Wolffsohn JS, Eperjesi F
Background: Prescribing magnification is
typically based on distance or near visual acuity. This presumes
a constant minimum angle of visual resolution with working
distance and therefore enlargement of an object moved to a
shorter working distance (relative distance enlargement). This
study examines this premise in a visually impaired population.
Methods: Distance letter visual acuity was
measured prospectively for 380 low vision patients (distance
visual acuity between 0.3 and 2.1 logMAR) over the age of 57
years, along with near word visual acuity at an appropriate
distance for near lens additions from +4 D to +20 D. Demographic
information, the disease causing low vision, contrast
sensitivity, visual field and psychological status were also
recorded.
Results: Distance letter acuity was
significantly related to (r = 0.84) but on average 0.1 ± 0.2
logMAR better (1 ± 2 lines on a logMAR chart) than near word
acuity at 25 cm with a +4 D lens addition. In 39.8 per cent of
patients, near word acuity was more than 0.1 logMAR worse than
distance letter acuity. In 11.0 per cent of subjects, near visual
acuity was more than 0.1 logMAR better than distance letter
acuity. The group with near word acuity worse than distance
letter acuity also had lower contrast sensitivity. The group with
near word acuity better than distance letter acuity was less
likely to have age-related macular degeneration. Smaller print
size could be read by reducing working distance (achieved by
using higher near lens additions) in 86.1 per cent, although not
by as much as predicted by geometric progression in 14.5 per
cent.
Discussion: Although distance letter and near
word acuity are highly related, they are on average 1 logMAR line
different and this varies significantly between individuals. Near
word acuity did not increase linearly with relative distance
enlargement in approximately one in seven visually impaired,
suggesting that the measurement of visual resolution over a range
of working distances will assist appropriate prescribing of
magnification aids.
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