Binocular Summation Is Intact in Intermittent Exotropia After Surgery.

Meiping Xu,Xinping Yu,Jiawei Zhou,Jun Jiang,Yiya Chen,Jia Qu,Yiyi Peng,Fang Hou,Zhifen He

doi:10.3389/fmed.2021.791548

Abstract

Purpose: To determine binocular summation of surgically treated intermittent exotropia (IXT) patients by measuring the contrast threshold.Methods: We recruited 38 surgically treated IXT patients aged 8–24 years and 20 age-matched healthy controls. All participants had normal or corrected-to-normal visual acuity (Snellen ≥ 20/20) in both eyes. The IXT patients had undergone the surgery at least a year prior to the study. Twenty-one of them obtained good alignment and 17 experienced a recurrence of exotropia. We measured the observers' monocular and binocular contrast sensitivities (CS) at six spatial frequencies (1.5, 3, 6, 12, 18, 24 cycles/degree) as an index of visual information processing at the threshold level. Binocular summation was evaluated against a baseline model of simple probability summation based on the CS at each spatial frequency and the area under the log contrast sensitivity function (AULCSF).Results: The exo-deviation of IXTs with good alignment was −6.38 ± 3.61 prism diopters (pd) at 33 cm and −5.14 ± 4.07 pd at 5 m. For the patients with recurrence, it was −23.47 ± 5.53 pd and −21.12 ± 4.28 pd, respectively. There was no significant difference in the binocular summation ratio (BSR) between the surgically treated IXT patients, including those with good alignment and recurrence, and normal controls at each spatial frequency [F(2,55) = 0.416, P = 0.662] and AULCSF [F(2,55) = 0.469, P = 0.628]. In addition, the BSR was not associated with stereopsis (r = −0.151, P = 0.365).Conclusion: Our findings of normal contrast sensitivity binocular summation ratio in IXT after surgical treatment suggest that the ability of the visual cortex in processing binocular information is intact at the contrast threshold level.

Highlights

Intermittent exotropia (IXT) is a disorder of ocular misalignment
Two recent studies [17, 18] directly address this issue and show that the eyes are much more imbalanced in surgically corrected IXT patients than those of normal controls using a binocular phase combination task. These findings indicate that an abnormal binocular vision can remain in IXT patients even after surgery
Our goal was to find whether surgically corrected IXTs had the normal ability to process visual information at the contrast threshold level, and whether such ability to process visual information could be associated with good alignment or stereopsis

Summary

Introduction

Intermittent exotropia (IXT) is a disorder of ocular misalignment It occurs in nearly 1% of children in the United States [1] and up to 3.5% of children in China [2]. It manifests frequently when one views a distant visual target or when one has illness or fatigue, but maintains a good alignment and binocular fusion when focusing on the visual target for near. Their otherwise normal vision function can be disrupted intermittently by a sudden outward deviation of one eye [1].

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