
0.15 to 3.4
0.2 to 5.0
Best corrected VA <6/36
Age 65-74
Age 75+
All (65+)
Best corrected VA <6/18
Age
Gender
Number measured
% Prevalence
95% CI
65-74
Both
101
0.99
0.03 to 5.4
37
75+
All (65+)
Men
Women
Both
Men
Women
Both
Men
Women
38
63
106
28
78
207
66
141
0
1.59
14.2
10.7
15.4
7.7
4.5
9.2
0.04 to 8.5
7.5 to 20.8
2.3 to 28.2
8.2 25.3
4.5 to 12.2
0.95 to 12.7
5.0 to 15.3
Visual acuity <6/12 and > 6/60
Age
Gender
Number measured
% Prevalence
65-74
Both
Men
Women
Both
Men
Women
Both
Men
Women
101
38
63
106
28
78
207
66
141
0.99
0.03 to 5.4
0
1.59
0.04 to 8.5
19.8
12.2 to 27.4
21.4
8.3 to 40.9
19.2
11.2 29.7
10.6
6.4 to 14.8
9.1
3.4 to 18.7
11.3
6.1 to 16.6
Best corrected visual acuity in better eye
6/18 to 6/36
<6/36
% prevalence
% prevalence
10.3
2.9
18.2
0.0
18.2
3.0
25.9
4.2
22.7
0.0
35.5
10.5
75+
All (65+)
Lavery et al
[30]
Age
76-79
80-84
85+
Gender
Men
Women
Men
Women
Men
Women
Number measured
68
99
66
143
22
76
38
95% CI
76+
RNIB 1991
[37]
Private
Household
RNIB/ONS 2000
[27]
Private Household
Men
Women
All
185,688
156
318
474
VA  6/24 or <N14
and eligible for
registration
<6/12
Blind registered or
visual acuity <6/60
Blind or partially
sighted registered or
visual acuity < 6/24
2
15.4
25.8
22.4
16+
16-59
60-74
75+
16-59
60-74
75+
16+
16-59
60-74
75+
16+
16-59
60-74
75+
2.6
4.4
3.8
1.72
0.3
2.3
15.2
0.55
2.10
10.52
1.7
0.12
0.49
2.0
0.35
0.31
1.10
5.4
16+
0.90
Since overall figure not given in Bruce et al 37. Bruce I, M.A., Walker E., Blind and partially sighted adults in Britain: the RNIB survey. 1991, Royal National Institute for the Blind: London.. this was
calculated as 757/1384 = 0.546 of OPCS prevalence estimate
39
Table 3.3 Prevalence % of visual impairment and blindness in different studies1
Visual acuity levels
<3/60
<6/36
<6/60
Wormald
Wormald
Age 65+
(95% confidence interval)
1.0 (0.15, 3.4) Best corrected, better eye
3.9 (1.7, 7.5) Best corrected better eye
Age 75+
(95% confidence interval)
Wormald
1.9 (0.2, 6.6) Better eye
MRC Assessment Trial
2.1 (1. 8, 2.4) Binocular presenting
Lavery (76+)
Wormald
3.8 (2.3, 5.9) 2 Best corrected
6.6 (2.7,13.1) Best corrected
VA  6/24 or <N14
RNIB 1990
15.2 Binocular presenting
Blind or partially sighted registered
or visual acuity < 6/24
RNIB 2000
5.4 Binocular presenting
Wormald
7.7 (4.5, 12.2)
Best corrected better eye
NDNS
9.9 (8.2, 11.6) Pinhole corrected
Wormald
14.2 (7.5, 20.8) Best corrected
<6/18
NDNS
MRC Assessment Trial
15.2 (12.4, 18.0) 4 Pinhole corrected
12.4 (10.8, 13.9) Binocular presenting
10..3 Pinhole corrected
Lavery
Wormald
NDNS
MRC Assessment Trial
RNIB 2000
26.2 (22.2, 30.2) 4 Best corrected
21.8 (14.4, 31.0) Best corrected
32.4 (28.8, 36.0) 3 Pinhole corrected
19.9 (17.8, 22.0) Binocular presenting
10.5 Binocular presenting
<6/12
North London
30.2 (24.8, 35.5) Both eyes
53.8 (48.4, 59.2) Either eye
NDNS
22.4 (20.0, 24.8) 4 Pinhole corrected
1
Unless otherwise stated results are for people living in the community
including registered and those who failed reading test and/or had field loss
3
Minimum estimate (calculated by adding estimate for VA<6/12 > 6/60 and VA <3/60)
4
No adjustment made for clustering when calculating the confidence interval
2
.
40
Chapter 4
Causes of Visual Impairment
41
Introduction
This chapter reports the results from five UK population-based prevalence studies that have
investigated the causes of visual impairment. The methods employed by each survey are shown
in Table 4.1 and the results are presented in Table 4.2.
A study of the prevalence of eye disease in the elderly in an English community (Gibson et
al 1985) [31]
Summary
484 subjects living in Melton Mowbray aged 76+ were examined by an ophthalmologist and an
ophthalmic optician. The prevalence of eye diseases associated with a visual acuity of less than
6/9 was 46.1%, for senile macular degeneration 41.5% and for open angle glaucoma 6.6%.
Methods
A randomly selected age-stratified sample from all those registered in the single general practice
in Melton Mowbray who were 75 and over on December 31 1980 and still living there on April
1st 1982. The data were obtained from the same survey that was discussed in Lavery et al.1988
in Chapter 3. 529 (out of the 677 eligible) subjects were examined, 55 at home or in hospital, the
rest were examined in a hospital outpatients department. Eye diseases were defined as follows:
1. Age-related cataract were defined as best corrected visual acuity of 6/9 or worse in the
affected eye attributable to lens opacities. Subjects with aphakia were included in this group, but
cataracts that could be ascribed to congenital or secondary causes were excluded.
2. Senile macular degeneration was diagnosed in those with best corrected visual acuity of 6/9 or
worse and the presence of degenerative changes. Degenerative changes were divided into dry
and exudative type. A history of secondary or congenital causes of macular disease excluded the
diagnosis of senile macular degeneration.
3. Open angle glaucoma
a) Glaucomatous cupping of the optic disc, defined as a cup-disc ratio of > 0.5 or the presence of
notching of the neural rim or asymmetry of the optic discs.
b) Intraocular pressure by applantation tonometry about 21 mmHg.
c) An open anterior chamber angle as judged by the method of Herick and Shaffer.
d) If the above criteria were met the subject was asked to attend an eye clinic where a repeat
ophthalmic examination included Goldman perimetry was performed. Glaucomatous field
defects were considered as enlargement of the blind spot, arcuate scotoma, paracentral scotoma,
nasal step and advanced field loss. A diagnosis of low-tension glaucoma was made if a, c, d but
not b were present.
Results
The prevalence rates of senile cataract, senile macular degeneration and open angle glaucoma by
age and sex in the sample are shown in Table 4.2. The overall prevalence of senile cataract was
46.1 % which increased with age. The odds of having senile cataract in the 85+ age group as
opposed to the 76-84 group was 2.6 (1.6,4.3). There was no significant difference between the
sexes. The overall prevalence of senile macular degeneration was 41.5%. There was no
significant difference between the sexes but the odds of having this disease in the older age
group was 1.8 (1.1, 2.9). The overall prevalence open angle glaucoma was 6.6% with no
significant difference between the sexes or age groups.
42
Comment
The results were based on rigorous clinical examination and clearly defined criteria. The
definitions were based on very minimal levels of visual acuity (<6/9) and, for macular
degeneration, degenerative changes which included very early maculopathy.
Visual problems in the elderly population and implications for services (Wormald et
al.1992) [32]
Summary
207 people age 65 and over, from an inner London health centre, were examined in order to
determine the prevalence of common eye problems. The majority of those with best corrected
visual impairment had cataract.
Methods
Sampling methods are described in Chapter 3. The central visual field was tested with the
Henson CFS2000.Intraocular pressure was tested with the Perkins mark 2 tonometer with the slit
lamp. After dilation with tropicamide 1%, the optic disc vertical cup to disc ratio was recorded.
Subjects were assessed clinically by one of two ophthalmologists. When indicated the retinal
periphery was examined with a binocular indirect opthalmoscope and 28 dioptre aspheric lens.
Results
The results are shown in table 4.2. Cataract was the main cause of vision impairment in 12/16
with best corrected visual acuity < 6/18.Ageing maculopathy was the cause of vision impairment
in 3/16. This disease was found only in the 75+ age group (2.8%). Glaucoma caused blindness in
one person and was prevalent in 4.4% of those aged over 65.
More than 72% of the bilateral visual impairment was potentially remediable by surgery or
glasses, nearly one in three had visually impairing cataract and 88% of these were not in touch
with the eye services. Eye problems were more prevalent in people living in relatively
underprivileged areas. 56 subjects (27%) would probably have benefited from refraction.
Comment
Clinical examination carried out by an ophthalmologist but results are based on very small
numbers.
The prevalence of eye disease in Leicester - a comparison of adults of Asian and European
descent (Das et a 1994) [38]
Summary
A random sample from two neighbouring inner Leicester general practices of approximately the
same size, one predominately European, the other predominately Asian was used in this survey.
Cataract prevalence was significantly related to age and differed markedly between the racial
groups. Asians had a significantly higher prevalence of age-related cataract. Age was significant
for age-related macular degeneration but sex and racial group were not.
43
Methods
The samples were stratified by age (40-59 and 60+). 369 subjects were measured at a specialist
eye clinic for visual acuity, near and distance vision , full refraction and split lamp
biomicroscopy. Perkins applanation tonometry and fundus examination with direct and binocular
indirect ophthalmoscopy after pupillary dilation. Examination for cataract was carried out by
direct ophthalmoscopy after mydriasis and direct and retroillumination with the slit lamp. Age
related cataract was said to be present when the best-corrected visual acuity was 6/9 or worse in
the affected eye and this was attributable to lens opacity. Age related macular degeneration was
defined by the presence of degenerative changes together with a best-corrected visual acuity of
6/9 or worse. People with a history of secondary of congenital cause of macular disease were
excluded from the age-related macular degeneration category. The criteria for the diagnosis of
diabetic retinopathy were a history of diabetes and the presence of microaneurysms, dot
haemorrhages, hard exudates, microvascular abnormalities or neovascularization. The diagnosis
of open-angle glaucoma was made if there was glaucomatous cupping of the optic disc and an
intra-ocular pressure above 21 mmHg and an open anterior chamber angle and glaucomatous
field defects.
Results
Of the 377 that attended for examination 165 were Asian. The pattern of response varied
between the two ethnic groups with the chance of response increased if the subject had a
previous contact with the local hospital service, especially if they had had a previous
appointment at the eye clinic. This effect was stronger in the Asian community than in the
European.
The prevalence of age-related cataract, aphakia, pseudophakia and age-related macular
degeneration are shown in table 4.2. Cataract prevalence was significantly related to age
(p<0.001). After adjustment for age, the prevalence did not differ significantly with sex (p=0.94)
but did differ markedly between the racial groups (p<0.001). Asians had a significantly higher
prevalence of age-related cataract: 30% compared to 3% for those aged under 60 and 78% vs.
54% in those age 60 and over, Age was significant (p<0.001) for age-related macular
degeneration but sex and racial group were not.
Comment
This was the only study to investigate eye disease in the Asian community. However the study
population was a small sample from a selected region which may not be representative of the
general ethnic population in Britain. There was a low response rate (42%).
