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TESTS OF VISUAL FUNCTIONS

Tests of visual functions are at the core of current disability determination practices for visually impaired claimants at the Social Security Administration (SSA), and the committee’s task required us to carefully review and evaluate these tests. This chapter presents the results of that review. Each fundamental function is discussed, beginning with acuity and visual fields, the functions currently tested by SSA for disability determination. Next we present the evidence on the testing of contrast sensitivity, followed by other visual functions that the committee judged worthy of consideration as candidates for testing by SSA—most of which were mentioned in the earlier NRC report (National Research Council, 1994). Some closely related functions are grouped in a single section.

For each function, we considered evidence on why the function is important in the evaluation of visual disability and reviewed and evaluated evidence of the relationships between that function and performance in the four daily living and work task domains selected (see Chapter 1). The committee established criteria for acceptable tests and then reviewed and evaluated currently available and emerging new tests of each function against these criteria. Each section describes the strengths and weaknesses of available and emerging tests



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Visual Impairments: Determining Eligibility for Social Security Benefits 2 TESTS OF VISUAL FUNCTIONS Tests of visual functions are at the core of current disability determination practices for visually impaired claimants at the Social Security Administration (SSA), and the committee’s task required us to carefully review and evaluate these tests. This chapter presents the results of that review. Each fundamental function is discussed, beginning with acuity and visual fields, the functions currently tested by SSA for disability determination. Next we present the evidence on the testing of contrast sensitivity, followed by other visual functions that the committee judged worthy of consideration as candidates for testing by SSA—most of which were mentioned in the earlier NRC report (National Research Council, 1994). Some closely related functions are grouped in a single section. For each function, we considered evidence on why the function is important in the evaluation of visual disability and reviewed and evaluated evidence of the relationships between that function and performance in the four daily living and work task domains selected (see Chapter 1). The committee established criteria for acceptable tests and then reviewed and evaluated currently available and emerging new tests of each function against these criteria. Each section describes the strengths and weaknesses of available and emerging tests

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Visual Impairments: Determining Eligibility for Social Security Benefits for a particular function. Finally, each section presents, with rationale, our recommendations for or against SSA’s testing that function for disability determination and describes further research required to improve testing or otherwise support better disability determination practices. A separate section discusses ways in which scores on tests of visual function could be mathematically combined to yield a single index of visual impairment for a claimant. The committee’s recommendations for the testing of visual functions are summarized in the final section. VISUAL ACUITY Description Visual acuity is a measure of the spatial resolving power of the visual system; it indicates the angular size of the smallest detail that can be resolved. Clinical tests of visual acuity determine a size threshold for a recognition task. The targets to be recognized are called “optotypes,” and typically they are letters, Landolt rings, or “tumbling E’s” designed so the width of the strokes and the gaps are one fifth of the height of the optotype character. An individual’s visual acuity is determined by measuring the angular size of the smallest optotypes whose identity (letters) or orientation (Landolt rings and tumbling E’s) can be recognized. Visual acuity is typically measured under conditions of high contrast, using printed or projected charts with optotypes like those described above. The results of visual acuity testing are usually expressed in Snellen notation, which is the ratio of the test distance to the distance at which the critical detail of the smallest optotype resolved would subtend 1 minute of visual angle. Thus, a minimum angle of resolution (MAR) of 1 minute of visual angle (or arc, sometimes abbreviated as “min arc”) when tested at 20 feet (6 meters) is expressed as 20/20 (6/6), whereas an MAR of 10 minutes of arc if tested at 20 feet is expressed as 20/200 (6/60). Alternative means of expressing visual acuity are the decimal notation (the reciprocal of the MAR or the

