Functional burden of strabismus: Decreased binocular summation (bis) and binocular inhibition

Abstract

BiS is defined as the superiority of visual function for binocular over monocular viewing. BiS decreases with age and large interocular differences in visual acuity. BiS has not heretofore been well-studied as a functional measure of binocularity in strabismus. Strabismus patients and normal controls underwent a battery of psychophysical and electrophysiological tests, including ETDRS VA, Sloan low contrast acuity (LCA, 2.5%, 1.25%), Pelli-Robson contrast test, and sweep visual evoked potential (sVEP) contrast sensitivity to determine BiS for each. BiS was calculated as the ratio between binocular and better eye scores. Sixty strabismic and 80 normal subjects were prospectively examined (age range, 8-60 years). Mean BiS was significantly lower in the strabismic patients than controls for the LCA charts (2.5% and 1.25%, P < 0.0001 for both). For 1.25% LCA, strabismics had a mean BiS score<1, indicating binocular inhibition. There was no significant BiS for contrast thresholds on the ETDRS, Pelli-Robson or sVEP. Regression analysis revealed a significant association between BiS and strabismus for 2.5% (P < 0.0001) and 1.25% (P < 0.0001) LCA accounting for age and interocular difference in visual acuity. BiS is significantly decreased in strabismus, and some measures of binocular function with two misaligned eyes viewing are worse than during monocular viewing. This may explain why strabismic patients who are not diplopic close one eye in visually-demanding situations. This finding represents an advancement in understanding of the visual deficits impacting quality of life in strabismic patients. Strabismic patients demonstrate sub-normal BiS and even binocular inhibition for low contrast viewing, suggesting that strabismus impairs visual function more than previously appreciated. BiS may represent a novel measure by which to evaluate and monitor function in strabismus.