Prevalence of serious eye disease and visual impairment in a north London population:
population based, cross sectional study (Reidy et al 1998) [33]
Summary
1,547 people aged 65+ sampled from populations in North London underwent an eye
examination conducted by ophthalmologists. The prevalence of VA <6/12 due to eye diseases
was : cataract 30%, refractive error 9%, age related macular degeneration 8%, glaucoma 3%, and
suspected glaucoma 7%.
44
Methods
The sampling methods were described in Chapter 3. Visual acuity, autorefraction, and field
examinations were carried out by a trained ophthalmic nurse. The remainder of the examination
was by ophthalmologists. Refractive status was ascertained by using the Humphrey 580
autorefractor. Visual fields were assessed in all subjects by the 76 point visual fields of the
Humphrey 730 screener, with reading correction. Anterior segment examination was carried out
using a slit lamp. Lens, vitreous, and retinal examinations were done after pupil dilatation.
Comparison with LOCS II standard photographs was used to record cataract type and density.
People were classified as having cataract causing visual impairment when the visual acuity in
one or both eyes was poorer than 6/12 and the impairment was attributable to a lens opacity.
Those with age related macular changes causing visual impairment (visual acuity <6/12) in one
or both eyes were classified as having age related macular disease causing visual impairment.
These two classes were not exclusive. So that the prevalence of either class would not be
underestimated, patients in whom both conditions contributed to the poor vision were included in
both classes.
Refractive error causing visual impairment was defined as reduced visual acuity of <6/12 in any
eye due only to refractive error. People were classified as “definite glaucoma cases” when there
was an absolute field defect and either a cup:disc ratio of 0.7 or larger or substantial asymmetry
of the cups (a difference in cup:disc ratio of 0.3 or larger) between the two eyes. “Glaucoma
suspect cases” were those who had an absolute field defect and either a cup:disc ratio of 0.5 but
<0.7 or asymmetry of 0.2 but <0.3.
Results
The results are shown in table 4.2. The prevalence of visual acuity <6/12 due to unoperated
cataract was 30%, due to age related macular degeneration was 8%, and due to refractive error
was 9%. The prevalence of definite chronic open angle glaucoma was 3%, and a further 7% of
subjects were suspected of having glaucoma. Impaired vision in one or both eyes was present in
more than half of the sample (53.8%) (815/1547). The proportion of VA < 6/12 in one or both
eyes due to cataracts was 55% and due to refractive error was 16.7 %; taking both causes into
account the authors report that the proportion of VA< 6/12 that was potentially remediable was
72 %.when both eyes were impaired and 69% when either eye was impaired.
Comment
This was a well conducted survey with high response rates and a detailed clinical examination.
Data on the causes of visual impairment were reported only for visual acuity <6/12.
Causes of visual impairment in people aged 75 years and older in Britain: an add-on study
to the MRC Trial of assessment and management of older people in the community. (Evans
et al. 2004) [39]
Summary
Causes of vision impairment were ascertained for 1,742 people who had been identified with
vision impairment (binocular VA<6/18) in the MRC Trial of Assessment [35](see chapter 3).
The main causes were refractive error (31.6%), age-related macular degeneration (32.6%);
45
cataract (20.4%), glaucoma (6.4%), myopic degeneration (2.6%) and diabetic eye disease
(2.1%).
Methods
The results on causes of vision impairment came from an add-on study to the main MRC Trial
The methods for ascertaining visual impairment have been described in detail in the previous
chapter [35]. 49 of the 53 practices who took part in the MRC trial took part in the cause of
visual impairment study. For those with VI <6/18 not corrected by pinhole, data regarding the
cause of vision loss were extracted from the general practice medical notes and additional
follow-up questionnaires were also sent to the hospital ophthalmologist to confirm the cause of
vision loss. This questionnaire was in the form of a checklist by eye that covered: age-related
macular degeneration (exudative, geographic atrophy), cataract (age-related, congenital, and
other), and glaucoma (primary open-angle, primary closed-angle, other), diabetes (diabetic
retinopathy, other), myopic degeneration, other (specify). The ophthalmologist was asked to
rank, if possible, any conditions ticked in order of their contribution to cause of visual loss.
Results
There were 1,742 (12.5%) people visually impaired in the 49 participating practices. Of these,
450 (26%) had a pinhole acuity 6/18 or better. Of the remaining 1292, the cause sought from GP
notes was obtained in 976 (76%) cause obtained and in 316 (24%) the cause was not known.
The causes of vision impairment are shown in Figure 4.1 and were: 31.6% due to refractive
error; 32.6% due to age related macular degeneration, 20.4% due to cataract, 6.4% due to
glaucoma, 2.1% due to diabetic eye disease, 2.6% due to myopic degeneration in 2.6% and due
to other rarer causes aggregated together in 3.8%.
Figure 4.1
31.6
Refractive error
AMD
Cataract
32.6
Glaucoma
3.8
Diabetic eye disease
2.6
Myopic deg
2.1
6.4
Other
20.4
Thus in the over 75s age group, a half of visual impairment can be attributed to remediable
causes: refractive error and cataract.
Comment
The results were based on a very large sample from the 75 and over population in Britain.
Difficulties with the pinhole in 40% of those with VA< 6/18 suggest that refractive error may
have been underestimated. Causes of vision impairment were extracted from GP notes or
46
hospital records and no direct eye exam was available. In around a quarter the cause was
unknown. The nursing home population was not included.
Chapter summary
This chapter summarises five surveys that provide population prevalence rates of (mainly agerelated) eye diseases. One study [39] is based on a sample covering the whole of Great Britain,
the others are from selected regions, two in London [32, 33]and two in Leicestershire [31, 38].
All except one study [38] are concerned with the elderly.
It is difficult to compare directly the results from the studies as they all use different definitions
of disease, and cut points for the group being investigated e.g. <6/18 in the MRC Trial and VA<
6/12 in the North London Study. It is clear that cataract, refractive error and age related macular
degeneration are the major causes of vision impairment in the elderly with the contribution from
age related macular degeneration increasing with increasing age. The data from the MRC
Assessment Trial also show that the relative contribution of different causes varies by level of
impairment with cataracts being most important for low vision, and age related macular
degeneration for blindness. Glaucoma and diabetic retinopathy make minor contributions to
vision impairment. However not all studies have included additional tests such as fields, or
funduscopic examination so it is possible that the contribution of glaucoma and diabetic
retinopathy to visual impairment may have been underestimated. In the North London study
which did include a detailed assessment for glaucoma, definite glaucoma was the reason for VA
<6/12 in either eye in 3% and possible glaucoma in 7%. In contrast the prevalence of cataract
associated with VA < 6/12 in one or both eyes was 30% and refractive error was 9%. In terms of
the proportion of visual acuity <6/12 in at least one eye which is due to remediable causes,
cataracts and refractive error account for 69%.
Based on the proportions with cataract and refractive error on the studies we have estimated the
prevalence of visual impairment due to non remediable causes (Table 4.3). This has the biggest
proportional reduction in the studies by Wormald et al and in the North London (over 65 age
group) where up to 75% of visual impairment was reported as due to these two causes. The
effect is smaller in the MRC Trial (52%) since age related macular degeneration is a more
important cause of vision impairment in the older age group.
In order to place these revised prevalence figures for visual impairment into context with the
registration figures, estimates of the prevalence rates for registered blind people in England were
calculated using the published figures for blind and partially sighted registration [12] and 2000
census (http://www.statistics.gov.uk/census2001/pop2001/england.asp). The estimates are shown
in Table 4.4. Overall 0.624% (306,500) of the population were registered rising to 5.7% of the
over 75s (211,070). We next examined how these estimates agreed with the results from the
studies reported in Table 4.3 and the impact on the numbers of the variation in prevalence. The
MRC Trial estimates and those from the ONS/RNIB 2000 are closest to the actual registration
figures (237,116 and 200,067) respectively. The estimates by Wormald (131, 526) are very low
but the numbers of over 75s in that study were small and the estimates show wide confidence
intervals. Finally the 1990 RNIB first Needs Survey [37] gave estimated numbers (563,152)
which are considerable higher than the actual registrations. The RNIB report gives the number of
people eligible for registration as blind or partially sighted. However, people are not usually
registered if their sight loss is remediable. If the visual acuity tests on which the estimates were
47
based were not corrected for refractive error, or included people with operable cataract, for
example, then these will be over-estimates of the number of people requiring registration. Since
35% of the over 75s in the RNIB study self reported the main cause of the vision problem as due
to cataracts it is likely that the high figures reported by RNIB are because cataracts and probably
also refractive error have been included. As no ophthalmological exam was conducted in the
RNIB sample this cannot be confirmed.
48
Table 4.1 Studies on causes of visual impairment – Methods
Name
Location
Aims
Gibson et al.
1985
[31]
Melton
Mowbray
Das et al 1994
[38]
Leicester.
To estimate
prevalence
of eye
disease
To estimate
prevalence
of eye
disease in
Leicester.
Comparison
of adults of
Asian and
European
descent
Prevalence
of main
serious eye
disorders
North London
Study
Reidy et al.
1998
[33]
North
London
Age 40+
Sample
size
Response
rates
Number
Included
Age/sex etc
677
71.5%
484
896
42%
369
Mean age 83
(SD 4.3)
147 males,
337 females
165 Asian,
204 European
Excluded 8
people of
West-Indian
origin
209 subjects
aged 60+
Age 65+
Registered at
GP practices
1840 from
7 GP
practices
84%
1547 from
7 GP
practices
65+
94.3% white
Great Britain
To ascertain
causes of
visual
impairment
Age 75+ .
Not in long
stay hospitals
or nursing
homes or
terminally ill
49 GP
practices
69%
1,742
visually
impaired
(12.5%)
from 49
GP
practices
75+.
mean age 81.3
percentage
male 39%
Inner
London
To
determine
prevalence
of common
eye disease
Age 65+
288 from
one
practice
72%
207
51% aged 75+
MRC
Assessment
Trial
Evans et al.
2004
[13]
Wormald et al.
1992
[32]
Inclusion
/Exclusion
Criteria
49
Table 4.2 Studies on causes of visual impairment – Results
Name of study
Gibson et al.
[31]
Estimated prevalence
of visual impairment
Not given
Causes (not necessarily mutually
exclusive).
Senile cataract
Best corrected VA <6/9
Age
76-84
Total number
examined = 484
85+
Senile macular degeneration
76+
76-84
85+
Open angle glaucoma
76+
76-84
85+
76+
50
Sex
Men
Women
All
Men
Women
All
All
Men
Women
All
Men
Women
All
All
Men
Women
All
N examined
132
260
392
15
77
92
484
132
260
392
15
77
92
484
132
260
392
% prevalence
37.1
43.8
41.6
60.0
66.2
65.2
46.1
40.9
37.7
38.8
40.0
55.8
53.3
41.5
9.8
5.8
7.1
Men
Women
All
All
15
77
92
484
4.2
2.6
2.9
6.6
Wormald et al
1992
[32]
Percentage in population. Age 65+ n=207
Blindness <6/36 3.9%
N. with condition
Ageing maculopathy
Glaucoma
Cataract
Ageing maculopathy
Optic atrophy
Cataract
VA < 6/18
7.7%
All aged 65 and over
Cataract
Aphakic
Glaucoma
Diabetic retinopathy
Ageing maculopathy
Name of study
North London
Study
[33]
1
1
6
2
2
12
Age
65+
65-74
75+
65+
65+
65+
65+
65-74
75+
% of visually impaired
with condition
12.5
12.5
75
13
13
75
Population prevalence %
5.8
1.0
10.4
5.8
4.4
1.0
1.4%
0
2.8
Estimated prevalence
of visual impairment
Causes (not necessarily mutually
exclusive).
Number of cases
Estimated prevalence
%
95% CI
53.8 % age 65+.
