Purpose
To describe the relationship between binocular summation and stereoacuity after strabismus surgery.
Design
Prospective case series.
Methods
setting : Stein Eye Institute, University of California Los Angeles. patient population : Pediatric strabismic patients who underwent strabismus surgery between 2010 and 2015. observation procedures : Early Treatment Diabetic Retinopathy Study visual acuity, Sloan low-contrast acuity (LCA, 2.5% and 1.25%), and Randot stereoacuity 2 months following surgical correction of strabismus. main outcome measures : The relationship between binocular summation (BiS), calculated as the difference between the binocular visual acuity score and that of the better eye, and stereoacuity.
Results
A total of 130 postoperative strabismic patients were studied. The relationship between binocular summation and stereoacuity was studied by Spearman correlation. There were significant correlations between BiS for 2.5% LCA with near and distance stereoacuity ( P = .006 and P = .009). BiS for 1.25% LCA was also significantly correlated with near stereoacuity ( P = .04). Near stereoacuity and BiS for 2.5% and 1.25% LCA were significantly dependent (Pearson χ 2 , P = .006 and P = .026). Patients with stereoacuity demonstrated significantly more BiS in 2.5% LCA of 2.7 ( P = .022) and 3.1 ( P = .014) letters than did those without near or distance stereoacuity, respectively.
Conclusions
These findings demonstrate that stereopsis and binocular summation are significantly correlated in patients who have undergone surgical correction of strabismus.
Strabismus affects approximately 2%–5% of the population. Over the past few decades, vision researchers have sought to better understand the concept of binocular summation, defined as superiority of visual function of binocular over monocular vision, and how binocular summation may contribute to reduced visual performance of strabismic individuals. Two general hypotheses have been offered to explain binocular summation: (1) “probability summation,” the statistical improvement provided by 2 independently functioning eyes; and (2) “neural summation,” generating improvement exceeding statistical “probability summation.” Visual task studies have since demonstrated that neural binocular summation likely arises from the cortical area V1 and that it generally provides approximately 40% improvement in visual function. In nonstrabismic subjects, several factors including advanced age and interocular differences in visual acuity have been shown to decrease binocular summation.
It is well known that strabismus patients have deficits in depth perception and fusion tasks. It was also recently shown that binocular summation is also adversely affected by strabismus. Given that binocular summation can be easily measured without monocular cues that confound testing of stereoacuity, and that binocular summation can also be measured in patients without potential for stereopsis, we sought to describe the relationship between stereopsis and binocular summation.
Methods
This study was approved by the University of California, Los Angeles Institutional Review Board and conformed to the Declaration of Helsinki and requirements of the US Health Insurance Portability and Accountability Act. Subjects were recruited from patients at their postoperative month 2 visit after strabismus surgery at the Stein Eye Institute from the clinics of 4 authors (J.L.D., S.J.I., S.L.P., F.G.V.) between the years 2010 and 2015. Exclusion criteria included history of amblyopia, age younger than 3 years or older than 65 years, dissociated vertical or horizontal deviation as the sole form of strabismus, pathologic nystagmus, neurologic disease, or any structural lesion causing an interocular acuity difference exceeding 0.3 logMAR. Subjects were included irrespective of the age at onset of strabismus. In order to obtain a wider range of stereopsis levels, all postoperative patients were included regardless of whether their strabismus was adequately controlled postoperatively, or if they were believed to have potential for stereopsis.
High-Contrast Visual Acuity
Visual acuity (VA) was tested using the Early Treatment Diabetic Retinopathy Study (ETDRS) protocol at 3 meters. The VA score was the number of letters identified correctly, with a maximum score of 70 (Snellen equivalent 20/12.5).
Low-Contrast Visual Acuity
Sloan acuity was tested (Precision Vision, LaSalle, Illinois, USA) at low-contrast levels of 2.5%, followed by 1.25%, using the ETDRS protocol at 3 meters in a dimly lit room. Sloan charts have a similar format to the ETDRS charts (5 letters per line), with each Sloan chart corresponding to a different contrast level. The low-contrast acuity (LCA) score is the number of letters identified correctly, with a maximum score of 70 (14 lines).
Stereoacuity
Stereoacuity was measured at both near (40 cm) and distance (3 m) using Randot Stereotest (Stereo Optical Company, Chicago, Illinois, USA). The stereoacuity score is recorded in seconds of arc distinguished by the subject, with the best score recorded at 40 seconds of arc and the worst score recorded at “nil.” Participants who were unable to discern the grossest level of stereopsis were assigned a score of 10 000 seconds of arc in order to statistically distinguish them from those with some form of stereoacuity.
