Binocular Summation and Control of Intermittent Exotropia

Abstract

Background: Binocular summation (BiS), or improvement in binocular vision exceeding the better eye alone, is affected by strabismus. Being easily measured, BiS may be a useful indicator for subjective outcomes like stereopsis in strabismus. This study aims to investigate the relationship between BiS and measures of control of intermittent exotropia (IXT). Methods: Patients with IXT were recruited before undergoing strabismus surgery and underwent tests of binocular and monocular high and low contrast visual acuity, stereopsis at distance and near, and Newcastle Score (NCS), a score developed by incorporating home control and clinic control criteria into a control rating scale. BiS was calculated using high-contrast Early Treatment of Diabetic Retinopathy Study (ETDRS) and Sloan low-contrast acuity charts (LCA) at 2.5% and 1.25% contrast as the difference between the binocular score and that of the better eye. The relationship between BiS and measures of IXT control (NCS and distance near stereoacuity disparity) was evaluated using a correlation analysis by Spearman correlation coefficients and the Kruskal-Wallis test. Results: Thirty-four patients were included (mean [± standard deviation (SD)] age 19±16years) having a mean (± SD) of 26±16Δ IXT at distance and 20±16Δ at near. Mean (± SD) BiS for ETDRS and Sloan LCA at 2.5% and 1.25% was 0.8±3.6, 1.9±6.0 and −2.3±7.2, respectively. The Spearman correlation coefficient of BiS and NCS was −0.53 (95% CI −0.85 to −0.25) for 2.5% LCA and −0.43 (95% CI −0.77 to −0.13) for 1.25% LCA. BiS at 2.5% LCA (p=0.006) and at 1.25% LCA (p=0.029) significantly differed between the groups based on NCS score groupings (1–3, 4–6, and 7–9), with patients who had better control scores having higher levels of BiS. BiS did not differ significantly between patients grouped according to the difference between stereoacuity measured at near versus distance. Conclusion: Significantly lower low contrast BiS in patients with higher NCS may suggest that decreased BiS is associated with less control in IXT. This finding suggests that BiS may reflect control in IXT across a population of patients with IXT.

Binocular Summation and Suppression of Contrast Sensitivity in Strabismus, Fusion and Amblyopia

“…We further show that binocular contrast summation is impaired in those SWA subjects who were unable to fuse at near distances; SWA subjects who were able to fuse, on the other hand, did exhibit significant binocular summation. These findings are in good agreement with studies showing that binocular acuity summation is greater in subjects with greater control over intermittent exotropia (Yulek et al, 2017) and that strabismus surgery to align the eyes can lead to improvements in binocular vision (Pineles et al, 2015a; Kattan et al, 2016; Chang et al, 2017).…”

Subnormal Binocular Contrast Sensitivity Summation in Patients with Intermittent Exotropia

“…Probability summation and neural summation attribute binocular summation, 22 and several methods have been used to assess binocular summation ratio. 9 20 23 Previous studies used visual acuity to evaluate binocular summation ratio, 9 20 23 however, we used the quadratic summation formula to investigate binocular summation status because our study was based on the CST. It is generally accepted that two eyes are better than one, but the extent of improvement is unclear.…”

“… 8 31 Previous studies revealed the relationship between control scale and stereoacuity in the patients with IXT. 5 10 32 In addition, Yulek et al 9 reported that significantly lower binocular summation in patients with IXT with higher control scoring (poor control state) using low-contrast acuity charts test, and they concluded decreased binocular summation was associated with less control in IXT patients. We also have similar results that binocular contrast sensitivity summation ratio have negative correlation with office-based control scale in the patients with IXT when low frequencies CST condition.…”

“… 2 3 Although stereoacuity testing is considered the standard method for clinically evaluating binocular vision of patients with IXT, there have been concerns about the usefulness of near stereoacuity because near stereoacuity is generally expected to be normal and it usually remains stable in patients with IXT, 4 5 whereas distance stereoacuity in patients with IXT varies depending on the characteristics of the test used. 6 7 There is increasing recognition of the importance of assessing control status in IXT, 8 which correlate to severity of XT and stereoacuity in the patients with IXT, 9 10 however there is no consensus regarding the best method by which to measure control in IXT patients. 11 As an alternative approach, binocular summation (the improved visual performance of binocular vision relative to monocular vision), as a measure of binocular vision may play an additional role in evaluating and monitoring of certain ocular diseases or conditions.…”

“… 11 As an alternative approach, binocular summation (the improved visual performance of binocular vision relative to monocular vision), as a measure of binocular vision may play an additional role in evaluating and monitoring of certain ocular diseases or conditions. 9 12 13 14 …”