Criterion VA < 6/12
in either eye
1. Cataract
451
30
25.1 to 35.3
2. Had cataract surgery (one or both
eyes)
162
10
5.5 to 10.8
8
5.8 to 10.8
3. Age related macular disease
133
3
2.3 to 3.6
4. Glaucoma (chronic open angle)
47
7
5.4 to 8.4
5. Suspected glaucoma
109
9
7.0 to 11.4
6. Refractive error causing visual
impairment (one or both eyes)
Age-related cataract, aphakia or
pseudophakia
136
1547 examined
Das et al. 1994
[38]
95% CI
3.0 to 9.9
0.03 to 5.8
4.6 to 16.2
3.0 to 9.9
2.0 to 8.0
0.1 to 3.4
0.3 to 4.2
0.5 to 8.0
Not given
Total number
examined =369
Asian
Age
Number examined
Prevalence %
Std.
error
40-49
50-59
60-69
17
49
69
17
49
69
5.2
8.2
6.7
51
European
70+
40-49
50-59
60-69
70+
93
39
32
69
64
93
0
6
30
64
4.9
4.3
5.5
6.0
Asian
60-69
20
0
-
70+
60-69
70+
13
28
15
23
0
7
11.7
6.4
60-69
32
3
3.1
70+
60-69
70+
39
37
25
28
3
16
7.2
2.7
7.3
Age-related macular degeneration
Women
Men
European
Women
Men
MRC Trial
[39]
Estimate of prevalence of visual
impairment for 75+
12.5%
Criterion binocular visual
impairment (VA < 6/18 )
N=1426
8.55%
Binocular VA < 6/18 excluding
those with refractive error.
N=976
Cause
N. with condition
% of with condition
95% confidence interval
Refractive error
450
31.6
28.3 to 34.8
AMD
516
36.2
32.9 to 39.5
Cataract
350
24.5
21.8 to 27.4
Glaucoma
113
7.9
6.2 to 9.6
Diabetic eye disease
33
2.3
1.5 to 3.1
Vascular occlusions
9
0.6
0.1 to 1.1
Myopic degeneration
41
2.9
1.9 to 3.8
Other
Cause
67
N. with condition
4.7
% of visually impaired with condition
3.7 to 5.7
95% confidence interval
Refractive error
AMD
Cataract
Glaucoma
Diabetic eye disease
Vascular occlusions
Myopic degeneration
-
-
516
52.9
49.2 to 56.5
350
35.9
31.7 to 40.1
113
11.6
9.1 to 14.0
33
3.4
2.2 to 4.6
9
0.9
0.2 to 1.6
41
4.2
2.8 to 5.6
-
52
Other
Cause
All aged 75 and over
N=14,403
Refractive error
AMD
Cataract
Glaucoma
Diabetic eye disease
Vascular occlusions
Myopic degeneration
Other
67
N. with condition
6.9
Population prevalence with va <6/18
5.5 to 8.2
95% conf. Int.
450
3.2
2.6 to 3.8
516
3.7
3.1 to 4.1
350
2.5
2.0 to 3.0
113
0.8
0.6 to 1.0
33
0.2
0.15 to 0.32
9
0.06
0.01 to 0.11
41
0.3
0.2 to 0.4
67
0.5
0.4 to 0.6
53
Table 4.3 Prevalence of visual acuity before and after exclusion of cataracts and refractive error (RE) using data reported from individual studies on the
causes of visual impairment as reviewed in the chapter
%Prevalence
% of visual
impairment1 due
to RE & Cataract
65+
30.2
72
8.5
<6/12 either eye
65+
53.8
69
16.7
<6/18 best corrected
65+
7.7
752
1.9
<6/18 presenting binocular
75+
12.4
52
6.4
75+
15.2
Not known3
Not Known
75+
5.4
Not known
Not known
Study
Visual impairment criterion
North London
[33]
< 6/12 both eyes
Wormald
[32]
MRC Assessment
Trial
[39]
RNIB 1990
[37]
RNIB 2000
[27]
1
Age Group
%Prevalence after
excluding
cataract & RE
VA  6/24 or <N14
Blind or partially sighted registered
or visual acuity < 6/24
Visual impairment defined in column 2
2
Cataract was the reason for visual impairment in 75% of people aged 65+ with best corrected VA < 6/18. The proportions with refractive error could not be estimated as best
corrected acuity “removes” those with refractive error
3
The main cause reported by 35% of those aged 75+ was cataracts but an ophthalmological examination was not conducted and therefore the figures are not included in the above
table. No information is available on the proportion with refractive error.
54
Table 4.4 Estimated prevalence of registered blind and partially sighted in England in 2001 (calculated by the authors)4.
Number of people on
2000 blind register
[12]
Number registered
Partially Sighted
Number registered
Blind and PS
Population of
England from 2001
census
Prevalence of
blindness registration
2000/2001
Prevalence of
Blindness and Partial
Sight registration
2000/2001
All ages
157,820 1
0-4
710
5-17
3,150
18-49
16,450
50-64
13,360
65-74
15,780
75+
108,360
148,6801
620
3,650
14,030
11,500
16,160
102,710
306,500
1,330
6,800
30,480
24,860
31,940
211,070
49,138,831
2,926,46
0
8,214,680
21,630,529
8,559,561
4,102,656
3,704,945
0.032%
0.024%
0.038%
0.076%
0.16%
0.39%
2.92 %
0.624%
0.045%
0.083%
0.141%
0.29%
0.78%
5.70%
1
Numbers in age groups do not add to total because of rounding (stated in report of the register) [12]
4
Prevalence estimates calculated using the 2001 census data and the blind and partially sighted register of 2000
55
Table 4.5 Actual registration figures for 2000 for people aged 75 and over in England [12] and expected numbers in England eligible for registration
from other studies/surveys
DOH registration
figures
Study
5.70%
211,070
Numbers (95% CI) likely to be eligible for
registration
RNIB 19902
Prev (95% CI) of those
likely to be eligible for
registration
6.4 %
95% CI 5.6,7.2
3.6%
95% CI (1.9, 5.2)
Not known
RNIB 20002
Not known
200,067
MRC Trial 1
Wormald 19921
237,116
207,477, 266,756
131, 526
69, 468 to 192, 657
563,152
based on estimates of prevalence adjusted for “treatable causes of vision impairment”
1
2
based on estimates of prevalence but without adjustment for treatable causes of vision impairment
56
Chapter 5
Prevalence of visual disability and visual impairment
in children in the United Kingdom
57
Introduction
This chapter summarises the literature relating to the prevalence of visual impairment in children
in the UK. The first section concentrates on nationwide surveys, a number of which RNIB
played a leading role in. Data from the larger local (rather than national) surveys are then
summarised.
The methods for each national survey are summarised in table 5.5 and the results in table 5.6.
The surveys from geographically defined areas are summarised in table 5.7.
There are no agreed definitions of visual impairment among children, an issue discussed in detail
elsewhere. [40] Terms such as visual impairment and visual disability have been used to mean
different things in different studies and contexts. As far as possible, we have therefore defined
what the outcomes were in the various studies.
The prevalence of disability among children, OPCS surveys of disability in Great Britain
(Bone et al 1989) [41]
Summary
This is the report of the 1985 OPCS disability surveys of children aged 15 and under in private
household and communal establishments in Great Britain. The sampling procedure was the same
as in Martin et al.1988 [25](see chapter 2). Questionnaires were sent to 100,000 private
households in 1985. Interviews were obtained with or on behalf of 2,025 of the 2,403 children,
identified as disabled from the sift forms. Interviews were obtained on behalf of 1,019 children
in communal establishments. The main questionnaires covered the same areas of disability as the
adult interviews and had the same basic structure, but were adapted for children. The estimated
prevalence rate of visual disability for children aged 15 and under both in private households and
resident in communal establishments was 0.2%. The estimated number of disabled children aged
15 and under in Great Britain was 22,000. Visual disability refers to not seeing well enough to
recognise things in a picture book or cannot see well enough to follow a TV programme sitting at
a normal distance to the TV set or cannot see well enough to recognise someone across a road
Methods
Private Households
The sample was the same as that for the adult disability survey [25]see chapter 2 for details of
the sampling procedure). A screening questionnaire was sent out to a random sample of
households. The design is shown in figure 5.1. The questionnaire contained detailed questions of
health problems and disabilities. There was a separate box for questions relating to children
asking if there were any children under 16 (including babies and toddlers) in the household and,
if the answer was yes, the following was asked:
“Is there any child in your household:
a) who is unable to do things which most children of the same age can do because of a
health development or behaviour problem?
b) who needs more help than usual for children of the same age with feeding, dressing,
toileting walking going up and down stairs or other daily activities?
58
c) who attends a special school or special or remedial unit of an ordinary school because of
health or behaviour problem , disabilities or learning difficulties?
d) who attends an ordinary school but is limited in taking part in school activities because
of health or behaviour problems or disabilities?
e) whose health , behaviour of development causes worry that he or she may have a longterm health problem, physical or mental disability or handicap?"
All households with the answer “yes” to one or more of these (sift) questions were included in
the sample for interview. The questions and severity scores are shown in table 5.1 for children
aged 6 and over and in table 5.2 for children younger than 6.
Interviews were carried out with, or in the presence of, one of the child’s parents, usually the
mother. Children were categorised as visually disabled if the answer was ``yes" to any of the
questions shown in table 5.1 or table 5.2 depending on their age.
59
Figure 5.1: Flow chart showing the numbers of participants at each stage of the OPCS 1985
survey of children in private households
60
Scale
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
Table 5.1: Seeing Severity scores for children aged 6-15 [Bone and Meltzer1989]
Description
Score
Cannot tell by the light where the windows are
12
Cannot see well enough to recognise someone he knows if close to his face
10.3
Cannot see well enough to walk around an unfamiliar room without bumping into things
9.7
Is aged at least 8 and cannot see will enough to read a large print book even if held close to his face.
9.2
Cannot see well enough to reach for and pass things at meal times
8.3
Cannot see well enough to enjoy a book of pictures unless held close to his face
8.0
Cannot see well enough to follow a TV programme even if sitting up close to the TV set
7.6
Is aged at least 8 and can only see well enough to read a large print book if held up close to his face.
6.8
Cannot see what was written on the blackboard if sitting at the front of the classroom
6.8
Cannot see well enough to recognise someone he knows across a room
6.3
Cannot see well enough to follow a TV programme sitting at a normal distance to the TV set
4.7
Is aged at least 8 and cannot see well enough to read ordinary newspaper print.
4.3
Cannot see well enough to recognise someone he knows across a road
2.3
61
Scale
S1
S2
S3
S4
S5
S6
S7
S8
S9
Table 5.2: Seeing Severity Scores for children aged 5 and under [Bone and Meltzer1989][41]
Description
Score
Is aged 5 and cannot tell by the light where the windows are
11.8
Is aged 5 and cannot see well enough to recognise someone he knows if close to his face
9.5
Is aged 6 months to 1 year and cannot see well enough to follow a moving object with his eyes
9.5
Is aged 5 and cannot see well enough to walk around an unfamiliar room without bumping into things 8.1
Is aged 5 and cannot see well enough to see things in a picture book held very close to his face.
7.9
Is aged 5 and cannot see well enough to follow a TV programme even if sitting up close up to the TV 5.7
Is aged 5 and cannot see well enough to recognise someone he knows across a room
4.8
Is aged 5 and cannot see well enough to recognise things in a picture book held at a normal distance to 4.7
his face.
