Maximum reading speed and binocular summation in patients with central vision loss

Abstract

ObjectiveVisual acuity is a poor predictor of the maximum reading speed of patients with central vision loss. This study examines the effects of binocular summation of acuity on the maximum reading speed of these patients. DesignProspective, observational case series. ParticipantsTwenty patients with central vision loss participated. MethodsMaximum reading speed was measured binocularly using the MNREAD acuity charts. Monocular and binocular acuities were measured with the Early Treatment Diabetic Retinopathy Study (ETDRS) chart. Binocular summation was evaluated with a binocular ratio (BR) calculated as the ratio between the acuity of the better eye to binocular acuity. Fixation stability and preferred retinal locus (PRL) distance from the former fovea were evaluated with the MP-1 microperimetre. ResultsSix patients experienced acuity summation (BR > 1.05), 5 experienced acuity inhibition (BR < 0.95), and 9 showed equality (BR = 1 ± 0.05). There were no differences in the mean acuity of the better eye, fixation stability, or PRL distance from the fovea of the 3 groups. Maximum reading speed was significantly slower (p < 0.05) for patients who experienced binocular inhibition (mean 42 ± 27 words/min [wpm], median 40 wpm) than for those who experienced binocular summation (mean 107 ± 39 wpm, median 108 wpm) or equality (mean 111 ± 62 wpm, median 90 wpm). BR correlated with the maximum reading speed for the overall sample (r[18] = 0.49, p = 0.03). BR together with PRL distance from the former fovea in the better eye explained 45% of the variance in maximum reading speed. ConclusionsBinocular summation of acuity rather than visual acuity alone affects maximum reading speed of patients with central vision loss. Patients with binocular inhibition read significantly slower than those with binocular summation or equality. Assessment of binocular summation is important when devising reading rehabilitation techniques.