Statistical Analysis
All statistical analysis was performed using STATA 13.0 (StataCorp, College Station, Texas, USA). Binocular summation was calculated by finding the difference between the binocular visual acuity score and the better visual acuity eye score (binocular score minus better eye score). As a conservative correction for test variability, a binocular summation score exceeding 5 letters (1 line) was required to demonstrate binocular summation. Similarly, binocular inhibition was considered to exist when the binocular summation score was less than −5 letters.
Stereoacuity scores were log transformed because of their non-normal distribution. The correlation between binocular summation and log stereoacuity was then calculated using a Spearman correlation. Stereoacuity scores were ranked as either (1) good stereoacuity (scores 40, 60, 80, and 100), (2) medium stereoacuity (scores 200 and 400), (3) low stereoacuity (score of 800), or (4) no stereoacuity. Binocular summation scores were categorized as either “summation” (for binocular summation >5) or “no summation” (for binocular summation ≤5). A 2 × 4 χ 2 analysis of binocular summation and ranked stereoacuity was then performed. Pearson χ 2 tests of independence were performed to demonstrate whether binocular summation and stereoacuity are dependent variables. To study the difference in binocular summation between individuals with vs without stereoacuity, a 2-tailed unpaired t test was performed. P values less than .05 were deemed to be statistically significant.
Results
A total of 130 patients with treated strabismus were studied, of whom postoperatively, 51% experienced diplopia, 44% demonstrated near stereoacuity, and 32% demonstrated distance stereoacuity. Subtypes of strabismus included infantile esotropia (12%), childhood-onset esotropia (7%), acquired esotropia (14%), intermittent exotropia (24%), consecutive exotropia (14%), acquired exotropia (2%), congenital trochlear palsy (12%), acquired hypertropia (13%), and combined horizontal and vertical deviations (2%). Demographics and visual acuity are summarized in Table 1 .
| Characteristic | Age at Visit, Months | Age at Onset, Months | ETDRS OU, Letters | LCA 2.5% OU, Letters | LCA 1.25% OU, Letters | Near Stereoacuity Present, % | Distance Stereoacuity Present, % | Diplopia Present, % |
|---|---|---|---|---|---|---|---|---|
| All patients (n = 130) | 442 (286) | 242 (301) | 55 (8) | 31 (12) | 20 (12) | 44 | 32 | 51 |
| Esotropia | ||||||||
| Infantile (n = 15) | 350 (234) | 11 (11) | 53 (7) | 33 (120) | 22 (12) | 13 | 4 | 27 |
| Childhood (n = 9) | 343 (329) | 43 (21) | 50 (5) | 25 (15) | 17 (13) | 33 | 22 | 11 |
| Acquired (n = 18) | 711 (216) | 591 (283) | 53 (10) | 25 (13) | 15 (13) | 55 | 29 | 100 |
| Exotropia | ||||||||
| Intermittent (n = 31) | 244 (195) | 85 (89) | 55 (10) | 32 (12) | 20 (13) | 74 | 52 | 19 |
| Consecutive (n = 18) | 354 (250) | 32 (54) | 58 (6) | 38 (6) | 27 (6) | 0 | 11 | 11 |
| Acquired (n = 3) | 364 (323) | 284 (379) | 53 (2) | 35 (1) | 21 (10) | 33 | 0 | 33 |
| Congenital SOP (n = 16) | 428 (201) | 243 (220) | 59 (6) | 37 (8) | 22 (10) | 63 | 50 | 94 |
| Acquired HT (n = 17) | 731 (179) | 658 (217) | 51 (7) | 24 (14) | 13 (13) | 0 | 35 | 100 |
| Combined V/H (n = 3) | 637 (304) | 502 (414) | 55 (2) | 28 (6) | 12 (13) | 33 | 0 | 67 |
Binocular Summation
The mean binocular summation scores are 0.34, 0.83, and −1.80 for ETDRS, 2.5% LCA, and 1.25% LCA visual acuity tests, respectively. The means were not significantly different from zero for ETDRS or 2.5% LCA testing, but was significantly less for the 1.25% LCA ( P = .002), indicating binocular inhibition. 5%, 16%, and 8% of patients demonstrated binocular summation greater than 5 letters for the ETDRS, 2.5% LCA, and 1.25% LCA visual acuity tests, respectively ( Table 2 ). Table 3 shows the results of 2-tailed t tests performed for mean binocular summation scores in those patients with and without stereoacuity for both distance and near. For 2.5% LCA, patients with stereoacuity had a significantly greater binocular summation of 2.7 ( P = .022) and 3.1 ( P = .014) letters than patients without near or distance stereoacuity, respectively.
| Test | Binocular Summation (BiS >5) | Intermediate (BiS −5 to 5) | Binocular Inhibition (BiS <−5) |
|---|---|---|---|
| ETDRS | 5% | 90% | 5% |
| 2.5% LCA | 16% | 76% | 8% |
| 1.25% LCA | 8% | 72% | 20% |
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