Is aged 5 and cannot see well enough to follow a TV programme sitting at a normal distance to the TV 1.5
set
62
Communal establishments
A list of institutions providing residential care for children was obtained from various
government departments. This list was then sent to the general managers of health authorities,
directors of social services, directors at the administrative headquarters of voluntary and
charitable organisations, and chief education officers asking the recipients to amend and update
it. The design is shown in figure 5.2. 3,421 establishments were found in this way. In order to
identify those establishments that contained children who were eligible a “census form” was sent
to each of these establishments asking for the name and age of each child and indicates whether
they were disabled. The form stated that:
“A child should be regarded as disabled if he/she has difficulties with everyday activities
because of a long-term health problem. This includes physical, mental and behavioural problems
which are chronic in nature.”
The same interview questions were asked as in the private households’ children's survey.
Interviews were obtained for a maximum of 4 children in each establishment. For the communal
establishments a weight was applied to compensate for inclusion of different proportions of
institutions according to the number of residents. In both surveys weights were applied to allow
for non-response and to bring the sample estimates up to the population estimates.
63
Figure 5.2: Flow chart showing the numbers of participants at each stage of the OPCS 1985
survey of children in communal establishments
64
Results
3,421 establishments were sent the census forms. 79% responded. The highest response rates
were from establishments administered by health authorities (85%) and the lowest from
establishments run by voluntary and charitable organisations (76%). The analysis of data in the
census forms indicated that 687 (25%) of institutions were eligible to be included in the survey.
The majority of the remaining 75% were short-term homes. All establishments with more than
11 eligible children were included in the sample and there was selective sub-sampling of those
with a smaller number than this. 332 institutions were selected for interviews. Of these 44 were
found subsequently to be ineligible (mainly because the children went home at weekends and
would therefore have been eligible for the private households’ survey. 1,019 Children from 288
co-operating institutions were interviewed. Prevalence estimates are not provided for institutions
on their own. The estimated prevalence rates for visually disabled children aged 15 and under
living in private homes and institutions was 0.2% for all age groups. The estimated number of
visually disabled children in communal establishments in Great Britain in 1985 was estimated to
be 1,000. The number of disabled children in the total population was estimated to be 22,000.
Comment
This survey included a random sample of the total population of Great Britain, a large sample
size and used its own questionnaire. The report includes the “sift” questionnaire and details the
disability scales.
Although detailed questions were asked on levels of seeing disability (see tables 5.1 and 5.2),
data on levels of sight disability are not provided in this report. Prevalence estimates are
provided only for those deemed to have some sight disability.
A specific weakness of this report is that it does not detail how children aged between 6 months
and 5 were identified as visually disabled. No figures were given for the number found to be
disabled in communal establishments, and thus it was not possible to calculate the number of
visually disabled in the sample.
Blind and partially sighted children in Britain: The RNIB survey (Walker et al 1992) [42]
Summary
This report is concerned with blind and partially sighted children and their needs. The survey
was based on interviews carried out in April and May 1988 with parents or guardians of 285
visually impaired children (aged 3 to 19) randomly selected from 33 local education authorities
(LEAs) in England, Scotland and Wales. The education authorities were selected to provide a
representative sample of LEAs in Great Britain. The estimated number of children in Great
Britain categorised as visually disabled (for educational purposes) by the local authorities was
10,000 (0.1%). The prevalence of visual disability was greater in boys than in girls (64% versus
36%), with a greater disparity among those aged 12 years and over (70% versus 30%).
Methods
A random sample of 37 LEA's were selected after splitting Great Britain into 8 geographic
regions and stratifying the LEA's within each region by metropolitan or county area, educational
policy and population density. Selection of the LEA's was made independently within each
65
region with a probability according to the total number of school children in the area. 33 out of
the 37 selected LEA's co-operated in the survey.
The prevalence estimates were based on the number of school children (aged between 3 and 19
years) who had been identified as having a visual disability by selected LEAs. A form was sent
to each LEA contact person (appointed by the LEA) asking for:
“Details on every child or young person for whom you have records or identified as having a
visual handicap, whether “educationally blind” or “partially sighted”, including those with
multiple handicaps. We are trying to assess the total population of visually handicapped children
(defined below), so include those children whom you may not have formally assessed, but you
acknowledge as meeting the criteria, providing as much information as you have available.
“Educationally blind” means a child or young person requiring education by means not
involving the use of sight (i.e. dependent wholly or partly on non-printed materials). “Partially
sighted” denotes someone who, although not blind, requires specialised help or attention, but
who can benefit from sighted methods of education.”
The prevalence figure was calculated by dividing the number of children aged 3-19 identified as
visually handicapped by the LEA's by the total number in the 33 LEA's. The ratio so obtained
was then applied to the total population of 110 LEA's to provide an estimate for the whole of
GB. Children not living in private households were excluded from this survey.
Results
The 33 LEA's identified 4,390 children as having some form of visual disability. Dividing this by
the total number of school children in the LEA's (4,265,300) gave a prevalence rate of
10.3/10,000 children. Applying this ratio to all the 9,516,000 children in the 110 LEA's gave an
estimate of 9,795. There was a large variation between LEA's in the prevalence rates. The
highest reporting authority had a prevalence rate of 2.6 per 1,000, which was more than fourteen
times that of the lowest at 0.18 per 1,000. There were more visually disabled girls than boys,
64% boys and 36% girls.
Comment
The prevalence estimates are based on information provided by 30% of the LEA's in the UK.
The fact that the rates vary widely between LEA's implies that different criteria are used. No
attempt was made out to identify the differences. The estimate of visually disabled children in
Great Britain was less than half that estimated to have a 'seeing disability' in the OPCS report
[41]. The authors conclude that the disparity was due to large-scale under-identification by the
LEAs. Their estimate was also half the estimate of the survey carried out by the RNIB in 1995
(also based on LEA records) only 7 years later (see the following report).
Where have all The Children Gone? An Analysis of New Statistical Data on Visual
Impairment Amongst Children in England, Scotland and Wales (Clunies-Ross 1997) [43]
Summary
This article reports findings from a study (carried out by the RNIB) of the numbers of visually
impaired (as defined by heads of visual impairment services) children aged 0-16 in England,
66
Scotland and Wales. There were over 19,000 visually impaired children in Great Britain known
to the local authorities in 1995.
Methods
Questionnaires, each with a covering letter, were sent to Heads of all LEA visual impairment
Services and also subsequently to Heads of special schools for children with visual impairment
during 1995. They were asked how many children with visual impairment were known to them
and to list the criteria they used to identify visually impaired children (table 5.3). The number of
children with visual impairment in Great Britain was estimated by extrapolation.
Table 5.3 Criteria used by Heads of visual impairment Services to identify children as visually
impaired (from Clunies-Ross 1997).
Criteria
Percentage
Visual acuity:
6/18 or less in best corrected eye
16.2
6/18 or less and significant field loss
3.9
significant visual field loss
11.9
other visual acuity criteria e.g. 3/60, 6/24, 6/60 7.9
Impaired functional vision
6.3
Vision not corrected by glasses
1.6
Named eye condition e.g. nystagmus
6.0
Degenerative eye condition e.g. RP
5.5
Temporary eye condition e.g. squint
1.2
Registered/registerable
3.5
Statement/record of need
0.8
Ophthalmological referral
1.6
Medical referral
4.7
Difficulty in accessing National Curriculum
17.8
Large print required
2.0
Mobility needs
1.6
Reference to other disabilities (not MDVI)
1.6
67
Results
Information was obtained from LEAs representing 76% of the population aged less than 16
years. Table 5.3 shows the criteria that were used by the Heads of visual impairment services to
identify children as visually impaired. Visual acuity and visual field were the key criteria. A few
services used WHO criteria as a basis for recording children with visual impairment. The number
of children aged 0-16 in GB was estimated to be 19,379.The estimated prevalence of visual
impairment was 2.1 children per 1,000. The highest rate recorded by an authority was 5.06
visually impaired children per 1,000 and the lowest 0.67 per 1,000. The percentage of multidisabled children aged 0-16 was 34.5% of all children with visual impairment known to the
LEA's.
Comment
This was a large study incorporating the whole of Great Britain. The study investigated the
criteria used by the LEA's to identify children with visual impairment.
However, there was no breakdown by age or sex. In addition, no attempt was made to
standardise for the different criteria for visual impairment used by different authorities.
Survey of educational provision for blind and partially sighted children in England,
Scotland and Wales in 2002 (Keil 2003)[44]
Summary
A postal questionnaire of local education authorities (LEAs) carried out by RNIB in 2002. A
prevalence estimate of 2.4/1000 children aged 0-16 years who received specialist educational
support to enable them to access the curriculum in Great Britain was obtained.
Method
Postal questionnaire to LEA visual impairment advisory services in England, Scotland and
Wales. The questionnaire asked about numbers of children, a detailed age breakdown and
presence of other disabilities.
Results
The response rate was 65%. A prevalence estimate of 2.4/1000 children aged 0-16 years who
received specialist educational support to enable them to access the curriculum in Great Britain
was obtained, giving a population estimate of 24,000. 50% of children receiving visual
impairment services had additional disabilities.
Comment
The marked variation in services provided and thresholds used was noted (see discussion at end
of chapter below).
The prevalence and nature of ascertained handicap in the National Child Development
Study (1958 cohort) (Peckham and Pearson 1976) [45]
68
Summary
The prevalence and causes of partial sight and blindness were investigated in 15,275 11-year-old
children from the 1958 birth cohort. The prevalence of blindness was estimated to be 0.13/1000
and of partial sight 0.46/1000. Classification as blind or partially sighted was based on the
descriptions used in educational and medical records.
Methods
16,000 children born in a single week in 1958 were included in the National Child Development
Study. By the age of 11 years, medical and/or educational data were available for 15,275.
Ascertained handicaps were based on a definition of:
“…any condition severe enough for the child to require formal ascertainment by the local
education authority.”
Children identified were thus those whose visual disability was severe enough for the child to
require formal ascertainment by the local education authority. Classification as blind or partially
sighted was based on the descriptions used in educational and medical records.
Results
The prevalence of blindness was estimated to be 1.3/10,000 and of partial sight 4.6/10,000, but
these estimates were based on a total of 9 children with a notified visual handicap.
Comment
A large proportion (of children born in one week in 1970 were included, thus producing a
representative sample of all children of that age in Great Britain. However, some underascertainment was likely to have occurred, the extent of which is difficult to quantify. Some
indication of under-ascertainment can be gained by considering the following report of a birth
cohort in which visual acuity was measured.
Partial sight and blindness in children of the 1970-birth cohort at 10 years of age (StewartBrown and Haslum 1988)[46]
Summary
The prevalence of partial sight and blindness were investigated in 14,907 10-year-old children
from the 1970 birth cohort. The prevalence of blindness was estimated to be between 3.4 and
4.0/10,000 and of partial sight between 5.4 and 8.7/10,000. The definitions were based on visual
acuity measurements: Blindness visual acuity <6/60 and partial sight visual acuity <6/24>=6/60.
Methods
An estimated 97% of all children born in the week of 5th-11th April 1970 were included. Of
these, 93% of the survivors were assessed in 1980. Corrected and uncorrected distance visual
acuities were measured by a clinical medical officer. Detailed instructions were given to the
medical officer and in most cases a Snellen chart was used. The medical officers were asked to
record the presence of any abnormal condition affecting the children's eyes. Causes were
ascertained by the medical officer from their own examination and a detailed examination of the
child's school medical records.
