Purpose: To determine the magnitudes of binocular summation for low- and high-contrast letter acuity in a multiple sclerosis (MS) cohort, and to characterize the roles that MS disease, age, interocular difference in acuity, and a history of optic neuritis have on binocular summation. The relation between binocular summation and monocular acuities and vision-specific quality of life (QoL) was also examined. Design: Cross-sectional observational study. Methods: Low-contrast acuity (2.5% and 1.25% contrast) and high-contrast visual acuity (VA) were assessed binocularly and monocularly in patients and disease-free controls at 3 academic centers. Binocular summation was calculated as the difference between the binocular and better eye scores. QoL was measured using the 25-item National Eye Institute Visual Functioning Questionnaire (NEI VFQ-25) and the 10-item neuro-ophthalmic supplement. The relation of the degree of binocular summation to monocular acuity, clinical history of acute optic neuritis, age, interocular acuity difference, and QoL was determined. Results: Binocular summation was demonstrated at all contrast levels, and was greatest at the lowest level (1.25%). Increasing age (P < .0001), greater interocular differences in acuity (P < .0001), and prior history of optic neuritis (P = .015) were associated with lower magnitudes of binocular summation; binocular inhibition was seen in some of these patients. Higher magnitudes of summation for 2.5% low-contrast acuity were associated with better scores for the NEI VFQ-25 (P = .02) and neuro-ophthalmic supplement (P = .03). Conclusion: Binocular summation of acuity occurs in MS but is reduced by optic neuritis, which may lead to binocular inhibition. Binocular summation and inhibition are important factors in the QoL and visual experience of MS patients, and may explain why some prefer to patch or close 1 eye in the absence of diplopia or ocular misalignment.
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