Evaluation of Ocular Disability

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Evaluation of Ocular Disability
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Optometrists are often called upon to determine
if patients have visual impairment or are legally
blind. Reasons for which such an assessment may
be necessary include Social Security benefits,
veteran's benefits, worker's compensation
benefits, eligibility for vocational
rehabilitation, deductions under the Internal
Revenue Code, or other insurance benefits.
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Under the Social Security lawthe most common law
citedlegal blindness exists if there is

• central visual acuity of 20/200 or less in the
  better eye with the use of a correcting lens
• an eye which has a limitation in the field of
  vision so that the widest diameter of the visual
  field subtends an angle no greater than 20
  degrees.

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Example If a 20/20 acuity monocular patient has
this 30-2 visual field, is the patient legally
blind?
Remember the widest diameter of the field must
be no more than 20 degrees.
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The legally blind population includes not only
persons who are totally blind but also those who
have a severe vision loss in both eyes. Estimates
are that more than 1.3 million persons in the
United States are legally blind (about 0.5 of
the population) and that approximately 8.7
million Americans (about 3.1 of the
population) have a visual impairment.
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For medicolegal purposes, a method of evaluating
permanent impairment has been developed by the
American Medical Association and has been
published in Guides to the Evaluation of
Permanent Impairment, 5th edition, which includes
a chapter on vision. The process requires
eight steps.
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The first two steps involve assessment of visual
acuity

• measurement of visual acuity and use of a table
  to convert results to a visual acuity score for
  each affected eye and for both eyes
  (binocularly)
• use of a formula to combine the acuity score from
  each eye and binocularly into a single functional
  acuity score, which represents loss of acuity

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The next two steps involve assessment of the
visual field

• measurement of visual fields for each affected
  eye and binocularly and conversion of results to
  a visual field score
• use of a formula to combine the field scores from
  each eye and binocularly into a single functional
  field score

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The next two steps involve combining the acuity
and field results

• combining the functional acuity score and the
  functional field score to obtain the functional
  vision score
• subtracting the functional vision score from 100
  to obtain a visual impairment rating

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The last two steps involve the determination of
total visual impairment and impairment of the
whole person

• adding any additional visual impairments (other
  than acuity or field) to obtain the total
  impairment of the visual system
• translating the impairment of the visual system
  into an estimated impairment of the whole person.

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Measured Impairment Estimated Functions
Global Impairmentof Each Eye of the Person
of the Visual SystemVisual acuity
score OD Visual acuity score OS ? Functional
acuity score (FAS) Visual acuity score
OU ? Functional vision score
(FVS) additional
impairments 100 FVS
impairment ratingVisual field score
ODVisual field score OS ? Functional field
score (FFS)Visual field score OU
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The equipment needed for evaluation of visual
impairment includes

• standardized letter chart for testing at 20 ft
  (ETDRS-type charts are recommended)
• standardized near reading tests
• (optional) visual field equipment
• (Goldmann kinetic perimetry is preferred).
• other functional tests, such as a
• contrast sensitivity test or a glare
• test, may also be used.

