Maximum Reading Speed and Binocular Summation in Patients with Central Vision Loss

Abstract

Purpose: Visual acuity is a poor predictor of the maximum reading speed of patients with central vision loss. We hypothesized that the maximum reading speed is a function of binocular summation of acuity for these patients.

Methods: Maximum reading speed of 20 patients with central vision loss (mean age = 77 ± 13 years) was measured binocularly using the MNRead charts. Monocular and binocular acuities were measured with the 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 PRL distance from the former fovea were evaluated with the MP-1 microperimeter.

Results: Six patients experienced acuity summation (BR > 1.05), 5 experienced acuity inhibition (BR < 1.05), and 9 showed equality (BR = 1 ± .05). There were no differences in the mean acuity of the better eye, fixation stability, or PRL distance from the fovea of the three groups. Maximum reading speed was significantly slower (p < .05) for patients who experienced binocular inhibition (mean 42 ± 27 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) = .49, p = .03). BR together with PRL distance from the former fovea in the better eye explained 45% of the variance in maximum reading speed.

Conclusions: Binocular summation of acuity rather than visual acuity alone predicts 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.

Keywords: 584 low vision • 672 reading • 754 visual acuity