Visual acuity in a large population of normal, strabismic, and anisometropic observers

Purpose. To understand the relationships among measures of acuity and sensitivity by analyzing their variation and covariation in a large population of normal and abnormal observers. Methods. We measured optotype (LogMAR) acuity, grating resolution, vernier offset threshold, edge contrast sensitivity and optotype (Pelli-Robson) contrast sensitivity in 336 observers aged 8-40 yr. 69 normal observers had no clinical abnormalities; 267 observers had anisometropia and/or constant strabismus. Results. Of the 267 abnormal observers, 151 (57%) were amblyopic (LogMAR >0.3 [20/40] viewing through the non-preferred eye). About 40% of observers with anisometropia or strabismus alone were amblyopic; 75% of observers with both strabismus and anisometropia were amblyopic. For the non-preferred eyes, LogMAR acuity, vernier acuity, and grating acuity were all strongly intercorrelated. Measures of contrast sensitivity were less strongly correlated with acuity measures, and with each other. In agreement with previous reports, we found that vernier and optotype acuity were more affected in amblyopes than grating acuity. The relationships among these measures, however, depended mainly on the severity of the amblyopia and not on whether the observer had anisometropia, strabismus, or both. Contrast sensitivity explained a small but significant additional portion of the variance in these relationships. Conclusions. Individuals with strabismus and anisometropia are more often and more profoundly amblyopic than those with either condition alone. Our data do not show other reliable differences in the overall pattern of visual loss associated with these three groups, but may contain subgroups of amblyopes with distinctive patterns of visual loss that are imperfectly associated with these three conditions.