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Visual Impairments: Determining Eligibility for Social Security Benefits Snellen fraction), logMAR notation (the common logarithm of the MAR), the visual acuity rating, VAR, where VAR = 100 − 50 (logMAR), and the Snell-Sterling visual efficiency (VE = 0.2(MAR-1)/9). Table 2-1 presents these alternative forms of measurement as a conversion table. The standard for normal acuity has traditionally been considered to be 20/20. However, individuals with normal, disease-free eyes often have acuity better than 20/20, provided that refractive error has been corrected (Elliott et al., 1995). Evaluation Why the Measurement Is Useful Ophthalmologists and optometrists routinely measure visual acuity for various purposes. In the measurement of refractive error, the lens power that permits the best visual acuity is often an important criterion. In the diagnosis and monitoring of eye diseases that may affect vision, changes of visual acuity are often taken to indicate the presence and magnitude of change in the medical condition. Ocular diseases and disorders that affect the transparency and optical regularity of the cornea, lens, or vitreous will degrade the optical image, with adverse effects on visual acuity. Diseases affecting the central region of the retina or the associated optic nerve pathways are likely to cause reductions in visual acuity. Visual acuity measurements are also used by some licensing authorities and employers as eligibility criteria for some occupations (e.g., airline pilot, police officer) and activities (e.g., driving). Visual acuity has traditionally been used as the primary indicator of the magnitude of functional impairment due to vision loss. Good spatial resolution is important for a variety of everyday tasks in the workplace, but probably most critically for reading text and interpreting symbols, key components of many jobs. Visual acuity also plays a central role in discriminating and recognizing small objects or the detailed features of objects. The visual acuity demand for a given task depends on the size of the critical detail in the task and the observation distance. For example, a person with good visual acuity might be expected to recognize faces at about 20 meters. To

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Visual Impairments: Determining Eligibility for Social Security Benefits TABLE 2-1 Conversion Table for Visual Acuity Notations Distance Vision   LogMAR notation MAR exact MAR notation* Decimal notation* VE% notation VAR notation −0.30 0.501 0.50 2.00 109.4% 115 −0.20 0.631 0.63 1.60 106.8% 110 −0.10 0.794 0.80 1.25 103.6% 105 0.00 1.000 1.00 1.00 100.0% 100 0.10 1.259 1.25 0.80 95.6% 95 0.20 1.585 1.60 0.63 89.8% 90 0.30 1.995 2.0 0.50 83.6% 85 0.40 2.512 2.5 0.40 76.5% 80 0.50 3.162 3.2 0.32 67.5% 75 0.60 3.981 4.0 0.25 58.5% 70 0.70 5.012 5.0 0.20 48.9% 65 0.80 6.310 6.3 0.160 38.8% 60 0.90 7.943 8.0 0.125 28.6% 55 1.00 10.00 10.0 0.100 20.0% 50 1.10 12.59 12.5 0.080 12.8% 45 1.20 15.85 16 0.063 6.8% 40 1.30 19.95 20 0.050 3.3% 35 1.40 25.12 25 0.040 1.4% 30 1.50 31.62 32 0.032 0.4% 25 1.60 39.81 40 0.025 20 1.70 50.12 50 0.020 15 1.80 63.10 63 0.016 10 1.90 79.43 80 0.013 5 2.00 100.0 100 0.010 0 *Note: Numbers rounded to simplify sequences. Rounding errors do not exceed 1.2 percent. Source: Ian Bailey, personal communication.

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Visual Impairments: Determining Eligibility for Social Security Benefits     Near Vision Snellen Fractions At 40 centimeters Based on 20 ft.* Based on 6 m.* Based on 4 m.* Snellen notation 0.40 meters* M Units * Points * x-height (mm) 20/10 6/3 4/2 0.40/0.20 0.20 1.6 0.29 20/12.5 6/3.8 4/2.5 0.40/0.25 0.25 2.0 0.36 20/16 6/4.8 4/3.2 0.40/0.32 0.32 2.5 0.47 20/20 6/6 4/4 0.40/0.40 0.40 3.2 0.58 20/25 6/7.5 4/5 0.40/0.50 0.50 4.0 0.73 20/32 6/9.5 4/6.3 0.40/0.63 0.63 5.0 0.92 20/40 6/12 4/8 0.40/0.80 0.80 6.3 1.16 20/50 6/15 4/10 0.40/1.00 1.00 8.0 1.45 20/63 6/19 4/12.5 0.40/1.25 1.25 10.0 1.82 20/80 6/24 4/16 0.40/1.60 1.60 12.5 2.33 20/100 6/30 4/20 0.40/2.0 2.0 16 2.91 20/125 6/38 4/25 0.40/2.5 2.5 20 3.64 20/160 6/48 4/32 0.40/3.2 3.2 25 4.65 20/200 6/60 4/40 0.40/4.0 4.0 32 5.82 20/250 6/75 4/50 0.40/5.0 5.0 40 7.27 20/320 6/95 4/63 0.40/6.3 6.3 50 9.16 20/400 6/120 4/80 0.40/8.0 8.0 63 11.6 20/500 6/150 4/100 0.40/10.0 10.0 80 14.5 20/630 6/190 4/125 0.40/12.5 12.5 100 18.2 20/800 6/240 4/160 0.40/16 16 125 23.3 20/1000 6/300 4/200 0.40/20 20 160 29.1 20/1250 6/380 4/250 0.40/25 25 200 36.4 20/1600 6/480 4/320 0.40/32 32 250 46.5 20/2000 6/600 4/400 0.40/40 40 320 58.2

