Visual Acuity and
Evidence-Based Medicine
A Failure in
Knowledge Translation?
Sound Bites of 2008
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Knowledge Translation
Knowledge Exchange
Knowledge Synthesis
Evidence-Based Medicine
Best Practice
The Parties Involved…
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Researchers
Clinicians
Students
Policy Makers / Politicians
Administrators
Funding Agencies
Industry…
An Example of “Best Practice”
in Action – Visual Acuity
Why measure VA?
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Refraction - Presbyopia
Clinical tool
Impairment benefits
Policy Guidelines
Driving License / Personal/Commercial
Airplane License / Personal/Commercial
Etc.
Historical Dates of Relevance
• Initial Ideas about measuring resolution of
the eye: Ancient Egypt
• Resolve Mizar and Alkor (Big Dipper) as
separate stars
• Later refined: Need 30 sec of visual arc to
resolve stars (Hooke, 17th Century)
Beginning of KT
• 1843: Küchler (ophthalmologist) calls for
standardized vision testing
• Jäger, Stellwagen von Cairon, Donders &
Snellen develop ideas
• 1862: Snellen publishes his chart design
Snellen’s originals
How to improve the original?
• 1886: Green
• Sans serif letters, geometric size
progression, equal target number
Recognition versus Resolution
• 1904: Landolt
• Landolt-Rings: advantages
Then 71 years of silence:
Two world wars
Optotypes
How to improve the original?
• 1952: Sloan
• Standardized Letters
• 1976: Bailey & Lovie
• Logarhythmic size progression, equal
target number, letter-by-letter scoring
(see 1886 Green)
How to improve the original?
• 1982: Ferris et al.
• Early Treatment for Diabetic Retinopathy
Study (ETDRS)
• OD OS OU
Improvements Continue
• Sheedy et al. 1984
– Optimal luminance parameters
– Retro-illumination (Lighthouse)
• Holladay, 1991
– Log MAR for statistics
ETDRS
• Gold Standard since 1993
• Mandatory for clinical randomized control
trials
• But: Who else is using it (or not) and
WHY?
Arguments
• Testing Distance measured for 6 meters
– But: ETDRS can be done at 4, 2 and 1 m
• Testing Time: 1 min for Snellen
– But: ETDRS can be done condensed
• Units
– log MAR expressed as Snellen fraction
Hussain et al., 2006
Arguments
• Consistency of measure
– Need to run in parallel for some time
• Cost: 1800.- CAN $
• Lesser measure comes at price of
accuracy and precision in evidence-based
medicine
Hussain et al., 2006
The “NEW” Discovery
• DeCarlo DK, et al., IOVS 2006; 47: ARVO E-abstract 3474; Relationship
Between ETDRS and Projected Snellen Visual Acuity in Patients With
ARMD
• Falkenstein I, et al., IOVS 2007; 48: ARVO E-abstract 2148; Comparison
of Visual Acuity Measurements in Macular Degeneration Patients
Tested With Snellen and LogMAR (ETDRS) Charts
• Kaiser PK, IOVS 2008; 49: ARVO E-abstract 2249; Comparison of
Snellen versus ETDRS Protocol Visual Acuities Performed by Certified
Vision Examiners
The “NEW” Discovery
• Hussain, B., Saleh, G. M., Sivaprasad, S., & Hammond, C. J. (2006).
Changing from Snellen to LogMAR: debate or delay? Clinical &
Experimental Ophthalmology, 34(1), 6-8.
• Beck et al. (2007). Visual acuity as an outcome measure in clinical trials of
retinal diseases. Ophthalmology, 114(10), 1804-1809.
• Falkenstein, et al. (2008). Comparison of visual acuity in macular
degeneration patients measured with Snellen and Early Treatment Diabetic
Retinopathy Study charts. Ophthalmology, 115(2), 319-323.
Subspecialty: Low Vision
• No news here:
– Augustus Colenbrander
– Measuring Vision and Vision Loss. In W.
Tasman & E. A. Jaeger (Eds.), Duane's
Ophthalmology (Vol. 5): Lippincott Williams
& Wilkins.
– Goal: Measure RESIDUAL Vision
– Must be done with attention to detail
Optometry & Acuity
• This is their domain!
• Bailey, I. L. (2006). Visual Acuity. In W.
Benjamin & I. M. Borish (Eds.), Borish's
Clinical Refraction (2nd ed.): ButterworthHeinemann
Meaningless Terms?
• counting fingers,
• hand-motion
• light perception
• Can be converted into useful scales
• Schulze-Bonsel, et al. (2006). Visual acuities
"hand motion" and "counting fingers" can be
quantified with the Freiburg visual acuity
test. Investigative Ophthalmology & Visual
Science, 47(3), 1236-1240.
Specialized Acuity Charts
• Macular Degeneration
• Macular Hole
Specialized Letters
• Rounded letters (S, O, C and D) are
generally more difficult to distinguish
– Can be language-dependent
• Plainis, S., Tzatzala, P., Orphanos, Y., &
Tsilimbaris, M. K. (2007). A modified
ETDRS visual acuity chart for Europeanwide use. Optom Vis Sci, 84(7), 647-653.
Optotypes
Special Populations
But: What does VA actually tell us?
• Not much!
• Resolution power at highest contrast at
highest spatial frequency
Spatial Contrast Sensitivity
VA Limitations
• Low/Middle Frequency loss is undetected
– i.e. MS
• VA rarely correlated with perceived visual
function (patient perspective) when
disease is involved
• Contrast loss can be with/without VA loss
VA and Face perception
• Common complaint in patients
• Face recognition involves multiple spatial
frequencies
• VA is only related when VA is “good”
Face Acuity
MARS Contrast Chart
Colenbrander
Low Contrast Chart
ETDRS Chart
How to test SCS clinically?
But: Not presently commercially available anymore!
Take-Home Message
Evidence-based best practice
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Apply the best tools
Thank you
Questions?
walter.wittich@mail.mcgill.ca