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I. L. Bailey, A. J. Jackson, H. Minto; Quantification of Functional Vision in Severe Visual Impairment. Invest. Ophthalmol. Vis. Sci. 2010;51(13):970.
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© ARVO (1962-2015); The Authors (2016-present)
The AMA Guides for the Evaluation of Permanent Impairment for Vision was promulgated in 2001 and has been followed by the WHO and ICO. It develops Functional Acuity Scores and Functional Field Scores by weighting both acuity and field scores (on 100-point scales) from monocular and binocular testing. Field and acuity scores are combined by multiplication to give a Functional Vision Score(FVS). We are developing methodologies to obtain a Functional Vision Score using methods that are easily applicable across wide ranges of populations and clinical environments.
We have developed the Berkeley Rudimentary Vision Tests(BRVT), for testing visual acuity, systematically simplifying the acuity tasks (LogMAR charts, Single tumbling E’s, and gratings) and testing distances (4m, 1m and 25cm) to measure acuities to LogMAR = 2.9 (20/16000, 6/4800). For visual field testing, we have developed a hand held perimeter for measuring peripheral fields, and novel computerized and tangent screen methods for assessing central fields. We have also experimented with using a standard tangent screen using test distances of 250 cm and 50 cm to test central and peripheral fields respectively
Applying these procedures to test subjects with severe visual impairment, we find acuity can be measure with good reliability (test-retest discrepancy SD = 0. 12 for newly trained testers). Single Tumbling E acuity can easily be measured up to LogMAR = 2.0 which gives zero as the Functional Acuity Score. Poorer acuities (LogMAR = 2.6 for E’s, 2.9 for gratings) can also be measured reliably. Field measurements are slower and more problematic. Size and location of binocular field defects depend on the angle of convergence, so that the short viewing distances used in all bowl and arc perimeters may give results that are not representative of peripheral defects at longer viewing distances. Tangent screens do not allow testing beyond 55 degrees (50 cm test distance). We obtain reasonably good test/retest reliability for the 50 points (SD = +/-7) obtained for the central fields, but we consider it is impractical to measure peripheral fields under both monocular and binocular viewing conditions.
It should be recognized that visual acuities poorer than LogMAR = 2.0 are easily measurable, so that FAS = 0 is not the lower limit of acuity. Mainly for practical reasons, we recommend discarding binocular testing of visual fields, and using a 3:1 weighting for the better eye to calculate the Functional Field Score.
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