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Visual Impairments: Determining Eligibility for Social Security Benefits recognize the same faces, a person with poor visual acuity would have to get significantly closer. In the workplace, there is a multitude of tasks in which it is important to see fine details. Some examples are reading labels, gauges, and dials; inspecting products for cracks, scratches, and foreign material; and visually guided manipulation, as in needle-threading, surgery, and fine assembly tasks. In mobility, acuity is important for recognizing environmental landmarks, avoiding small obstacles, and reading highway signs during driving (Hofstetter, 1976). Acuity is also a strong predictor of self-reported vision-related quality of life. Value as a Practical Measure In 1865, Hermann Snellen designed the first letter chart for the clinical measurement of visual acuity. It had a large letter at the top, and below it there were 6 rows of letters and numbers in progressively smaller sizes. The chart was viewed from a standard distance, and the size of the smallest letters that could be read provided the measure of visual acuity. Since then, numerous modifications have been made to Snellen’s original chart design, with changes being made to the selection and design of the letters or symbols, the range of sizes, the progression of sizes, the number of letters in the rows, and the spacing between letters and between rows (see Figure 2-1 for a sample chart). While the design has evolved to improve the validity and reliability of visual acuity measurement, Snellen’s letter chart approach has prevailed for more than a century. Letter charts are used almost universally for visual acuity testing of literate adults and school-age children in clinical and research settings. Alternative charts and other test procedures are sometimes necessary for testing infants and preschool children and other individuals who are unable to identify or respond appropriately to the letters or symbols on the chart. While there may be some further modifications to chart design or test procedures, it can be expected that letter chart testing will remain the standard means of measuring visual acuity.

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Visual Impairments: Determining Eligibility for Social Security Benefits FIGURE 2-1. Snellen-type acuity chart. Source: National Eye Institute, National Institutes of Health.

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Visual Impairments: Determining Eligibility for Social Security Benefits Quantifying Performance The current SSA standard defines Impairment of Central Visual Acuity as best-corrected Snellen acuity of 20/200 or worse in the better eye, measured with a distance visual acuity chart (Social Security Administration, 1999). Specific recommendations for visual acuity chart design and testing conditions have been made by several bodies (American National Standards Institute International Standards, 1986a, 1986b; Consilium Ophthalmologicum Universale Visual Functions Committee, 1988; National Research Council, 1980, 1994). On the basis of these recommendations, we identify four weaknesses in the current SSA standard: In the SSA standard, the type of chart(s) to be used for testing visual acuity is specified only as “Snellen.” There is no standardized Snellen chart. The most commonly used projector charts and panel charts differ significantly from Snellen’s original chart design, but they are still referred to as Snellen charts. Commonly these charts have few letters at the larger sizes, the size progression varies from one chart to the next, and there is a pattern of having more letters and relatively closer spacings at the smaller sizes. As emphasized by the 1980 report of the Committee on Vision (National Research Council, 1980), the design of the chart used (including optotype, the number and spacing of optotypes on a line, the range and progression of optotype sizes, the chart luminance, and the nominal contrast between the optotypes and their background) has an important influence on the results of visual acuity measurement. The present standard does not specify chart design requirements and permits the use of charts that may produce very different visual acuity scores. For example, the standard is met when someone fails to read any optotypes that are smaller than the 20/200 optotype. On the most commonly used Snellen charts, the next smallest size optotype is in a 20/100 row, but on others it may be 20/160 or even 20/180. Applying the SSA criterion of “20/200 or worse distance acuity” to such different charts has the functional effect of making the cutoff less than 20/100, less than 20/160, or less than 20/180, according to the chart being used.

