Strabismus Study Reveals Visual Function Deficits

Damian McNamara

September 23, 2013

A new study has shed light on why some people with strabismus sometimes close 1 eye to see better, even in the absence of diplopia, and offers a clinically accessible way to measure and monitor binocular visual function in patients with strabismus.

"This study reveals that patients with strabismus may not see as well with both eyes together as patients without strabismus," first author Stacey L. Pineles, MD, from the Jules Stein Eye Institute and Department of Ophthalmology at the University of California, Los Angeles, told Medscape Medical News.

The findings, published online September 19 in JAMA Ophthalmology, also potentially expand understanding of how strabismus can affect individual patients. "In adult patients with strabismus but without double vision, we previously thought that they did not have any functional deficit from their eye misalignment and lack of stereopsis," Dr. Pineles said.

Not satisfied with the conflicting results provided by previous studies (which they criticize for small sample sizes and use of measures not practical in the clinical setting), Dr. Pineles and colleagues assessed 140 children and adults, including 60 patients with strabismus and 80 control patients. They enrolled people with many different types of strabismus, including early-onset esotropia with onset before age 1.5 years, childhood esotropia with onset between ages 1.5 and 8 years, esotropia acquired after 8 years of age, and intermittent exotropia.

The researchers calculated a binocular summation (BiS) score by dividing the better eye score into the binocular vision score to reflect the superiority of binocular over monocular viewing. They found no significant differences between patients with strabismus and control participants in sweep visual evoked potential testing or high-contrast testing, using the Early Treatment Diabetic Retinopathy Study protocols.

However, the BiS score was significantly lower for patients with strabismus, using 2.5% (P = .005) and 1.25% (P < .001) low-contrast Sloan charts, compared with control participants. The findings remained significant even after accounting for patient age and the angle and type of strabismus.

This novel strategy to diagnose and monitor binocular vision function within strabismus based on BiS score and low-contrast Sloan charts may be more practical in the clinical setting than measures such as evoked potentials, the authors note. The results also may be more valid than current fusion and stereoacuity measures, they add.

"The take-home message is that this study proves that there are additional functional abnormalities of binocular vision in patients with strabismus, even those without diplopia or amblyopia," Dr. Pineles said. "Other ophthalmologists can implement this strategy if they want to obtain a functional measurement of binocularity other than stereopsis.

"This is a unique and interesting paper," Sue Elliott, head orthoptist in the Eye Clinic at Salisbury District Hospital in the United Kingdom, told Medscape Medical News. "We often find it difficult to quantify the benefits of strabismus surgery, for example, quality of life benefits, advantages,... from trying to restore binocularity, etc." She added, "This is a novel way of providing such information. Certainly here in the UK we are always looking for ways to prove that what we are doing makes a difference, as the funding by the [National Health Service] for interventions is tightly monitored."

A total of 9 control participants also demonstrated binocular inhibition on the 1.25% Sloan low-contrast acuity measure, which may be a potential limitation of the study. Test–retest variability and/or patient fatigue may have contributed to this finding, the authors note. In addition, they found a clinically small but statistically significant difference between strabismus and control patients on interocular differences and high-contrast visual acuity measures.

Dr. Pineles and associates are recruiting larger numbers of patients with specific subtypes to confirm BiS is impaired regardless of the specific form of strabismus.

The National Institutes of Health National Eye Institute, the Knights Templar Eye Foundation, and the Oppenheimer Family Foundation supported the study. The authors and commentator have disclosed no relevant financial relationships.

JAMA Ophthalmol. Published online September 19, 2013. Abstract

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