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The test distance for persons with 20/60 or
better acuity should be 20 feet (6 meters). The
patient's visual acuity should be recorded
monocularly with best correction, and then
binocularly. Refraction may be necessary to
ensure that the patients best correction is
being used.For persons with acuity worse than
20/60 a 1 meter test distance is recommended.
When recording results, the Snellen fraction
numerator is 1 and the denominator is the
smallest letter the person can read, noted in M
units. Thus a persons ability to read 8 M
letters would be recorded as 1/8 M (20/160).
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For counting fingers acuity, test size is deemed
to be 200, so that if fingers could be counted at
3 feet, the acuity would be noted as 3/200. For
hand motion acuity, the test size is assumed to
be 1000, so hand motion seen at 5 feet would be
written as 5/1000.Once a visual acuity
measurement is obtained, a table may be used to
replace acuity results with a visual acuity score
(VAS) for OD, OS, and OU.
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Example Best corrected acuities are OD 20/40, OS
20/25, and OU 20/30. The table shows that the
VAS is 85 OD, 95 OS, and 90 OU. Visual
Acuity Visual Acuity US notation M
notation Score (VAS) Impairment
Rating 20/20 1/1 100 020/25 1/1.25 95
520/30 1/1.6 90 1020/40 1/
2 85 15 20/50 1/ 2.5 80 2020/60 1/
3.2 75 25
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These three scores must next be converted into a
single functional acuity score (FAS). A
calculation is performed, in which the VAS for OD
and OS are multiplied by 1, the VAS for OU is
multiplied by 3, and the total is divided by 5.
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Measured Snellen Values Calculated Visual Acuity
ScoresOU 20/___ VAS VAS x 3
____ OD 20/___ VAS VAS x 1 ____OS 20/___
VAS VAS x 1 ____Add OU, OD, OS
____Divide by 5 ____ functional acuity
score (FAS)
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Example A patient has best corrected acuities of
OD 20/60, OS 20/800, and OU 20/60. The VAS for
these acuities is OD 75, OS 20, and OU 75. The
functional acuity score is calculated by
multiplying OD VAS x 1, OS VAS x 1, and OU VAS x
3, adding them together, and dividing by 5 75 x
1, 20 x 1, and 75 x 3 equals 320, and 320 divided
by 5 is 64, which is the FAS.
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The last step is to convert the FAS to the
impairment rating. This is done by subtracting
the FAS from 100. Example A patient has best
corrected acuities of OD 20/20, OS NLP, and OU
20/20. The VAS for these acuities is OD 100, OS
0, and OU 100. The FAS is OD 100 x 1, OS 0 x 1,
and OU 100 x 3, which when added equals 400, and
when divided by 5 is 80. The impairment rating is
100 80, or 20 percent visual impairment (even
though monocular!)
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Measurement of reading acuity is optional and is
undertaken only when near acuity is significantly
different from distance acuity. Continuous text
reading material is preferred (rather than
letters), and M-units should be used for letter
size. Testing is performed binocularly unless one
eye is preferred over the other. Once an acuity
measurement is obtained, it can be converted to a
near acuity functional acuity score. The distance
acuity FAS and near acuity FAS can then be
averaged and an impairment rating obtained.
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The testing of visual field should be performed
with a Goldmann perimeter using a kinetic
technique in which the examiner moves the target
from the periphery into the central field.
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In the Goldmann bowl all testing distances are
equal, while the background and stimulus
luminances are varied. With this technique the
results are plotted as isopters, contour lines
that represent the areas where stimuli of
different intensity can be seen. The Goldmann
III4e isopter should be plotted.
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An automated perimeter can only be substituted if
full field plots (Humphreys 60-2 field) are
available, or if the radius of the field of
vision is less than 20 degrees from fixation.
Plots of the central 30 degrees with a 10-dB
stimulus can then be employed (i.e., a Humphreys
30-2 field).
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The visual field is composed of 10 meridians, 2
in each of the upper quadrants and 3 in each of
the lower quadrants. Along each of these
meridians 5 points (spaced 2 degrees apart) are
assigned to the central 10 degrees, while 5
points (spaced 10 degrees apart) are assigned
from the central 10 degrees to the periphery.
When this is done, the field size will, on
average,consist of 100 points.
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Measurement of field loss begins with visual
field plots for each eye. Based on these results,
the number of points within the field are counted
for each of the 10 meridians and added together.
If scotomas are present, a test grid overlay
may be constructed to ensure accuracy. (There are
special rules that apply to the calculation of
field loss from scotomata.)
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The total for each eye comprises that eyes
visual field score (VFS). The binocular field
is determined by superimposing the two monocular
fields points seen binocularly and monocularly
are included, while points not seen by either eye
are not. The resulting count is the binocular
visual field score.
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The next step is to convert the three visual
field scores into one functional field score