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Visual Impairments: Determining Eligibility for Social Security Benefits The current SSA criterion cannot be applied consistently unless there are specific constraints on the design of the test charts. In particular, the optotype size that is next smallest to 20/200 should be specified. The recommended visual acuity chart design has two steps of size 20/125 and 20/160 between the 20/100 and 20/200 levels, and it is also recommend that credit be given for partial success in reading the sample of letters at each size. The SSA standard for Impairment of Central Visual Acuity is that the visual acuity should be 20/200 or worse. With charts of the recommended and more modern design, the literal application of the SSA criterion is that the standard is met when no letters at all can be read at the 20/160 level or smaller. However, the common practice has been and remains testing acuity with charts that have no intermediate sizes between 20/100 and 20/200. As it has been most commonly applied, this means that the SSA standard is met when no letters at all can be read at the 20/100 size or smaller. We are not recommending a change from the criterion for Impairment of Central Visual Acuity. We do, however, recommend standardization of chart design, which would raise policy issues for SSA. The literal application of the 20/200 or worse criterion with a recommended chart would mean that a sizable group of people who currently qualify would be no longer classified as having Impairment of Central Visual Acuity. These are the people who would be able to read all or some of the letters at the 20/160 or 20/125 sizes while being unable to read any at the 20/100 level. Alternatively, SSA could choose to continue allowing the most commonly applied criterion: no letters can be read at the 20/100 size. This would lead to a sizable group of people’s meeting the criterion, even though their visual acuities could be anywhere in the range from 20/125 to one letter better than 20/200. The standard does not specify the conditions under which visual acuity should be tested. The level of illumination and the testing environment are important factors affecting performance. Inadequate illumination leads to poor performance, as does glare from extraneous light sources. The standard does specify that visual acuity should be tested with best correction; thus, care should be taken to ensure that refractive error is properly corrected prior to visual acuity testing.

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Visual Impairments: Determining Eligibility for Social Security Benefits The standard does not specify the use of standardized testing procedures. There is no universal standard procedure for measuring performance on different lines of a chart, and there are no standard procedures for scoring performance; for example, what should be done when a subject is correct for some elements on each of two adjacent lines? It is common clinical practice to assign a score that indicates the smallest size at which a certain proportion of the optotypes can be read (often the required proportion is “greater than 50 percent”). Scores can depend on whether guessing is encouraged or is obligatory when letters are difficult to read. The absence of standard testing and scoring methods reduces the reliability of measurements. The standard deals only with the performance of the better eye. Everyday vision, however, is based on simultaneous viewing of the world with both eyes. Monocular acuity of the better eye may sometimes underestimate binocular acuity, for example, under conditions in which binocular summation occurs (Cagenello et al., 1993; Home, 1978; Pardhan, 1993) or in subjects with latent nystagmus, a condition in which rhythmic eye movements occur in the unoccluded eye when the other eye is occluded (Helveston & Ellis, 1984). Alternatively, monocular acuity of the better eye may sometimes overestimate binocular acuity, for example, under conditions in which inhibition is produced by the worse eye (Pardhan, 1993; Taylor et al., 1991). Thus, monocular acuity of the better eye is not always an adequate predictor of binocular acuity and therefore of visual resolution in everyday life. Standardizing Visual Acuity Measurement Chart Design There is general agreement that the design of a visual acuity chart should be such that the visual task is the same at each size level, so that size remains the only significant variable from one size level to the next (Bailey & Lovie, 1976). For this principle to be satisfied, the size progression should be logarithmic, there should be the same