69
Results
Five children had best corrected visual acuity of less than 6/60. All were registered blind. One
additional child who had never been ophthalmologically assessed was also included (in the upper
estimate for prevalence) who was thought to be blind following measles encephalitis and 6
month of age and who attended a school for educationally severely abnormal. Eight children had
best corrected visual acuity of between 6/24 and 6/60 inclusive. Only three had been registered
as partially sighted and none as blind. A further three children had presenting VA <6/24 that
improved to VA 6/24 after correction. Two children were reported as having refractive errors,
no diagnosis was determined for the other child. These three children were included in the upper
prevalence estimate of partial sight.
Comment
The study included a large proportion (over 90%) of children born in one week in 1970, thus
producing a representative sample of all children of that age in Great Britain. Children had their
vision measured.
Severe visual impairment and blindness in children in the UK (Rahi et al, 2003)[47]
Summary
National active surveillance schemes were used to identify children newly diagnosed with severe
visual impairment or blindness during 2000. The annual incidence was highest in the first year
of life, being 4.0 (95% CI 3.6-4.5) per 10 000, with a cumulative incidence by 16 years of age of
5.9 (5.3-6.5) per 10 000. The study was restricted to those children with a severe visual
impairment or blindness (described below)
Methods
In the UK, children aged younger than 16 years newly diagnosed with severe visual impairment
or blindness (SVI/BL, WHO criteria) during 2000 were identified through national active
surveillance schemes in ophthalmology and paediatrics. Eligible children were identified through
both the British Ophthalmological and the British Paediatric Surveillance Units, whose reporting
bases consist of all consultant ophthalmologists and paediatricians, respectively, participating in
national active surveillance schemes. Every month clinicians reported all children younger than
16 years who were newly diagnosed as severely visually impaired or blind (SVI/BL) as a result
of any disorder. Such a definition included:

children with corrected distance visual acuity of worse than 6/60 in the better eye
(SVI/BL by WHO international taxonomy);

those eligible for notification to the national registers of blindness;

those with clinical features consistent with SVI/BL (e.g. cannot fix or follow a light) but
whose acuity could not be measured formally.
From these data, yearly age-group specific incidence and cumulative incidence rates were
calculated. Only children with permanent SVI/BL at 1 year of follow-up were included: children
whose vision exceeded this level after treatment or spontaneously were not included. Children
with unilateral visual loss and those born outside the UK were not included.
70
Results
Of 439 newly diagnosed children, 336 (77%) had additional non-ophthalmic disorders or
impairments (SVI/BL plus). Total yearly incidence was highest in the first year of life, being 4·0
(95% CI 3·6-4·5) per 10 000, with a cumulative incidence by 16 years of age of 5·9 (5·3-6·5) per
10 000. 10% (44) of all children died within 1 year of diagnosis of blindness. The main results
are shown in table 5.4.
Age-specific incidence
Age (years)
<1
1-4
5-15
0-15
Cumulative incidence
Age (years)
1
5
16
Incidence per 10 000 (95% CI)
4·04 (3·56-4·52)
0·32 (0·28-0·38)
0·06 (0·04-0·08)
0·35 (0·31-0·38)
4·04 (3·56-4·52)
5·30 (4·76-5·84)
5·90 (5·33-6·47)
Table 5.4 Annual age-specific incidence and cumulative incidence of SVI/BL
Comment
The study was a rigorously designed and executed national survey. The case definition was
intended to include all children with impairment of vision sufficient to interfere significantly
with development, or have major educational implications, or both, and the study thus
concentrated on the severe end of the clinical spectrum. The definition was used in the absence
of an existing standard definition of SVI/BL that was applicable to all children, including those
too young, or who cannot participate in formal visual function tests for other reasons, such as
developmental delay.
The study depended on children being known to the relevant health services and on clinicians
reporting newly diagnosed children. A small degree of under-ascertainment was therefore likely.
Annual Schools Census (DfES 2004)[48]
Summary
The Department for Education and Science now collects data on every pupil in maintained
schools in England. At January 2004, from a population of 7.7 million pupils in England in
maintained primary, secondary and special schools, there were 7200 children with visual
impairment as their primary special educational need. This gives a prevalence of 9.35/10,000
children.
71
Method
Analysis of routinely collected data. Based upon survey returns from over 99% of primary,
secondary and special schools in England (around 22,300). Children identified are those who
have a special education need status of School Action Plus or who have a statement of special
educational need.
Results
At January 2004, from a population of 7.7 million pupils in England in maintained primary,
secondary and special schools, there were 7200 children with visual impairment as their primary
special educational need. This gives a prevalence of 9.35/10,000 children. However, an
additional 900 children had multi-sensory impairment, giving a prevalence of visual disability of
10.5/10,000 children. Children with visual impairment or multi-sensory impairment as their
primary type of need comprised 1.3% of children with special educational need status.
Comment
Only data on primary type of need are so far available. Given the frequency with which visual
disability occurs with other needs, it is likely that a substantial number of children with other
primary special needs also have a visual disability. Data on secondary needs will be published in
late 204 or early 2005.
Summary tables
Tables 5.5 and 5.6 below summarise the national surveys of visual impairment and visual
disability among children in Great Britain
72
Table 5.5 Methods used in the national surveys of visual impairment in children
Name
Location
Inclusion
Design and
Visual
and
/Exclusion
methods
measurement
date of
Criteria
survey
Great
Age 15 and
Cross sectional
None
OPCS
Disability
Britain
under
survey
Survey
1985
(Bone et
al.1989) [41]
RNIB survey Great
Aged
Questionnaires sent None
(Walker et al. Britain
between 3-19 to random sample of
1985) [42]
1988
years.
LEA’s , stratified by
Excluded
region and size.
those not
living in
private
households.
Clunies-Ross Great
Children
Questionnaires sent None
1997 [43]
Britain
aged up to 16 to a heads of all
1995
LEA visual
impairment services
in England, Scotland
and Wales.
Keil 2003
Great
Children
Questionnaires sent None
[44]
Britain
aged up to 16 to a heads of all
2002
LEA visual
impairment services
in England, Scotland
and Wales.
73
Sample size Response
rates
Number
Included
100,000
households
81%
2,025
children
identified as
disabled
37 LEA’s
33/37 LEA’s
4,390
children
identified as
having some
form of
visual
disablement
Not
applicable
“Reliable data”
obtained from
76% of the LEA’s
Not
applicable
Response rate of
65%
Name
Location
and
date of
survey
Great
Britain
1969
Inclusion
/Exclusion
Criteria
StewartBrown 1988
[46]
Great
Britain
1980
Rahi
2003[47]
Great
Britain
2000
England
Peckam
1976[45]
DfES
2004[48]
Design and
methods
Visual
measurement
Sample size Response
rates
Number
Included
Birth cohort study
none
95% (approx)
15,275
Children
born in week
one week in
April 1970.
Birth cohort study
All notified
cases
Active surveillance
system
Corrected and
uncorrected
monocular
distance Snellen
visual acuities
not applicable
approx
16,000
children
aged 11
years
14,907
children
surveyed
aged 10
years
not
applicable
not applicable
not
applicable
Children at
maintained
schools
Routine data
collection
7.7 million
children
data from over
99% of schools
not
applicable
none
74
93% (health data), 13,871 health
85% (visual
data, 12,853
acuity)
visual acuity
measured
Table 5.6 Prevalence estimates obtained from the surveys of visual impairment in children
Survey
What the prevalence estimate refers to
OPCS
Disability Survey
(Bone et al.1989) [41]
RNIB survey (Walker et al. 1985)
[42]
Clunies-Ross 1997 [43]
Keil 2003 [44]
Peckam 1976[45]
Stewart-Brown 1988 [46]
Cannot see well enough to recognise things in a picture book or
cannot see well enough to follow a TV programme sitting at a
normal distance to the TV set or cannot see well enough to
recognise someone across a road.
Identified by LEA as having a visual handicap, i.e.
“educationally blind” or “partially sighted”
Criteria as used by heads of services for visually impaired
children to identify visually impaired children
Criteria as used by heads of services for visually impaired
children to identify visually impaired children
Visual handicap severe enough for the child to require formal
ascertainment by the local education authority. Classification as
blind or partially sighted based on the descriptions used in
educational and medical records
Blindness: visual acuity <6/60
Age
0-15 years
Prevalence
estimate
/10,000
20
3-19 years
10.3
0-16 years
21
0-16 years
24
11 years
1.3 (blindness)
4.6 (partial sight)
10 years
Partial sight: visual acuity <6/24>=6/60
Rahi 2003[47]
DfES 2004[48]
Corrected distance visual acuity of worse than 6/60 in the better
eye; those eligible for notification to the national registers of
blindness; and those with clinical features consistent with
severe visual impairment or blindness
Children with visual impairment or multi-sensory impairment
as their primary special educational need
75
3.4 (blindness)
5.4 (partial sight)
Cumulative
incidence by
age 16 years
5.9
All school ages
10.5
Locally based surveys
There have been a number of estimates of visual impairment in geographically defined
populations in the United Kingdom. The main characteristics and findings of the larger and more
rigorous studies are presented in table 5.7. The Flanagan study is notable because it comes from
Northern Ireland: the national surveys presented above were all restricted to Great Britain.
These studies have used a range of methods, sources, and definitions, outcome measures,
limiting the scope to generate a summary estimate. However, the findings are broadly similar to
the national surveys of visual impairment, with prevalence estimates around 10-20/10,000
children. While taking these caveats into account, the estimate from Northern Ireland (Flanagan)
appears similar to estimates from the rest of the United Kingdom.
76
Table 5.7. Characteristics and findings of surveys in geographically defined populations
Study and
Setting
Method
Main findings
year
Crofts 1998
Berkshire, Bucks, Children identified as
Visual acuity  6/18 or poor
[49]
Oxfordshire and
visually impaired from the behavioural responses, born
Northamptonshire Oxford Register of Early
1984-1987 and diagnosed by
Childhood Impairment
age 5 years, 10.3/10,000
Rogers
Liverpool
Children identified as
Corrected vision sufficiently
1996[50]
visually impaired from the abnormal to interfere with
Liverpool vision
development or to have
assessment team database
ongoing education
implications in children aged
0-16 years, 18.1/10,000.
Flanagan
Belfast
Children identified as
Best-corrected visual acuity
2003 [51]
having visual impairment
6/18 in the better eye or
from multiple sources
significant decrease in visual
including paediatricians,
field size, ages 0-18 years,
teachers, social workers
16.1/10,000
and statutory registers,
visual acuity and fields
measured
Visual
Scotland
Information sent to all
Rates not presented and
Impairment
heads of special education
cannot be calculated as no
Scotland
services in each local
denominators given.
2003 [52]
authority plus to
community paediatricians
333 children met criteria for
asking them to identify
“significant visual
possible children and
impairment” (see text for
disseminate information on details).
the project to families.
Notification was by
parents.
Welsh
Wales
Audit of LEA visual
Among people aged 0-19
Assembly
impairment services
years, 20/10,000 people
Government
received specialist support
2004 [53]
from visual impairment
services
77
Overall comments
Definitions used
A key issue is the variation in definitions used for visual impairment and in the criteria used by
local education authorities to identify children in need of services. One survey asked visual
impairment services how they identified visually impaired children and multi-disabled visually
impaired children. The criteria are described in tables 5.8 and 5.9 below.
Table 5.8: criteria used by visual impairment services to identify a visually impaired child
from a sample of 152 providers (Clunies-Ross 1997)[43]
A child who is ‘educationally significantly visually impaired’.
Generally this means children with a visual acuity of 6/18 or less in the best
correct eye.
6%
Visual acuity of 6/18 or less and/or restricted visual fields.
4%
A child who experiences difficulties in gaining access to the National
Curriculum due to impaired vision. This includes children with visual acuity
of 6/18 or less in the best corrected eye, also significant field loss.
