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Evaluation of Ocular Disability

Evaluation of Ocular Disability.

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Evaluation of Ocular Disability

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  1. Evaluation of Ocular Disability

  2. 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.

  3. Under the Social Security law—the most common law cited—legal blindness exists if there is: • central visual acuity of 20/200or 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.

  4. 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.

  5. 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 percent of the population) and that approximately 8.7 millionAmericans (about 3.1 percent of the population) have a visual impairment.

  6. 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.

  7. 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;

  8. 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;

  9. 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;

  10. 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.

  11. 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) 100 – FVS = impairment ratingVisual field score ODVisual field score OS → Functional field score (FFS)Visual field score OU

  12. 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.

  13. 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 patient’s 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 person’s ability to read 8 M letters would be recorded as 1/8 M (20/160).

  14. 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.

  15. 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.VisualAcuity 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

  16. 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.

  17. 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)

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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).

  24. 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.

  25. 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.)

  26. The total for each eye comprises that eye’s 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.

  27. 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.

  28. Measured Field PlotsCalculated 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) = ____

  29. 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.

  30. 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.

  31. Example: a monocular patient has a scotoma in his good eye and no lightperception in the other eye. Testingyields the following VFS:OU 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.

  32. 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.

  33. 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.

  34. 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).

  35. 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).

  36. 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).

  37. Based upon the functional vision score, and visual system impairment rating, impairment of the whole person may be estimated, using a table.

  38. 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.

  39. 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.

  40. 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.

  41. 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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