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Visual Impairments: Determining Eligibility for Social Security Benefits number of optotypes at each size level, the spacings between optotypes within a row and between rows should be proportional to the size of the optotype, and the average recognition difficulty should be approximately the same for each row of optotypes. The Committee on Vision (National Research Council, 1980) recommended the Landolt ring as the reference standard for optotypes, and it considered the Sloan family of 10 nonserif letters (CDHKNORSVZ) designed on a 5 × 5 grid (Sloan, 1959) to be acceptable. Another widely accepted family of optotypes is the British Standards family of 10 nonserif letters (DEFHNPRUVZ), which are designed on a 5 × 4 grid (British Standards Institution, 1968). Both of these charts use letters with a stroke width (critical detail) equal to 1/5 of the letter height. In clinical research today, there is almost universal use of the Early Treatment for Diabetic Retinopathy Study (ETDRS) chart (Figure 2-2) (Ferris et al., 1982), which uses Sloan letters, and the Bailey-Lovie (1976) chart, which uses the British family of letters. These two charts were found acceptable in the 1994 Committee on Vision report (National Research Council, 1994). Both charts have five letters per row, one letter width separating adjacent letters, with the spacing between adjacent rows equal to the height of the letters in the smaller row. Both charts have 14 rows covering a 20-fold range of letter sizes, and both follow a logarithmic (geometric) size progression with a ratio of 0.1 log unit (1.2589×) between each row and the next. Observation Conditions For assessment of distance visual acuity, test distance should be 3 meters (10 feet) or more, to minimize the need for the use of accommodation to bring the optotypes into focus. The traditional test distance is 6 meters (20 feet); however, the Committee on Vision (National Research Council, 1980) recommended that the standard test distance be changed to 4 meters because this distance presents an accommodation demand of exactly 0.25 D; it is also conveniently 10 times longer than 40 cm, which is a commonly used distance for testing near vision (Hofstetter, 1973). The ETDRS clinical research protocols use a 4-meter standard test distance, with a recommendation

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Visual Impairments: Determining Eligibility for Social Security Benefits Combining Measures SSA regulations prescribe a method for computing the overall loss of visual function that might result from impairments of both acuity and visual fields. Overall efficiency (visual efficiency) is calculated as Visual efficiency = central visual efficiency × visual field efficiency. This measure gives equal weight to the component scores. The standard for severe impairment is an overall efficiency in the better eye of 20 percent or less. Two important issues arise: Is the equal weighting of component scores reasonable, and do the impairments act multiplicatively? Because impairments of acuity and fields limit performance in quite different ways and generally in different domains of activity, their relative importance is likely to be task-dependent. One approach to understanding their relative importance would be to establish what levels of impairment on each have equivalent effects on higher-level performance indicators, such as quality of life measures. At present there is insufficient evidence on what might be the appropriate weightings of acuity and fields in any composite measure to be used for disability determination. We recommend that research be done on this question. In the meantime, we recommend that SSA continue its current practice of giving equal weight to the measures of visual acuity and visual fields. Recommendations Beyond establishing commensurate scales for characterizing acuity, visual fields, and contrast sensitivity, we need to be able to compute an aggregate indicator of impairment. We know too little about the interactions between multiple visual impairments to recommend a change in SSA’s current practice of deriving an overall measure by multiplicative combination of component measures. Some recent evidence (Rubin et al., 2001) suggests that impairments act independently in affecting overall visual performance. We

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Visual Impairments: Determining Eligibility for Social Security Benefits recommend that further research be undertaken to examine directly how different kinds of impairments interact in determining overall visual performance. In the meantime, the committee recommends that, with the modification noted below, SSA continue its current practice of computing an overall measure of performance as the product of the component measures. This principle implies that different tests measure independent aspects of visual function, which cannot be completely true. For example, a central scotoma will be reflected in both an acuity score and a visual field score, but its weight in the visual field score will be slight. The problem is consequential for acuity scores and contrast sensitivity scores, which are generally well correlated. For this reason, we recommend that when contrast sensitivity has been measured (which would be done only when a loss is suspected beyond that captured by an acuity score), the contrast sensitivity score should supplant the acuity score in the calculation. Because our recommended measures of acuity, visual fields, and contrast sensitivity are already logarithmic measures of impairment, we need only add the scores to compute an overall measure in order to achieve a combined multiplicative score. Recognizing that we must give equal weight to visual fields and to visual acuity (or contrast sensitivity), but that we use either a measure of acuity or a measure of contrast sensitivity, we recommend that the overall measure of impairment should be computed as: aggregate impairment = logMAR + |MD|/22 when the scores to be combined are visual acuity and visual field and aggregate impairment = (2 − CS) + |MD|/22 when the scores to be combined are contrast sensitivity and visual field. The current standard for disability would be met when the aggregate impairment equals or exceeds 1.0.