Combination of factors, including named eye condition, prognosis, visual
acuity and field of vision. Criteria are as much decided by functional vision
as by clinical assessment of vision.
14%
74%
Other
2%
100%
Table 5.9: criteria used by visual impairment services to identify a multi-disabled visually
impaired child from a sample of 152 providers (Clunies-Ross 1997)[43]
Any child who has significant visual impairment with additional or
multiple difficulties.
Children with visual impairment who have additional handicaps which
combine to cause severe learning difficulties, including profound and
multiple difficulties.
Children with multiple disabilities including visual impairment who are
not able to benefit from education in mainstream.
Pupils with a visual impairment who have a major additional
physical/medical/emotional/behavioural/learning difficulty, normally
falling into the SLD or PMLD range.
Other
28%
20%
13%
38%
1%
100%
78
The variation in definitions and policies was highlighted in the recent report from Wales
(summarised in table 5. 7 above) carried out by the Welsh Assembly Government in 2003. This
report noted that “…there are considerable differences in policies for referrals across the 22
Welsh LEAs.” and that “In some areas the service for visually impaired children and young
people has included children and young people with a slight reduction in visual acuity who may
only be seen by them twice-yearly or annually. In other areas, these children/young people
would not meet the descriptors for intervention.” [53]. The Visual Impairment Scotland project
[52]used a modified version of a visual impairment classification system first developed in
Scandinavia, called the NORDSYN system [54]. The target population was “children with
significant visual impairment” under the age of 16 years who had any of:

best corrected binocular acuity equal to or worse than 6/18

any form of visual field loss

an eye movement disorder which affects visual function

any form of cognitive visual dysfunction due to disorders of the vision parts of the brain.
These definitions were chosen because they were felt to represent thresholds of relevance to the
educational and developmental needs of children.
Multiple disability
A consistent finding has been that a substantial proportion of children with visual impairment
have additional disabilities/needs. The 2002 RNIB survey of LEA visual impairment services
[44] found that around half of children with visual impairment had additional disabilities. In the
Visual Impairment Scotland project, 57.1% of children identified as having “significant visual
impairment” were found to have another disability in addition to a visual problem [52]. The
Thirty per cent were classified as having severe or profound and multiple learning difficulties in
addition to a visual impairment, while the remaining 20% had additional disabilities excluding
severe or profound learning difficulties. In the study by Rahi et al[47] ; 77% of children newly
diagnosed with severe visual impairment or blindness had additional non-ophthalmic disorders
or impairments. A re-analysis of the 1989 OPCS child disability survey used cluster analysis to
group together the children into different groups or clusters according to shared characteristics,
with the aim of taking into account and “make(ing) more visible the type, severity and
combination of all the disabilities experienced by a child” [55]. The study showed that children
were likely to either have a mild to moderate visual impairment with few other disabilities, or to
have visual impairments of a more severe nature, along with several other disabilities also of a
severe or profound nature.
Summary
This chapter summarises the literature relating to the prevalence of visual impairment and visual
disability in children in the UK. Most of the available data about visual loss in children comes
from registers or from surveys of providers of health care, social care or educational services to
children with visual loss. Thus the available estimates rely on children with visual loss being
known to the relevant services. This may be a reasonable assumption for children, because
substantial visual loss is very likely to be detected, particularly at school.
79
Because of the variation in definitions and service provisions, great caution is required in
interpretation of any single estimate of the prevalence of visual impairment or blindness among
children in the UK. However, using a broad and pragmatic definition of visual loss sufficiently
bad as to mean a child is identified as being in need of special educational or social services, the
existing data suggest a prevalence of visual impairment in the region of 10-20/10,000 children.
For severe visual loss, the estimates from the study by Rahi et al are likely to be much the most
reliable. In this study, the cumulative incidence of blindness or severe visual impairment by the
age of 16 years was estimated to be 5.9/10,000 children. The term “cumulative incidence” is
correctly used because the data were not derived from a true prevalence survey, but can be
interpreted as similar to prevalence at the age of 16 years.
Around half the children receiving support from visual impairment services may have additional
disabilities, and this proportion may be even higher for children with severe visual loss.
80
Chapter 6
Visual acuity and vision related quality of life
81
Introduction
Visual acuity is a good indication of the degree of vision loss, but may not reflect an individual’s
own perception of how vision affects their everyday life, from the ability to perform every day
tasks or the psychological and social consequences of reduced vision. A number of
questionnaires have been developed, primarily in the US, to measure visual function / vision
related quality of life [56]. Only two scales- the National Eye Institute Visual Function
Questionnaire (NEI-VFQ) [57] and the Indian Vision Function Questionnaire (IND-VFQ) [58]
were developed from patient elicited problem statements collected from focus groups, in
contrast to other scales developed from clinical judgement. The need for a specific measure of
visual function led to the development of A 51-item questionnaire was originally devised in the
US from focus groups of people with major causes of eye disease The NEI 51-item original
questionnaire was later shortened to 25 items, based predominantly on the responses from those
with eye disease and visual impairment, and also from a minority group without eye disease [59].
The NEI-VFQ 25 has been used in used to show that those with age-related macular disease
[60], glaucoma [61], optic neuritis [62], diabetes mellitus [63], central retinal vein occlusion
[64], subfoveal choroidal neovascularisation and accompanying visual impairment [65] have
lower scores compared to a reference group without ocular disease or visual impairment. This
questionnaire has also been used to show the longitudinal benefit of vision rehabilitation
amongst a group without correctable visual impairment.[66, 67] Despite some concerns about
the range of measurement obtained with the NEI-VFQ [68], it has also been used in adult
populations (aged 40 years or more) to show that those with visual impairment have lower scores
compared to those without reduced visual acuity [69, 70]. Use of the NEI VFQ in non US
populations is more limited especially among the older population who are likely to experience
higher levels of visual difficulties than younger age groups. To our knowledge there are no
published vision function data from a population based UK sample. A nested trial within the
MRC Elderly Trial permitted the possibility of collecting these data. Full details of the nested
trial design and results have been published elsewhere [71] .
Methods
Surviving participants in 20 randomly selected general practices from each arm of the main trial
were invited to have an assessment of their vision and complete the 25-item NEI-VFQ.
Research nurses measured presenting monocular and binocular visual acuity (defined as using
their habitual distance correction) on the LogMAR scale using Glasgow acuity charts [11].
NEI-VFQ questions are grouped in 12 sub-scales ( general health, general vision, ocular pain,
near activities, distance activities, social functioning, mental health, role difficulties,
dependency, driving, colour vision and peripheral vision), as well as a combined total score
(Table 6.1). We followed the NEI VFQ scoring method. Each sub-scale was assigned a score
from 0 to 100, where 0 is the worst and 100 represent no disability related to vision (that is,
ratings of excellent or no difficulty).
2589 people were invited of whom 1807 participated (69.8%), 670 refused (25.9%) and 112
(4.3%) were too ill. The mean age of participants was 83 years (ranging from 77 to 101 years);
and the majority were female (64%). Complete data, including a measure of visual acuity and a
completed NEI-VFQ, were available for 1745 participants. The prevalence of presenting visual
acuity less than 6/12 in the better eye was 31% (n=545/1745), and 12% (n=203/1745) for visual
82
impairment (acuity less than 6/18 in the better eye). The cause of visual impairment was only
available for a minority, and hence is not considered further.
With the exception of general health, few respondents reported at the extreme ends of the scales
in the NEI-VFQ subscales. For this reason responses were categorised into “no problems” or
“any problems” for all scales except for General Health and the Total score. The proportions are
shown in the second column of Table 6.2. For General Health only 8% reported no problems and
30% reported having poor health. In contrast, for general vision 68% reported having good to
excellent eyesight. The percentage reporting problems for the other scales (ranked by highest
proportion of reporting problems was: 53% for near activities, 51% with distance activities, 45%
for driving, 45% for ocular pain, 46% mental health, 41% difficulties in carrying out roles, 25%
peripheral vision, 22% reported some level of dependency, 20% had some problems in social
functions, and 10% with colour vision. For the overall VFQ score, only 25% scored 100 i.e. no
problem on any of the sub-scales; and at the top end 25% had high scores. The third column of
Table 6.2 shows the proportion with visual acuity <6/12 in each problem category for each of the
sub-scales and overall scale and the fifth columns gives the proportion for VA <618. For
example in the General Vision scale 59% of those reporting problems had a VA <6/12 compared
with 20% with VA<6/12 in those who reported no general vision problems. For VA <6/18 the
differences are greater, i.e. 33% of those reporting problems on the General Vision scale have
VA <6/18 compared with only 4% in those who report no problems in general vision. Columns 4
and 6 present the comparisons between the proportions with poor visual acuity according to
whether they have problems or not on the sub-scales. The analyses are given as odds ratios
(ORs) and in all the comparisons the effects of age and gender were taken into account. The OR
for General Vision is 5.75 for VA <6/12 and 11 for VA <6/18 i.e. there is a five fold chance of
having VA <6/12 for people who reported problems on the General Vision sub-scale and an
eleven fold chance of having VA <6/18. The P-value shows that this result is highly statistically
significant (unlikely to have occurred by chance). Significantly increased ORs were seen for all
sub-scales except ocular pain. Another way of reporting the results is shown in the last two
columns of the table. The mean LogMAR score and the equivalent level of Snellen chart acuity
is presented according to the reporting of problems.
Because sub-scales are correlated with each other i.e. a person who reports problems on one subscale is likely also to report problems on another scale, further analyses investigating which of
the sub-scales was the most important in terms of vision impairment was carried out. The results
showed that difficulties with general vision, near activities, and social functioning remained
strongly associated with visual impairment. Difficulties with dependency and colour vision were
of borderline significance. A combined score was derived from the average of these sub-scales.
Although visual acuity is clearly strongly associated with vision function we investigated how
much of the combined score was accounted for by visual acuity. If there was a perfect
correlation for visual acuity with vision function, the variation in the score accounted for by
visual acuity would be 100%. Our results showed that 20% of the combined score was explained
by visual acuity, and only an additional 4% by age and gender and area of residence. Measures
of deprivation were not related to the scores. We do not know what other factors contribute to
the variation in the score. It is likely that psychological attributes such as coping strategies, or
being a pessimistic or optimistic type of person may influence the way in which people with the
same levels of visual acuity report their general vision function. For example, of 242 people
with VA <6/18, 50 (21%) reported no problems with their General Vision. For specific problems
such as Near Activities the proportion was much lower, with only 27 (11%) of those with VA
83
<6/18 reporting no problems. We used visual acuity alone as a “clinical” measure. Although
useful in population vision testing, visual acuity is a relatively limited measure of vision
performance and it is possible that inclusion of a broader spectrum of tests, such as contrast
sensitivity, colour vision and assessment of the visual field would provide a more comprehensive
picture.
Chapter Summary
In a general population sample of elderly people (mean age 83), the prevalence of presenting
visual acuity less than 6/12 in the better eye was 33% and 14% (n=243/1785) had acuity less
than 6/18 in the better eye). Using the NEI-VFQ 25, 68% reported their vision was excellent or
good and only 32% reported their vision as fair or poor. Around half of the reported problems
were with near or distance activities. Visual acuity levels were strongly associated with all subscales except ocular pain. The most important scales related to visual acuity were general vision,
near activities, and social functioning and, to a lesser extent, dependency and colour vision.
However visual acuity only explained about a fifth of the variation between people in their
reporting on these scales. Of concern were the 21% of visually impaired people reporting no
problems with vision although this proportion was lower when considering specific activities.
We conclude that NEI-VFQ scores should be considered as an additional measure of visual
disability and not as a substitute for visual acuity.
