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Original Investigation | Clinical Sciences

Functional Burden of Strabismus:  Decreased Binocular Summation and Binocular Inhibition

Stacy L. Pineles, MD1; Federico G. Velez, MD1; Sherwin J. Isenberg, MD1; Zachary Fenoglio, BA1; Eileen Birch, PhD2; Steven Nusinowitz, PhD1; Joseph L. Demer, MD, PhD1,3,4,5
[+] Author Affiliations
1Jules Stein Eye Institute and Department of Ophthalmology, University of California, Los Angeles
2Retina Foundation of the Southwest, Dallas, Texas
3Department of Neurology, University of California, Los Angeles
4Neuroscience Interdepartmental Program, University of California, Los Angeles
5Bioengineering Interdepartmental Program, University of California, Los Angeles
JAMA Ophthalmol. 2013;131(11):1413-1419. doi:10.1001/jamaophthalmol.2013.4484.
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Importance  Binocular summation (BiS) is defined as the superiority of visual function for binocular over monocular viewing. Binocular summation decreases with age and large interocular differences in visual acuity. To our knowledge, BiS has not heretofore been well studied as a functional measure of binocularity in strabismus.

Objective  To evaluate the effect of strabismus on BiS using a battery of psychophysical tasks that are clinically relevant and easy to use and to determine whether strabismus is associated with binocular inhibition in extreme cases.

Design  Case-control study.

Setting  University-based eye institute.

Participants  Strabismic patients recruited during 2010 to 2012 from a preoperative clinic and control participants with no history of eye disease other than refractive error.

Intervention  A battery of psychophysical and electrophysiological tests including Early Treatment Diabetic Retinopathy Study visual acuity, Sloan low-contrast acuity (LCA) (2.5% and 1.25%), Pelli-Robson contrast sensitivity, and sweep visual evoked potential contrast sensitivity.

Main Outcome and Measure  Binocular summation was calculated as the ratio between binocular and better-eye individual scores.

Results  Sixty strabismic and 80 control participants were prospectively examined (age range, 8-60 years). Mean BiS was significantly lower in the strabismic patients than controls for LCA (2.5% and 1.25%, P = .005 and <.001, respectively). For 1.25% LCA, strabismic patients had a mean BiS score less than 1, indicating binocular inhibition (ie, the binocular score was less than that of the better eye’s monocular score). There was no significant difference in BiS for contrast thresholds on Early Treatment Diabetic Retinopathy Study visual acuity, Pelli-Robson contrast sensitivity, or sweep visual evoked potential contrast sensitivity. Regression analysis revealed a significant worsening of BiS with strabismus for 2.5% (P = .009) and 1.25% (P = .002) LCA, after accounting for age.

Conclusions and Relevance  Strabismic patients demonstrate subnormal BiS and even binocular inhibition for LCA, suggesting that strabismus impairs visual function more than previously appreciated. This may explain why strabismic patients who are not diplopic close 1 eye in visually demanding situations. This finding clarifies the visual deficits impacting quality of life in strabismic patients and may represent a novel measure by which to evaluate and monitor function in strabismus.

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Figure.
Scatterplot Representations for Linear Regression Models of Binocular Summation (BiS) Accounting for Age and Strabismic vs Control Status

R values are depicted for each visual outcome. Strabismic vs control status was the only statistically significant variable for 2.5% Sloan low-contrast acuity (LCA) (P = .007), 1.25% Sloan LCA (P < .001), and Pelli-Robson contrast sensitivity (P = 0.02). Neither age nor strabismus vs control status were significantly associated with increased BiS for high-contrast Early Treatment Diabetic Retinopathy Study visual acuity chart (ETDRS VA) or sweep visual evoked potential (VEP) contrast sensitivity.

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