Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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336 Surgery and Healing in the Developing World
Figure 1. Symbol Snellen chart.
Snellen charts are designed so that a normal-sighted individual can read the top
line at 60 meters, the next line at 36 meters, the third at 24 meters, and so on
through 18, 12 and 9 meters. The final line should be readable at 6 meters. Using
this method acuity is recorded as 6/60 if only the first line is described correctly and
6/6 if all lines are correct (i.e., the same as 20/20 of measured in feet). The first
figure of this nomenclature denotes the standard distance from the chart (NB a
half-size chart at 3 m would be recorded as a standard chart at a distance of 6 meters),
the second is the last line accurately read (i.e., 60,36,24,18,12,9 or 6).
If the top line is read incorrectly at 6 meters (i.e., vision is worse than 6/60), the
patient should be brought closer to the chart in stages of 1 meter. If the patient only
identifies the top symbol correctly at a distance of 3 meters the correct result would
be recorded as 3/60. If the vision is below 1/60 the patient should be asked to count
fingers at 0.5 meters. If this also fails, try perception of hand movement. If hand
movement is not recognized, close the curtains (or find a dimly-lit room) and record
light perception.
If a patient has subnormal vision (i.e., less than 6/6) their vision should be checked
for refractive errors (Fig. 2). To do this a card with a pinhole should be placed in
front of one eye and vision should be retested. An improvement indicates that there
is a refractive error that should be treated with the prescription glasses. Acuity measurement
should only be perfomed by qualified personnel. For further details, see
standard texts.