84
Table 6.1 NEI VFQ 25 Questions and sub-scale scores
NEI-VFQ sub-scale
Number of
questions used to
derive sub-scale
score
1
1
2
3
NEI-VFQ question
numbers used
Distance activities
3
8,9,14
Vision specific:
Social functioning
Mental health
2
4
11,13
3,21,22,25
2
3
3
1
1
17,18
20,23,24
15c,16
12
10
All
General health
General vision
Ocular pain
Near activities
Role difficulties
Dependency
Driving
Colour vision
Peripheral vision
Total VFQ score
1
2
4,19
5,6,7
Questions asked
Perception of overall health (5 levels)
Perception of eyesight (6 levels)
Pain and discomfort in and around the eyes and the degree of ocular pain
Difficulty reading ordinary print in newspapers, performing work or hobbies requiring
near vision, and finding something on a crowded shelf
Difficulty reading street signs or names of shops, going down steps, stairs or kerbs in
poor light and visiting people in unfamiliar surroundings
Difficulty seeing how people react and visiting people in unfamiliar surroundings
Worries and frustration about eyesight, embarrassment and loss of control caused by
eyesight
Lack of accomplishment and limitations caused by eyesight
Need to stay at home, reliance on others, and need of help
Difficulty driving during the daytime and at night
Difficulty picking out and matching clothes
Difficulty noticing objects off to the side whilst walking
A total score averaged for the subscale scores listed above
85
Table 6.2 NEI-VFQ 25 scores and association with visual acuity
NEI VFQ subscales
Responses by
Number (%) with
OR VA<6/12 (95%
Number (%) with
OR VA<6/18 (95%
category
VA<6/12 in each
CI)
VA<6/18 in each
CI)
N (%)
category of NEI-
category of NEI-
VFQ subscale
VFQ subscale
Mean logMAR
Snellen equivalent
General health
No problems
142 (8)
41 (29)
1.00
16 (11)
1.00
0.22 (0.18, 0.26)
6/10
Few problems
493 (28)
129 (26)
0.78 (0.50, 1.22)
44 (9)
0.68 (0.36, 1.29)
0.20 (0.18, 0.22)
6/10
Moderate health
614 (34)
198 (32)
1.03 (0.67, 1.59)
91 (15)
1.16 (0.64, 2.11)
0.23 (0.21, 0.25)
6/10
Poor health
534 (30)
216 (40)
1.43 (0.93, 2.22)
91 (17)
1.25 (0.68, 2.28)
0.26 (0.24, 0.28)
6/11
Total
1783
584 (33)
P-value †
242 (14)
<0.001
1744
0.018
0.001
General vision
No problems
1206 (68)
244 (20)
1.00
50 (4)
1.00
0.16 (0.15, 0.17)
6/9
Problems
577 (32)
339 (59)
5.75 (4.53, 7.31)
192 (33)
11.0 (7.72, 15.5)
0.37 (0.35, 0.39)
6/14
Total
1783
583 (33)
P-value
242 (14)
<0.001
1744
<0.001
<0.001
Ocular pain
No problems
982 (55)
316 (32)
1.00
120 (12)
1.00
0.23 (0.21, 0.24)
6/10
Problems
801 (45)
267 (33)
1.04 (0.83, 1.29)
122 (15)
1.22 (0.91, 1.64)
0.23 (0.21, 0.24)
6/10
Total
1783
583 (33)
P-value
242 (14)
0.751
1744
0.176
0.891
Near activities
No problems
838 (47)
156 (19)
1.00
27 (3)
1.00
0.16 (0.14, 0.17)
6/9
Problems
940 (53)
422 (45)
3.48 (2.75, 4.40)
213 (23)
8.35 (5.44, 12.8)
0.29 (0.28, 0.31)
6/12
86
Total
1778
540 (31)
P-value
240 (14)
<0.001
1740
<0.001
<0.001
Distance activities
No problems
839 (50)
185 (22)
1.00
40 (5)
1.00
0.18 (0.16, 0.19)
6/9
Problems
856 (51)
339 (40)
2.11 (1.67, 2.66)
178 (21)
4.91 (3.37, 7.14)
0.26 (0.25, 0.28)
6/11
Total
1695
524 (31)
P-value
218 (13)
<0.001
1658
<0.001
<0.001
Social function
No problems
1420 (80)
357 (28)
1.00
95 (7)
1.00
0.19 (0.18, 0.20)
6/9
Problems
355 (20)
219 (62)
4.71 (3.60, 6.16)
145 (41)
9.38 (6.81, 12.9)
0.39 (0.37, 0.42)
6/15
Total
1775
576 (32)
P-value
240 (14)
<0.001
1736
<0.001
<0.001
Mental health
No problems
954 (54)
218 (23)
1.00
56 (6)
1.00
0.18 (0.17, 0.19)
6/9
Problems
829 (46)
365 (44)
2.54 (2.03, 3.17)
186 (22)
4.46 (3.20, 6.23)
0.29 (0.27, 0.30)
6/12
Total
1783
583 (33)
P-value
242 (14)
<0.001
1744
<0.001
<0.001
Role difficulties
No problems
1058 (60)
246 (23)
1.00
52 (5)
1.00
0.18 (0.16, 0.19)
6/9
Problems
721 (41)
33 (46)
2.79 (2.23, 3.49)
188 (26)
6.77 (4.81, 9.55)
0.30 (0.29, 0.32)
6/12
Total
1779
579 (33)
P-value
240 (13)
<0.001
1740
<0.001
<0.001
Dependency
No problems
1381 (78)
335 (24)
1.00
91 (7)
1.00
0.19 (0.17, 0.20)
6/9
Problems
400 (22)
246 (64)
4.80 (3.70, 6.24)
150 (38)
7.45 (5.44, 10.2)
0.38 (0.36, 0.40)
6/14
Total
1781
581 (33)
241 (14)
87
1742
P-value
<0.001
<0.001
<0.001
Driving *
No problems
299 (55)
29 (10)
1.00
3 (1)
1.00
0.11 (0.08, 0.13)
6/8
Problems
246 (45)
79 (32)
4.63 (2.81, 7.62)
38 (15)
24.5 (6.57, 91.1)
0.21 (0.18, 0.23)
6/10
Total
545
108 (20)
P-value
41 (8)
<0.001
534
<0.001
<0.001
Colour vision
No problems
1578 (90)
445 (28)
1.00
146 (9)
1.00
0.20 (0.19, 0.21)
6/10
Problems
178 (10)
121 (68)
5.39 (3.74, 7.78)
89 (38)
9.86 (6.77, 14.3)
0.45 (0.41, 0.49)
6/17
Total
1756
566 (32)
P-value
235 (13)
<0.001
1718
<0.001
<0.001
Peripheral vision
No problems
1300 (75)
326 (25)
1.00
94 (7)
1.00
0.19 (0.18, 0.20)
6/9
Problems
440 (25)
227 (52)
3.02 (2.37, 3.87)
95 (24)
5.03 (3.68, 6.87)
0.32 (0.30, 0.34)
6/13
Total
1740
553 (32)
P-value
189 (11)
<0.001
1703
<0.001
<0.001
Total VFQ score
No problems
442 (25)
75 (17)
1.00
12 (3)
1.00
0.16 (0.13, 0.18)
6/9
Few problems
447 (26)
103 (23)
1.45 (1.02, 2.07)
21 (5)
1.72 (0.82, 3.58)
0.18 (0.16, 0.20)
6/9
Moderate vision
456 (26)
136 (30)
2.01 (1.43, 2.83)
41 (9)
3.37 (1.72, 6.60)
0.21 (0.19, 0.23)
6/10
Poor vision
438 (25)
269 (61)
6.93 (4.93, 9.75)
168 (38)
19.5 (10.5, 36.5)
0.39 (0.37, 0.41)
6/15
Total
1783
583 (33)
P-value
242 (14)
<0.001
1744
<0.001
<0.001
1.00
0.16 (0.13, 0.18)
Total VFQ score
No problems
442 (25)
75 (17)
1.00
12 (3)
88
6/9
Few
447 (26)
103 (23)
1.47 (1.03, 2.09)
21 (5)
1.78 (0.85, 3.71)
0.18 (0.16, 0.20)
6/9
Moderate
456 (26)
136 (30)
2.02 (1.43, 2.85)
41 (9)
3.42 (1.74, 6.72)
0.21 (0.19, 0.23)
6/10
Lots
399 (23)
230 (58)
5.93 (4.20, 8.38)
129 (32)
15.1 (8.04, 28.5)
0.39 (0.37, 0.41)
6/15
Total
1744
544 (31)
<0.001
<0.001
P-value †
203 (12)
<0.001
89
Chapter 7
Conclusions and recommendations
In this chapter we summarise the evidence relating to prevalence estimates, discuss how the
estimates relate to registration data and identify gaps in the knowledge base where further
research is required.
As described in Chapter 1 the methods of measuring visual problems use two different
approaches: self reported visual difficulties (reviewed in Chapter 2) and clinical measures of
visual acuity (reviewed in Chapter 3).
Prevalence of visual difficulties
The estimates of visual difficulties (described as visual disability in some studies) derive
predominantly from large government surveys and show, when using similar survey methods
and criteria for visual disability, (as in the OPCS surveys of 1985 [25]and 1996/7 [26]) some
consistency in the results in the older population: 22% of the over 75s in private households
reported difficulties with reading newsprint or worse and 26% when people in communal
establishments were included. There was less consistency in the results for the younger age
groups where the two surveys differed over 2 fold in their estimates for the population aged 1664 from 0.8% to 2.0 %. If the proportion of the younger population with visual difficulties is
over estimated this will substantially inflate the numbers with sight difficulties in the population
since the numbers of people in the population aged 16-64 is much greater than in the older age
groups. Thus the estimated numbers with sight difficulties in private households in England and
Wales based on the 1996/7 [26] survey can be estimated at nearly 2 million compared to 1.4
million from the 1985 survey[25].
There are doubts concerning the reliability of all estimates based on the criteria of “difficulties
with reading newsprint” or worse. The lack of reliability is based on findings from the
OPCS/RNIB 1998/1999 survey [27] . The sample for this study came from a re- survey of
respondents who had reported sight difficulties in the 1996/7 survey quoted above and in the
Omnibus survey. A substantial proportion of respondents who initially self reported with a sight
problem on re-survey denied they had had a sight problem. The main change in response was to
the questions: “Difficulty in seeing a friend across the road” and “Difficulty reading ordinary
newsprint”. With more severe task difficulties the level of misclassification was minimal. The
estimates of visual disability adjusted for misreporting in the OPCS/RNIB 1998/1999 study.
were considerably lower than either of the two previous surveys with a prevalence for difficulties
with reading newsprint or worse of 14% for the over 75s and 1.1% for the 16-59 age group.
Estimates from OPCS/RNIB 1998/1999 study may also be too low because some people who
initially had a problem had subsequently received treatment.
Confidence in the results is further limited by the lack of detailed information on response rates
especially in the government surveys. The government surveys on disability used a “sift”
approach to screen out people with no apparent disability with successive reductions in response
with successive sifting. Although the final response rates in the sample from whom the estimates
90
are derived are not clearly presented they are of the order of 50% or less in most government
surveys. The representativeness of the surveyed population is therefore difficult to assess.
We advise against the use of data on self reported sight problems to estimate the levels of visual
impairment in the population.
Prevalence of visual impairment
Studies using visual acuity measurements have used various criteria for definitions and cut points
of visual impairment. The results from two studies: National Diet and Nutrition Study (NDNS),
and MRC Assessment Trial that were nationally representative of the older population and use
uncorrected presenting VA and similar cut points are given in the table below (unpublished data
from NDNS provided by Dr van der Pols and unpublished data on VA <6/12 for MRC
Assessment Trial using same LogMar criteria as NDNS).