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Visual Impairments: Determining Eligibility for Social Security Benefits Disability Criteria The acuity standard for statutory blindness (central acuity of 20/200 or worse) appears to have resulted from an examination (Snell, 1925) of the fitness for work of individuals with varying degrees of visual impairment. Snell found that the “threshold for incapacity” lay between 20/200 and 20/400. There is little evidence that this criterion reflects current employment rates of people with visual impairments or that it provides a reliable characterization of their visual capabilities in the workplace. The studies we have reviewed and also clinical consensus (American Medical Association, 1993, 2001) suggest that the current acuity standard and the current visual field standard represent severely impaired vision. Nevertheless, the evidence also shows that overall visual performance varies continuously, and roughly linearly, with the measures of visual acuity, visual fields, and contrast sensitivity that we have recommended. Evidence about visual function therefore provides no guidance on where it might be appropriate to place a criterion for eligibility for disability benefits. While recognizing that a criterion is required, we make no recommendation about where it should be placed. A decision about where to place the criterion involves many policy factors, including considerations of overall cost. At present, it is not possible to estimate the cost increases or savings that would result from changing the disability criteria now in place, for we do not know how the measures of acuity, fields, and contrast sensitivity are distributed among the population of working age. We therefore recommend that research be undertaken to establish the distributions of our recommended measures of acuity, fields, and contrast sensitivity in the working-age population. Should SSA implement our recommendations for testing and scoring acuity, fields, and contrast sensitivity and at the same time retain a disability standard equivalent to the current one, we make these observations:

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Visual Impairments: Determining Eligibility for Social Security Benefits From measurement of visual acuity, a logMAR score of 1.0 or greater meets the current standard. From measurement of visual fields, an MD score of −22db or worse meets the current standard. It is not necessary to recommend a criterion for contrast sensitivity alone, because such a score will be used only in combination with a visual field score when visual acuity is 20/50 or worse. An aggregate impairment score of 1.0 or more corresponds to the current standard of 20 percent visual efficiency. Children An aggregate impairment score may be calculated for children who are old enough to be tested using the instruments designed for adults. RECOMMENDATIONS FOR TESTS OF VISUAL FUNCTIONS The committee’s recommendations for the testing of visual functions for determination of disability focus on strengthening the testing of visual acuity and visual fields and adding one additional test, of contrast sensitivity, under certain circumstances. Another important recommendation is to consider establishing formal methods to ensure the quality of test administration and to evaluate new tests as they are proposed for use. In addition, the committee recommends that SSA support specific research efforts that will provide a firm scientific basis for future decisions about disability determination for people with visual impairments.

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Visual Impairments: Determining Eligibility for Social Security Benefits Acuity Testing Our recommendations concerning assessment of visual acuity are similar to those of the Committee on Vision in its 1980 and 1994 reports (National Research Council, 1980, 1994). We therefore recommend that visual acuity charts should contain the same number of optotypes in each row; the space between optotypes in a row should be at least as wide as the optotypes in that row; and the size of the optotypes should decrease in 0.1 log unit steps from row to row. Chart luminance should be at least 80 cd/m2, with 160 cd/m2 optimal, free from glare, with a level of contrast between optotypes and background that is above 80 percent. The person being tested should be encouraged to read as many optotypes on the chart as possible and to guess at an optotype if he or she is unsure. Acuity results should be scored on an optotype-by-optotype basis, since this scoring procedure produces lower test-retest variability than does row-by-row scoring. For disability determination, visual acuity should be tested under binocular conditions, since this provides the most representative measure of an individual’s everyday vision. However, if monocular acuity is tested rather than binocular acuity, the acuity of the better eye should be used for disability determination. Testing should be performed with the subject wearing the best tolerable refractive correction. Given the history and legislation behind the current SSA standard of “20/200 or worse distance acuity” as the principal criterion for visual disability, the committee recommends continuation of the 20/200 cutoff criterion. Since we recommend a visual acuity chart design that would include optotypes at the 20/160 level, applying the “20/200 or worse” criterion literally to scores obtained with such a chart would set the effective criterion to “worse than 20/160 distance acuity.” The scoring of the charts currently used in disability determination sets the effective criterion at “worse than 20/100.” The recommended charts have a 20/100 line that would allow SSA to maintain the criterion at the current effective acuity level, but SSA must make the decision on whether this should be done.

