Psychophysics of reading--XIII. Predictors of magnifier-aided reading speed in low vision

A key clinical problem in low-vision assessment is finding simple measures that are predictive of real-world performance. Our purpose was to determine whether a set of clinical variables could predict how well low-vision subjects read with their magnifiers. The variables were Snellen acuity, presence/absence of central scotomas, clear/cloudy ocular media, age, magnifier type, and score on a standardized test of reading speed (Minnesota Low-vision Reading Test). Forty low-vision subjects identified the magnifier they most "preferred" to use, based on the frequency and length of time used, ease of use, or any other factor considered important by each subject. All subjects were highly experienced with their magnifier (3 months to 21 yr of use). We classified the magnifiers into five categories: (1) standard correction only; (2) hand-held magnifier; (3) spectacle-mounted magnifier; (4) closed-circuit TV; and (5) stand magnifier. Subjects used their preferred magnifiers to read aloud printed paragraphs of about 150 words from which their reading speeds were calculated. By far the best predictor of the magnifier-aided reading speed was the score on the standardized reading test which accounted for 79.7% of the variance. 43.7% of the variance was accounted for by age, and 42.3% by magnifier type. Snellen acuity, central visual field status, and ocular media status were not significantly correlated with magnifier-aided reading speed. We conclude that a standardized clinical reading test can give a valid prediction of the reading speed a low-vision patient is likely to achieve with a magnifier.