Clinical Relevance of Straylight Assessment with the C-Quant.
What is retinal straylight?
We know that disturbances to the eye media may cause vision loss of small
detail. This can be determined with visual acuity assessment using a letter
chart. But eye media disturbance can do much more harm, because it may
cause light scattering. Light scattering results in a veil of straylight over the
retinal image. The patient complaints may include hazy vision, increased
glare hindrance, loss of contrast and color, etc. These problems are much
enhanced if visual function is already low from retinal pathology, such as
in macular degeneration or glaucoma.
pedestrian
car headlight
Figure 1 Visualization of retinal straylight. The optical components of the eye form an image of the
outside world (left picture) on the retina (right picture). In the case of such a street scene, the picture
on the retina is much degraded. Street objects are much less visible compared to the original picture.
This is caused by the fact that part of the light coming from the car headlight is scattered in all
forward directions (represented by the white arrows in the figure), projecting a veil of light over the
retinal image which causes a decrease in the contrast of this image. This veil of light is called stra ylight.
In an ideal eye there would be no light scattering at all, but because
the eye media are not optically ideal, there will always be some light scattering. This light scattering reduces the contrast of the image projected on
the retina, thus decreasing the quality of vision (Figure 1). In short, more
straylight means worse vision.
It is important to realize that the effect of straylight on vision is totally
different from the effect of decreased visual acuity on vision. In many dai-
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ly life circumstances, increased light scattering has a much stronger effect
on the quality of vision than decreased visual acuity.
What are the causes of retinal straylight?
The amount of retinal straylight is different for each individual, and may
even differ between both eyes. It depends on age, pigmentation, pathologies such as cataract, and may change due to human interventions such as
refractive surgery. Also the young healthy eye suffers from straylight, as
we all know from the hindrance from headlights while driving at night. But
this is very mild compared to the straylight level of many patients. Within
the normal eye there are four major sources that contribute to the total
amount of straylight: the cornea, the iris and sclera, the eye lens, and the
fundus. Each of these contributions can increase to very disturbing levels
in pathology.
Straylight in patients
- Early Cataract. If cataract starts to develop the earliest complaints often
are from increased straylight. In fact, most often the first effect of cataract
is that patients stop driving at night. Other complaints may include hazy
vision, loss of contrast and color, halos around bright lights, and difficulties with against-the-light face recognition.
- Most corneal disturbances as e.g. in corneal dystrophies cause strong
increase in straylight. In some cases, visual acuity can be remarkably maintained while straylight deterioration is strong, such as in corneal edema.
- In refractive surgery there is a chance of haze in the cornea. Visual acuity may hardly suffer, but complaints from straylight such as glare are of
considerable concern.
- Turbidity in the vitreous can cause large increases in straylight, often
also without much effect on visual acuity. Other factors that have been
documented include contact lenses, pigmentation defects, IOLs, etc.
How can straylight be measured?
Straylight can be measured with the C-Quant instrument from Oculus (figure 3). It takes about 1.5 minutes per eye. The patient sees two semicircular fields with some flickering in it. He/she must press the left or the right
button, depending on the side that he/she sees to flicker strongest. There
are 25 short presentations that the patient must respond to. The strength of
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his straylight is given as “the straylight parameter” ( s ), but it is better to
use the logarithm, denoted as log(s). A higher log(s) value means more
straylight and thus worse vision.
Figure 3. The C-Quant instrument from Oculus for measuring the amount of straylight in patient eyes.
Population studies have shown that the average log(s) value for
young, healthy eyes is around 0.85. Above 40 years of age this value starts
to increase to 1.2 at 70 yrs and 1.4 at 80 yrs. In the true population much
more increased values can be found because of cataracts, cornea conditions, etc. Figure 4 gives results of a European multi-center study. A
young, healthy, non-Caucasian eye may have a value as low as 0.6, whereas a cataract may cause the log(s) value to hit 2.0 or more. This is an increase in straylight intensity of a factor of 25! Because of the logarithmic
scale, a 0.3 increase in the log(s) value means in fact a doubling of the
amount of straylight, and a log(s) increase of 1 means a ten-fold increase in
straylight.
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2.2
Serious straylight hindrance:
straylight increase >4x compared to young eye
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straylight value (log(s))
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
20
30
40
50
60
70
80
90
age [years]
Figure 4 Log(s) values as a function of age for a population of European drivers.
Reliability of a straylight measurement
The C-Quant straylight meter measures straylight as the subject actually
sees it (and is disturbed by it). So, it gives a functional measure, as opposed to an optical measure that may be ill related to the functional problem. The measurement outcome also includes a parameter that gives the
accuracy of the respective measurement (Esd). Normal accuracy of a CQuant measurement is Esd ≈ 0.07 log units. This accuracy is very good
compared to the effects to be measured (see above).
When to use straylight in the clinic?
Refractive lens exchange
Straylight must be measured before making a decision, because if straylight is nicely low, the postoperative outcome may be disappointing, in
case his straylight increases as side effect of the surgery (see vdBAJO07).
If preop straylight is high, you are quite safe.
