Vision loss is a complex phenomenon that cannot be fully understood unless many different aspects are considered (see Chapter 25). Yet, for certain applications it may be desirable to reduce this complex reality to a single number. Administrators prefer the oversimplification of the single number approach when they have to decide on eligibility for benefits or for worker's compensation cases, where the outcome also is a single number: the amount of compensation.
Formulas to calculate what was then called “Visual Economics” were first proposed in Germany in the late 1800s. In 1925 Snell proposed to the American Medical Association (AMA) a simpler formula for “Visual Efficiency.” This formula, reflecting an 80% loss of employability for a visual acuity loss to 20/200, served until 2000. In its fifth (2001) and sixth (2008) editions the AMA Guides to the Evaluation of Permanent Impairment adopted the “Functional Vision Score” (FVS), which reflects an estimate of the ability to perform Activities of Daily Living (ADL).
On the new scale, 20/200 acuity is rated as an estimated 50% loss of ADL ability, rather than as an 80% loss of employability. Other changes include no longer considering the two eyes as separate organs, vision with both eyes open being the normal condition. The new scale has been shown to correlate well with other measures of ability.1
1 Fuhr PSW, Holmes LD, Fletcher DC et al. The AMA Guides Functional Vision Score is a better predictor of vision-targeted quality of life than traditional measures of visual acuity or visual field extent. Vis Impairment Res 2003;5:137-146.
Figure A–1 represents the steps in calculating the functional vision score and its use in calculating an AMA impairment rating.
Steps in calculating the Functional Vision Score (FVS) and the AMA impairment ratings.
The first step is measuring the visual acuity. Use of an ETDRS type chart with a logarithmic progression of letter sizes and 5 letters on each line is preferred. The best corrected acuity is measured for each eye and with both eyes open.
According to the Weber–Fechner law, visual ability is proportional to the logarithm of the visual acuity value. This is reflected in the visual acuity score (VAS) (Table A–1). On an ETDRS type chart the VAS increases by 1 point for every letter read correctly; the scale is anchored at 20/20 = 100.
Table A-1. Visual Acuity (VA) and Corresponding Visual Acuity Score (VAS) ||Download (.pdf)
Table A-1. Visual Acuity (VA) and Corresponding Visual Acuity Score (VAS)