65-74
NDNS
community
sample1
VA <6/18
VA<6/12
MRC Assessment
Trial2
VA <6/18
VA<6/12
475
5.6
15.8
Age group
not
included
95% CI
75-84
95%CI
429
13.3
28.3
3.5, 7.6
12.6, 19.1
11500
8.5
18.7
85+
95% CI
10.1, 16.5
24.1, 32.6
222
31.7
54.0
25.5, 37.8
47.4, 60.6
7.1, 9.8
16.5, 20.9
3100
26.8
45.8
23.9, 29.7
42.2, 49.5
1
Best score (Glasgow Acuity Cards) of two eyes without pinhole correction
2
Binocular acuity without pinhole correction
These results show that the point prevalence estimates of vision impairment (VA <6/18) are : 6%
NDNS only) at ages 65-74, 8% to 13% at ages 75-84 and 27% to 32% at ages 85+. Estimates of
the prevalence of minor visual loss (visual acuity <6/12) are 16% (NDNS only) at ages 65-74,
between 19% to 28% at ages 75-84 and 46% to 54% at ages 85+.
Considering the upper and low bounds of the 95% confidence intervals the results show that the
estimates of vision impairment (VA <6/18) range from 4 to 8 % (NDNS only) at ages 65-74, 7%
to 16% at ages 75-84 and 24% to 38% at ages 85+. Similarly the range of estimates of the
prevalence of minor visual loss (visual acuity <6/12) are from 13% to 19% (NDNS only) at ages
65-74, between 16% to 33% at ages 75-84 and 42% to 61% at ages 85+.
Estimates for the nursing home population from NDNS show high levels of vision impairment
(VA <6/18): 12.1% at ages 65-74 (95% CI 9.7%, 23.2%), 30.0% of those aged 75-84 (95% CI
20.5%, 39.5%) and 46.9% of those aged 85+ (95% CI 37.7%, 56.1%). However as these are the
only recent estimates available for the nursing home population we recommend that they are
viewed with some caution.
91
Two studies provided estimates of blindness using international criteria of VA < 3/60. In the
MRC assessment trial the estimates for the 75+ age group were 2.1 (95% CI 1.8, 2.4) and in a
small study in London the estimates for blindness were very similar (1.9%) although the
confidence intervals were very wide (0.2, 6.6)
Information on visual impairment alone is only partially useful and is enhanced by knowledge of
the causes of vision impairment. Similarly if the focus of interest is in identifying people who
could benefit from services and treatments, measures of vision impairment alone are inadequate.
In Chapter 4 we reviewed the few UK studies which have provided some information on the
causes of vision impairment. Prevalence estimates of different eye diseases/conditions differed
between studies because of differences in the visual acuity cut points in the studies e.g. <6/12 in
North London Study[33], <6/18 in the MRC Assessment Trial [39] and differ also in the age
range, 65+ in the North London Study and 75+ in the MRC Trial. All studies highlighted
untreated refractive error and cataracts as major remediable causes of vision impairment in older
people ranging from 70% to 50% as a proportion of visual impairment. With increasing age the
proportion of treatable vision impairment declines as age related macular degeneration becomes
more prevalent.
In the UK studies, consistently higher estimates of poor visual acuity were reported for women
compared to men. This reflected higher prevalence rates for untreated cataracts[13, 33]and for
age related macular degeneration[39]. The seven country EUREYE study reported higher rates
of vision impairment and for age related macular degeneration in women compared with men
[72]. A pooled analysis of studies conducted mainly in the US found that women were 30% more
likely to have VA < 6/12 compared with men of the same age [73]. Gender differences in the
prevalence of vision impairment in the UK may reflect lower access to services and aetiological
factors for age related macular degeneration. In the North London Study, a higher prevalence of
refractive errors and cataracts in people was found for those living in the more deprived areas.
What proportion of the visually impaired population eligible for registration are unregistered?
The RNIB/ONS 1990 report [37] raised concerns that a large proportion of the visually impaired
population were not accessing services and support via the registration system. As the results for
visual impairment from that report were based on sight tests alone without excluding possible
remediable conditions (such as refractive error and cataracts) the registerable component is likely
to be overestimated by around two fold. We showed that, based on other studies which have
collected data on the causes of vision impairment the numbers likely to be registered because of
permanent vision loss, and those registered were of a similar magnitude (Table 4.5). Even if we
consider that some of the prevalence estimates of vision impairment may be biased towards the
low side, and therefore there is some under reporting, it is extremely unlikely that it is of the
order of magnitude suggested by the RNIB/ONS 1990 estimates. For example using data from
the MRC Assessment trial we estimate that around 237000 aged over 75 were likely to have
visual impairment eligible for registration. Comparing this with the actual figures from the DOH
of 211000 registered suggest that approximately 10% of the over 75s may not be registered. If
we take the lower figure from the 95% confidence interval of 207,000 then the registration
numbers are slightly higher than those we estimate and if we take the upper confidence interval
the extent of under registration is 20%.
92
International data also provide some reassurance that the prevalence of registerable vision in the
UK is similar to that expected from the population studies. In a recent pooling of studies
predominantly from the USA [73] the prevalence of blindness based on best corrected VA <3/60
was very similar to the percent prevalence for registration figures for England in the comparable
age groups (as shown in Table 4.4).
Estimates of registerable visual impairment due to age related macular degeneration in the UK
also suggest good agreement between predictions and observations [74, 75]. Owen at al [74]
pooled the data from international studies to provide age specific prevalence rates for visually
impairing age related macular degeneration and applied these rates to the UK population to
obtain expected numbers of those eligible for registration. The estimated numbers were 214,000
(95% CI 151,000 to 310,000) with visual impairment caused by AMD (visual acuity 6/18 or
worse). Evans et al using prevalence rates for AMD from the MRC Assessment Trial[75]
estimated the numbers as slightly lower at 192,000, 95% CI 144,000 to 239,000). There was a
small difference in the definition of visual impairment (<=6/18 in Owen and <6/18 in Evans).
We conclude that, although registration rates may show some under ascertainment in the older
age groups, the evidence does not support substantial under-reporting.
Gaps in evidence
Most studies have been done in the older population and there is a scarcity of data in younger
adult age groups in the UK. A review by Nissen et al[76] of epidemiological studies performed
in Western Europe, North America and Australia covering the age group 20 to 59 years found
the prevalence of blindness was 0.08 and of visual acuity 6/24 to 6/48 was 0.07%. These figures
agree well with the prevalence of registrations in a similar age range and we conclude that
registration data provide reasonably accurate estimates of the prevalence of serious vision
impairment in the younger adult age groups. Information is lacking on less severe levels of
visual impairment.
For children, agreed criteria to define visual disability, visual impairment and the need for
specialist support are needed. The needs of children with visual disability who also have other
disabilities should also be a priority.
There are very little data on vision impairment in different ethnic groups in the UK population.
Two small studies attempted to examine ethnic differences (Asian) in the prevalence of
registerable blindness/partial sight [36] or cataracts [38]. These studies suggested that rates might
be higher in Asian groups. The OPCS/RNIB 2000 [27]report also examined prevalence rates by
responses to the sift questions on seeing difficulty for a variety of ethnic groups and found that
the highest levels were in Black Caribbean populations with low rates for Asians. At present the
data are too sparse and unreliable to comment on the prevalence of visual impairment or specific
types of eye disease in different ethnic groups.
The reasons for higher levels of vision impairment in women requires further understanding, in
particular the extent to which this reflects under utilisation of services.
The reasons for high levels of treatable conditions in the older population requires further
investigation. Some indications of possible barriers was provided by our randomised trial of
93
screening visual acuity by a practice nurse [71]. Although visual impairment was common, few
people benefited from subsequent intervention, and screening did not reduce levels of visual
impairment. Key explanations for the lack of effect identified were under-detection of
uncorrected refractive error and that only around half the recommendations for referral to an
ophthalmologist resulted in referral by the general practitioners. In addition participants
themselves reported an unwillingness to self-refer to optometry services for further assessment,
citing fear of costs and lack of perceived need.
Recommendations
Policy
The estimates provided from the studies of visual acuity for the older population are sufficiently
consistent for general purposes of planning and estimating the numbers of people in the UK with
vision impairment. However we caution that these estimates are seen as approximations and not
as “tablets of stone” as prevalence estimates will vary from place to place and over time and
because all estimates carry a range of uncertainty (as expressed by the 95% confidence interval).
Studies in the older population have also consistently shown that about 50% of visual
impairment is due to treatable conditions i.e. cataract and refractive error. This proportion
diminishes with increasing age due to the higher prevalence of age related macular degeneration
as a cause of vision impairment. Increased awareness of these facts is important for agencies
dealing with visually impaired people. Appropriate action e.g. media campaigns and advocacy is
required to ensure that both eye care providers and older people themselves are aware of these
conditions and of the current unacceptably high levels of untreated conditions.
Strategies to identify the most cost effective way of targeting the older population, including
novel approaches to screening older people, are urgently needed
Standards for Reporting and Measurement
We have shown the considerable variation in definitions and cut points used in the UK based
studies. This applies even more at the international level especially when comparing data with
the US where very different definitions on blindness and low vision are used. A recent WHO
Informal Consultation Working Group [6] has made recommendations for measuring acuity and
it is hoped that this will go some way to improving the consistency of reporting across studies.
The Working Group recommended that vision assessment in population-based studies should
include a measurement of visual acuity using LogMAR charts at distance and near under
standardized conditions. Information collected should record: (i) monocular and binocular
distance presenting visual acuity, whether a method of vision correction is used (e.g. spectacles)
and, if so, the type and power of vision correction device; (ii) monocular and binocular near
presenting visual acuity at 40 cm, whether a method of vision correction is used (e.g. spectacles)
and, if so, the type and power of vision correction device; (iii) monocular and binocular bestcorrected visual acuity at distance and near, following refraction using an age-appropriate
addition for near acuity. Few UK studies have used best corrected visual acuity largely because
of the extra resources required to collect these data. Presenting binocular vision impairment
94
probably represents the measurement of vision most closely related to vision experienced by the
individual. However, an additional measurement of best corrected visual acuity would clearly
identify the level of refractive error and the remaining vision loss due to eye diseases. The
Working Group also suggested that revisions should be made to the categorisation of visual
acuity as in ICD-10 (Table 7.1). The categories should be based on presenting (not best
corrected vision as currently applies).
We recommend the use of validated questionnaires or scales for measuring self reported vision
problems or vision related quality of life. We emphasize the need to thoroughly test all questions
before use in surveys.
There was a substantial lack of clarity and detail in some of the reports that we reviewed. We
recommend that reports which present novel research findings (but are not submitted for
publication in academic journals) should conform to the same standards of reporting as the
formal peer review process of academic journals; for example the guidelines of the International
Committee of Medical Journal Editors [77] (www.icmje.org).
95
Table 7.1 Recommendations for categorisation of visual acuity [6]
Category
Presenting distance visual acuity
Worse than:
Equal to or better than:
Mild or no visual
impairment
0
Moderate visual
impairment
1
6/18
3/10 (0.3)
6/19
3.2/10 (0.3)
20/63
6/60
1/10 (0.1)
20/200
Severe visual
impairment
2
6/60
1/10 (0.1)
20/200
3/60
1/20 (0.05)
20/400
Blindness
3
3/60
1/20 (0.05)
20/400
1/60*
1/50 (0.02)
5/300 (20/1200)
Blindness
4
1/60*
1/50 (0.02)
5/300 (20/1200)
Light perception
Blindness
5
No light perception
96
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