Figure A–1 represents the steps in calculating the functional vision score and its use in calculating an AMA impairment rating.
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) |Favorite Table|Download (.pdf)
Table A-1. Visual Acuity (VA) and Corresponding Visual Acuity Score (VAS)
|CF < 2 ft||0|
|HM < 10 ft||0|
Next, the three VAS values—both eyes (OU), right eye (OD), left eye (OS)—are combined to provide a single functional acuity score (FAS), 60% weighting being given to the acuity with both eyes open and 20% to each of the monocular values.
In a similar way, a visual field score (VFS) and functional field score (FFS) are calculated. The VFS is determined with a grid (Figure A-2), which allocates 50 points to the central 10° area and 50 points to the remainder of the visual field. This division reflects that the representation of the central 10° of visual field occupies about 50% of the primary visual cortex. It also divides the score evenly between the central area, which is important for reading and detailed vision, and the outer area, which is important for orientation and mobility.
VFS grid, showing the total number of points in each region (left half) and how the points are allocated along the 5 meridians (right half). The radius of the circle is 10 degrees.
The points are allocated along two meridians in each of the upper quadrants and three meridians in each of the lower quadrants. On each meridian 5 points (2° apart) are assigned to the central area and 5 points (10° apart) to the outer area, their distribution being approximately logarithmic. The lower visual field is weighted 50% more than the upper visual field because of its greater importance in functional vision. The primary meridians are not used, to avoid the need for special rules for hemianopias.
The VFS is determined by counting the number of points seen within the visual field delineated by the Goldmann III4e (or equivalent, eg, Humphrey 10 dB) isopter.
The FFS is calculated from the three VFS values using the same weighted formula for calculating the FAS from the three VAS values (60% OU + 20% OD + 20% OS).
Finally, the FAS and FFS are combined into a single functional vision score (FVS).
Thus far, the calculation follows strict mathematical rules. If there are other vision problems that are not reflected in a visual acuity or visual field loss, the examiner may apply an adjustment of maximally 15 points. Such an adjustment must be properly argued and documented.