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Visual Impairments: Determining Eligibility for Social Security Benefits Visual Field Testing The committee recommends that the current SSA standard should be revised so that disability determinations are based on the results of automated static projection perimetry rather than Goldmann (kinetic, nonautomated) visual fields. We propose the following criteria for any perimeter to be used by SSA for disability determination: The automated static perimeter should be capable of performing threshold testing using a white size III Goldmann target and a 31.5 apostilb (10 cd/m2) white background. The perimeter should be capable of measuring sensitivity for the central 30° radius of the visual field with equal numbers of target locations in each quadrant of the field, and target locations no more than 6° apart. The perimeter should be a projection perimeter or should produce measures that are equal to those obtained on a projection perimeter. The perimeter should have an internal normative database for automatically comparing an individual’s performance with that of the general population. The perimeter should have a statistical analysis package that is able to calculate visual field indices, particularly mean deviation or mean defect (MD), which is the average deviation of visual field sensitivity in comparison to normal values for the central 30° radius of the visual field. The perimeter should demonstrate high sensitivity (ability to correctly detect visual field loss) and specificity (ability to correctly identify normal visual fields). The perimeter should demonstrate good test-retest reliability.

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Visual Impairments: Determining Eligibility for Social Security Benefits The perimeter should have undergone clinical validation studies by three or more independent laboratories with results published in peer-reviewed ophthalmic journals. At present, the Humphrey Field Analyzer and the Octopus perimeters are known to meet these criteria. For qualified devices, we recommend that a threshold procedure should be employed for visual field determinations (for example, Full Threshold, Fastpac, SITA, and SITA Fast are all suitable alternatives for the Humphrey; Threshold, TOPS, and TOPS Plus are suitable alternatives for the Octopus). Ideally, one would have a measure of the binocular visual field serve as the basis for disability determinations because the binocular visual field is what people use for daily activities. However, simple procedures for determining the binocular visual field empirically, or deriving it from monocular visual field results, are not currently available. Until such procedures can be implemented, we recommend that the visual field results for the better eye should be used for disability determinations. To account for scotomas and normal visual field locations between major meridians, we recommend that an index of the overall visual field status, such as mean deviation or mean defect, should be used for disability determinations. MD provides the best overall indication of visual field status, taking into account both the spatial extent and the localized sensitivity variations that are present in the visual field. An MD of −22 dB approximately corresponds to a visual field extent of less than 10° radius (the current SSA standard) and would serve as a reasonable criterion for disability determination. Contrast Sensitivity Testing The committee recommends that contrast sensitivity be assessed as an additional basis for disability determination for claimants who do not meet the current medical listing criteria for disability, but who have acuity between 20/50 and 20/200 and show other evidence or self-