Straylight lens exchange
The second entry point for lens exchange may be straylight itself. If straylight is high, you can offer surgery to relieve the patient of his straylight.
What we propose here is to stretch the concept of lens exchange, to include
a third line: the old type was visual acuity driven, next came refraction
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driven, and now straylight driven. The third type has been underrated because straylight was not accessible. This can change with the introduction
of the C-Quant. An extra beauty of straylight is that you know for sure that
it is caused by the optical media (for visual acuity that is often uncertain).
We studied for routine cataract surgery the relative benefits. It was an improvement of a factor 1.7 for visual acuity, but an improvement of a factor
3 for straylight. Of course in practice one considers a combination of all
three benefits.
An anecdote: A patient (about 60 year) had cataract surgery on one
eye, and no complaints from the other (UCVA 1.0). As it happened, the
cataract surgery resulted in an eye with very low straylight values.
Better than normal for that age (as he had in his other eye). As result
the patient demanded surgery on his good eye! The lesson to be learned is
that awareness is an important issue. On the side of the ophthalmolgists
as well as the patients. This ophthalmolgist granted his patient the
operation, because he could understand his patient's wish. On the side of
the patients; if you relieve patients of their straylight, they notice it.
With time, clarity of the optics and quality of vision will become more
and more a general demand, now that you can reliably assess it with the
C-Quant.
What is the relation between straylight measurement and other tests?
Straylight and visual acuity
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2.2
2
Serious straylight hindrance:
straylight increase >4x compared to young eye
straylight value (log(s))
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
-0.4
Visual Acuity <0.5 (<10/20)
logMar>0.3
-0.2
0
0.2
0.4
0.6
0.8
visual acuity (logMar)
Figure 5 Straylight value as a function of visual acuity for a European driver population. Log(s) values > 1.47
and visual acuities < 0.5 (logMAR > 0.3) are considered serious visual impairments. A lot more individuals in
this population are impaired by increased straylight than by decreased visual acuity. Only a very small
subgroup suffers from both impairments.
There is only a weak relation between straylight and visual acuity. This is
because straylight is determined by light scattering over larger angles (1 to
90 degrees), whereas visual acuity is determined by light deflections over
small angles (<0.1 degree, more commonly known as aberrations). Moreover, the physical processes that cause these light deflections are different
for the two angular domains. Therefore, changes in the one domain do not
necessarily mean changes in the other domain. For example, putting a +2
dpt trial lens in front of a subject’s eye will definitely change the subject’s
visual acuity, whereas his straylight value will stay precisely the same. On
the other hand, putting a fog filter in front of the subject’s eye will show a
dramatically increased straylight value, whereas visual acuity will hardly
decrease. This independence is also illustrated in the practical population
by
Figure
5
for
the
large
European
driver
study.
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Straylight and contrast sensitivity
Straylight has its effect, because it reduces the contrast of the image on the
retina. However, studies have shown that you cannot (reliably) test for
straylight by performing a contrast sensitivity test. This may seem odd, but
has been explained: Contrast sensitivity is a relatively coarse measurement
and dependent on refractive aberration type errors. Contrast sensitivity is
more like visual acuity. So, contrast sensitivity can not be used as a valid
means to assess the amount of straylight, but straylight can be used as an
accurate way to assess contrast loss.
Straylight and glare sensitivity
As mentioned above, the primary effect of straylight is reduction of retinal
contrast. But in a particular manner. The point to make here is that in our
normal surroundings huge intensity differences exist. Because of straylight, high intensity areas influence the contrasts in low intensity areas. The
most well known practical example is blinding by head lamps at night.
This has been called “disability glare”. The international standards committee CIE, has defined disability glare to be synonymous with straylight,
because it was found that straylight is the solid basis of these phenomena.
There have been attempts to measure glare sensitivity via contrast sensitivity or visual acuity with so-called glare testers. Studies have shown the
results of glare testers to be unreliable and not to be sufficiently related to
the patients’ complaints.
Straylight and patient complaints
Patient complaints from increased straylight may be voiced in a variety of
ways. It is important to note that straylight defines a functional condition
of the eye in a straightforward quantitative way, but the complaints may
include such variety as hazy vision, increased glare hindrance, loss of contrast and color, halos around bright lights, and difficulties with against-thelight face recognition. It may strongly depend on the individual subject
which of these complaints are mentioned, or even what words are used to
describe the complaints. Moreover, it must be mentioned that in the field
of glare a particularly subjective type of patient response has been identified, called “discomfort glare”. As opposed to disability glare, which is the
functional effect of glare, discomfort glare is a description of subjective
glare experience. Complaints may be expressed in terms of discomfort,
annoyance, fatigue, and even pain. On average, increased disability glare
will also lead to more discomfort, but in some cases, such as with the nowadays abundant blue-light high-intensity discharge (HID) car headlamps,
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people might be severely annoyed by light sources that have only a moderate functional glare effect.
TJTP van den Berg, PhD
Netherlands Inst. Neuroscience, Royal Academy
www.nin.nl
t.j.vandenberg@nin.knaw.nl
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