Binocular summation is the superiority of visual function for binocular over monocular viewing. BiS was calculated as a ratio between the binocular score and the better-eye score, which was close to the estimated 1.4 in normal subjects in laboratory measures,[8]. The improvement in visual acuity for binocular viewing at high contrast is 7% in average [8]. BiS for low contrast improves approximately 40% or greater in normal participants. 10% contrast is thought a more sensitive measure of both monocular and binocular visual function in multiple sclerosis and retrobulbar neuritis, which is associated with brain MRI lesion burden. Reductions in vision specific quality of life were associated with lower scores for the low contrast[9, 10]. In children, 65.7% had binocular summation and 28.6% had binocular equivalency and 5.7% had binocular inhibition,[11]. The age, visual acuity of the better eye, interocular difference of monocular vision and stereoacuity affect BiS,[3, 10, 12].
Binocular interaction was affected by the presence of strabismus. The BiS was more sensitivity in the low contrast than that in the high contrast. Pineles et al found the mean BiS was lower in the strabismic patients than control participants for 2.5% and 1.25% contrasts. But there was no significant difference in BiS for 100% contrast. The mean BiS of 0.9 for 1.25% contrast in strabismic patients demonstrated binocular inhibition,[4]. Anika reported that the average BiS score of -2.14 letters for 1.25% contrast in the strabismic patients. It might explain blur vision complained by the strabismic patients that cannot be demonstrated by the routine examinations. Binocular inhibition for1.25% contrast was associated with decreased quality of life,[5]. Jaffer et al reported BiS for high contrasts was not associated with stereoacuity but that for the 2.5% contrast wad significant related with both near and distance stereoacuity,[13]. Melinda found that subnormal BiS for 1.25% contrast in Strabismic amblyopes was similar to that in strabismic patients before surgeries. Strabismus surgery also had effect on the improvement of low-contrast BiS in Strabismic amblyopes,[14]. The proportion of patients with BiS was increased from 21–30% for 2.5%contrast and from 13–26% for 1.25%contrast after surgeries, respectively. The proportion of patients with binocular inhibition was decreased postoperatively for all contrast levels,[6].
Although stereoacuity and BiS for low contrast are correlated, they might be differentially affected by strabismus. Having measurable preoperative stereoacuity, older onset of strabismus, and larger preoperative deviation were factors of improved BiS after the surgeries,[14]. In these previous studies, the enrolled subjects had all subtypes of strabismus. BiS might be more affected in esotropia than that in exotropia,[15, 16]. The patients with infantile esotropia and childhood-onset strabismus had deeper suppression than patients with other subtypes so that they had less effect on BiS scores from strabismus surgery,[6]. The patients with IXT could maintain some degree of binocular visual function. Stereoacuity at distance was variable in 42% patients,[17]. Deterioration in near stereoacuity at 1to 2 years follow-up is infrequent in IXT [18]. Chang reported binocular inhibition presented in 39.2% patients with IXT for 100% contrast. Distance BVA correlated with a decreased distance stereoacuity but not with the size of the deviation. It might be helpful in assessing the deterioration of alignment in IXT,[14]. Jeong found binocular CS summation ratio was correlated to control scale in IXT and might be a useful method in assessing binocular visual function in IXT,[20]. We supposed BiS in IXT should be effect on more obviously than other type of strabismus. However, The improvement of BiS for the high and low contrasts in IXT, as the functional benefit from the surgical treatment has not been investigated well in the previous study.
In our series, the proportions of patients with binocular summation were significantly increased for all the contrasts (100%, 10%, 5% and 2.5% ) after surgeries. More patients obtained binocular summation at 5% and 2.5% contrast after surgeries. We considered that our results of surgical effect on BiS were better than that in the previous studies because only the patients with IXT were enrolled, excluding other subtypes of strabismus, amblyopia, anisometropia, nystagmus, and mental maldevelopment. The postoperative improvement of BVA for 2.5% contrast differed significantly among three groups of the patients with different distant RDS and between the groups with and without central fusion. We suggested that BiS for 2.5% contrast might be sensitive for binocular vision in all the contrasts.
In our series, the binocular summation also presented in some patients without stereopsis or central fusion. Viewing at 10° eccentric to the fovea reduced VA about 8 ETDRS lines. A manifest strabismus causing an image to fall extra fovea (non-corresponding retinal location) in the deviated eye, might lead to interocular difference and therefore decrease BiS, or even produce binocular inhibition [6, 12]. Therefore, we supposed that good alignment after strabismic surgeries might improve BiS in those without stereopsis or central fusion. Although the improvement of BiS was most evident at low contrasts, it was also present at high contrast in some patients with a good alignment after surgery.
There are several limitations in our study. We didn’t analyze the relationship between BiS and the result of Bagolini lens tests because there were only four patients had suppression tested by Bagonili lens after surgeries. We didn’t investigate the relationship between BiS and the near stereopsis tested by Titmus. The postoperative examinations were taken at 2 to 3 month after surgeries so all the patients had good outcomes after surgeries. Further study is required to investigate BiS improvement in long term.
In conclusions, the BiS for the high and low contrasts could be improved in IXT after surgical treatment. It might be associated with obtaining central fusion, recovering stereopsis at distance and good alignment after the surgeries. Improvement of BiS, particularly for 2.5% contrast, has benefit for the daily activities in the real environment and increasing the quality of life. We suggest the BiS could be as supplementary assessment of binocular function for the patients with and without stereopsis and central fusion before and after surgeries to evaluate the improvement of vision function in the real environment.