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Visual Impairments: Determining Eligibility for Social Security Benefits report of serious visual impairment. Contrast sensitivity testing of this subset of claimants can provide useful information not captured by high-contrast acuity testing. A contrast sensitivity test should be simple to administer, requiring no sophisticated electronic or computer equipment, well-standardized, reliable, valid, sensitive to visual loss, and relatively insensitive to changes in focus, viewing distance, and illumination. It should provide a single score that is meaningful and can easily be compared with extensive normative data, and it should provide information about visual function not captured by other tests (such as high contrast acuity). One available test, the Pelli-Robson, is known to meet these criteria for a satisfactory test, as may others now available or emerging in the future. Testing of Other Visual Functions The committee recommends not testing visual functions other than acuity, fields, and contrast sensitivity at this time. Our review of the evidence has not shown that testing of color vision is justified by the additional information it would provide. Testing of binocularity, visual search, and adaptation to glare and luminance change, although worthy of further study because of their potential importance to visual task performance, are not recommended with the tests now available. Well-documented severe impairments of visual functions other than acuity, fields, or contrast sensitivity could be taken into account as “adjustments” in the disability determination process. Combining Scores to Derive a Measure of Impairment We know too little about the interactions between multiple visual impairments to recommend a change in SSA’s current practice of deriving an overall measure by multiplicative combination of

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Visual Impairments: Determining Eligibility for Social Security Benefits component measures. We recommend that research be undertaken to examine directly how different kinds of impairments interact in determining overall visual performance, so that the appropriate rule may be developed for combining component measures. In the meantime, the committee recommends that, with the modification noted below, SSA continue its current practice of computing an overall measure of performance as the product of the component measures. For reasons fully explained in the chapter, we recommend that when contrast sensitivity has been measured (which would be done only when a loss is suspected beyond that captured by an acuity score), the contrast sensitivity score should supplant the acuity score in the calculation. Because our recommended measures of acuity, visual fields, and contrast sensitivity are already logarithmic measures of impairment, it is not necessary to calculate central visual efficiency and visual field efficiency. We need only add the scores to compute an overall measure. Recognizing that we must give equal weight to visual fields and to visual acuity (or contrast sensitivity), but that we use either a measure of acuity or a measure of contrast sensitivity, we recommend that the overall measure of impairment should be computed as: aggregate impairment = logMAR + |MD|/22 when the scores to be combined are visual acuity and visual field and aggregate impairment = (2 − CS) + |MD|/22 when the scores to be combined are contrast sensitivity and visual field. The current standard for disability would be met when this aggregate impairment equals or exceeds 1.0. Assurance of Test Quality The committee recommends that SSA should develop standards for the selection of tests to be used in disability determination and consider establishing a formal body of experts on vision testing to

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Visual Impairments: Determining Eligibility for Social Security Benefits implement these standards. This could take the form of a standing advisory board or a panel of consultants with both clinical and scientific expertise; it would review proposed new tests and changes to tests now used, approving those that meet the standards. The committee also recommends that SSA consider developing standards for test administration, in consultation with the ophthalmological and optometric communities, exploring ways to ensure that such standards are met by professionals testing SSA claimants, while respecting the value of practitioners’ clinical judgment. This could greatly improve the reliability of testing. Implementation possibilities range from initiating an accreditation or certification system for providers and their test facilities to establishing dedicated test centers that would operate under SSA supervision. These quality assurance recommendations apply to tests of visual task performance (discussed in Chapter 3) as well as to tests of basic visual functions. Research Recommendations Research is needed relating the outcome of visual assessment using such tools as visual acuity charts to an individual’s ability to function in the workplace and in society. The results of such studies would allow future evaluation of the adequacy of the traditional cutoff of 20/200. There is currently very little information available on the relationship between the status of the peripheral visual field and the performance of daily activities, occupational demands, and task performance. This area merits further research to support the validity of using visual field measurements as predictors of functional capabilities. Another important area is the development of techniques for providing valid and reliable measures of binocular visual field sensitivity. The relationships of contrast sensitivity to performance on tests of mobility, social participation, and tool use/manipulation are areas

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Visual Impairments: Determining Eligibility for Social Security Benefits clearly in need of further study, as is the contrast sensitivity testing of young children (see Chapter 4). Glare and vision in poor and changing lighting are exacerbating factors for seeing low-contrast objects. There is no standard, widely available test for glare. Thus research is needed on glare testing and on the impact of exacerbating factors, including glare, on task performance, and methods for documenting these problems. New tests are available to test low luminance, low-contrast acuity, but they have not been extensively used among working-age visually impaired people and therefore require further study, which would also be useful for tests of binocularity and of visual search and related functions.