Decreased Binocular Summation in Strabismic Amblyopes and Effect of Strabismus Surgery

Abstract

Purpose: Binocular summation (BiS) occurs when binocular visual function surpasses that of the better eye alone. We sought to evaluate whether strabismic amblyopia reduces BiS more than strabismus alone, and determine whether BiS improves in strabismic amblyopes after strabismus surgery. Methods: We prospectively recruited 15 patients with strabismic amblyopia who then underwent strabismus surgery. Thirty age-matched normal subjects and 30 non-amblyopic strabismic patients served as controls. Subjects underwent binocular and monocular visual acuity testing on high-contrast Early Treatment Diabetic Retinopathy Study (ETDRS) as well as 2.5% and 1.25% Sloan low contrast acuity (LCA) charts. BiS was calculated as the difference between better eye and binocular scores. Results: Strabismic amblyopes and strabismic controls did not significantly differ in preoperative BiS, but both had subnormal BiS preoperatively on LCA charts. Among 11 strabismic amblyopes with preoperative and postoperative BiS measurements, average postoperative BiS was not significantly different from preoperative. Improved LCA BiS postoperatively occurred in some patients and was associated with measurable preoperative stereoacuity (P=0.02), older age at strabismus onset (P=0.02), and larger preoperative angle of strabismus (P=0.0043).Conclusions: In this preliminary study, strabismic amblyopes experienced subnormal BiS, but amblyopia generally did not further impair BiS beyond that due to strabismus alone. Some strabismic amblyopes experienced improved low-contrast BiS after strabismus surgery. This suggests that further investigation in larger groups of patients should be undertaken to analyze a previously unrecognized functional benefit of strabismus surgery in strabismic amblyopes.