(FFS). In this calculation, the visual field
score for each eye is multiplied by 1, the
binocular field score is multiplied by 3, and the
sum is divided by 5.To determine the field
impairment rating, the FFS is subtracted from 100.
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Measured Field Plots Calculated Field
ScoresBinocular field plot (OU) VFS ____ x 3
____Right eye field plot (OD) VFS ____ x 1
____Left eye field plot (OS) VFS ____ x 1
____Add OU, OD, and OS
____Divide by 5 ____ functional
field score (FFS)Field Impairment Rating
(100-FFS) ____
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Example visual field plots for a patient with
temporal field loss are illustrated top is OS,
which has the temporal loss bottom is OD, which
is full middle is OU, which is created by
superimposing the two fields on one
another. Where the field of the OD overlaps the
missing field of the OS, points are counted, but
where missing points OS are not covered by the
OD, field points are lost.
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Example a patient with a nasal defect in the
left eye and a normal right eye istested and has
the following VFS OU 105OD 100OS
79The FFS is 105 x 3, 100 x 1, 79 x 1, or494,
and when divided by 5 yields an FFS of 99. The
impairment rating is 100 - 99, or 1 percent.
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Example a monocular patient has a scotoma in
his good eye and no lightperception in the other
eye. Testingyields the following VFSOU
75OD 0OS 75The FFS is 75 x 3, 0 x 1, 75
x 1, or 300, and when divided by 5 results inan
FFS of 60. The impairment rating is 100 - 60, or
40 percent.
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The impairment of the visual system is based upon
both visual acuity loss and visual field loss. Of
course, in many instances a patient may suffer
only visual acuity loss or only visual field
loss, but loss of both acuity and field may also
occur. In either event, the functional acuity
score and the functional field score must be
combined to obtain the impairment rating for the
visual system.
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To calculate the functional vision score (FVS)
the FAS and FFS are multiplied together and the
result is divided by 100. Then the FVS is
subtracted from 100 to obtain the visual
impairment rating.Example The FAS is found to
be 80 and the FFS is found to be 90. The
functional vision score is 80 x 90 720, divided
by 100, or 72. The visual impairment rating is
100 72 or 28 percent.
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If there is visual acuity loss but no visual
field loss (FFS 100), then the functional
vision score is the same as the acuity score (FVS
FAS). Example The FAS is 60 and there is no
visual field loss. The FVS is 60 x 100 600
divided by 100, which equals 60. This results in
an impairment rating for the visual system of 40
percent (100 60).
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If there is visual field loss but no visual
acuity loss, then the functional vision score is
the same as the visual field score (FVS
FFS).Example The FAS is 100 and the FFS is 82.
The FVS is 100 x 82 820, divided by 100, or 82.
The impairment rating for the visual system is 18
percent (100 82).
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Individual adjustments can be made, based upon
the determination that for the individual
involved there are significant factors other than
acuity and field loss that affect vision
function. These aspects of vision include
contrast sensitivity, glare sensitivity (veiling
glare), delayed glare recovery, photophobia,
reduced or delayed light and dark adaptation,
color vision defects, binocularity, stereopsis,
suppression, and diplopia. Adjustments due to
these conditions should be determined and
combined with functional loss (but they can
constitute no more than a 15 percent impairment
of the visual system).
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Based upon the functional vision score, and
visual system impairment rating, impairment of
the whole person may be estimated, using a table.
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Example A 65-year-old man with advanced glaucoma
has peripheral vision to 32 degrees OD, 28
degrees OS, and 36 degrees OU his acuity is
reduced to OD 20/40, OS 20/50, OU 20/40.The VAS
is OD 85, OS 80, OU 85. His FAS is 85 x 1, 80 x
1, 85 x 3 420, divided by 5 84.His
peripheral field loss results in a VFS of OD 32,
OS 28, OU 36. His FFS is 32 x 1, 28 x 1, and 36 x
3 168, divided by 5 34.
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His functional vision score (FVS) is FAS x FFS,
divided by 100, or 84 x 34/100 29.His
impairment rating for the visual system is 100 -
29 or 71 percent.Impairment of the whole person
(FVS 29, impairment rating 71) is Class 5.
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Example A 75-year-old woman with age-related
macular degeneration has reduced acuity at
distance and near of OD 20/30, OS 20/25, and OU
20/25. Visual field testing shows that all field
points at 3 and 5 degrees are missed in both eyes
due to paracentral scotomas (20 points lost),
resulting in a visual field score of 80 for each
eye and 85 binocularly.The VAS is OD 90, OS 95,
and OU 95. Her FAS is 90 x 1, 95 x 1, 95 x 3
470, divided by 5 equals 94.
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Her central field loss results in a VFS of OD 80,
OS 80, OU 85. Her FFS is 80 x 1, 80 x 1, 85 x 3
415, divided by 5 equals 83.Her functional
vision score (FVS) is FAS x FFS/100, or 94 x
83/100 78.Her impairment rating for the
visual system is 100 78 or 22
percent.Impairment of the whole person (FVS
78, impairment rating 22) is Class 2.
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