Ophthalmologists may be one step closer to having a metric to measure improvement in visual acuity in both eyes after surgery to correct crossed or misaligned eyes, according to an article published online January 2 in JAMA Ophthalmology.

Stacy L. Pineles, MD, from the Jules Stein Eye Institute, University of California, Los Angeles, and colleagues prospectively analyzed outcomes for 90 patients ranging in age from 2.5 to 90 years with a mean (SD) age of 35.5 (25) years (range, 2.5-90 years) who underwent strabismus surgery between September 1, 2011, and January 31, 2014.

All patients underwent visual acuity testing preoperatively and 2 months after surgery. Investigators tested for high- and low-contrast visual acuity binocularly and monocularly.

The investigators then calculated binocular summation (BiS), a measure of improvement in visual acuity using both eyes compared with using the better eye alone. They calculated BiS for high contrast, using Early Treatment of Diabetic Retinopathy Study Charts. They also calculated BiS for low contrast, using Sloan low-contrast acuity charts at 2.5% and 1.25% contrast as the difference between the binocular score and that of the better eye.

All 90 patients experienced improvement in BiS scores in two low-contrast levels. Seventy-five patients for whom the surgery successfully restored binocular alignment experienced improvement at all contrast levels.

Overall, the mean improvement in BiS scores came to 1.3 (95% confidence interval [CI], −0.1 to 2.7) to 2.8 (95% CI, 1.4 - 4.3; P = .01) letters for the 2.5% chart, to −1.6 (95% CI, −3.1 to −0.05) to 1 (95% CI, −0.6 to 2.4; P = .01) letter for the 1.25% chart. For all contrast levels (including high-contrast ETDRS charts) for the 75 participants who achieved successful alignment by surgery (P = .02, .04, and .03 for the ETDRS chart, 2.5% low-contrast acuity charts, and 1.25% low-contrast acuity charts, respectively).

Decreased Binocular Inhibition

Notably, the proportion of patients whose BiS score was worse by at least five letters than the better eye decreased postoperatively at all contrast levels. Among the 90 patients, 19 had more than a five-letter improvement in high-contrast vision, 24 had that improvement in the 2.4% low-contrast acuity chart, and 28 had that improvement in the 1.25% low-contrast acuity chart.

"The most important point from these results is that they demonstrate that patients with strabismus may have diminished vision compared to one eye alone due to their strabismus, and that in some cases, strabismus surgery may improve this for them," Dr Pineles told Medscape Medical News.

"The types of patients who could benefit most from this would be patients who have certain subtypes of strabismus that are intermittent," she said.

The study included several strabismus subtypes, including infantile esotropia with onset before age 1 year, consecutive exotropia after surgery for infantile esotropia, and acquired hypertropia after age 1.5 years.

Although the study was not powered to evaluate subtype differences, the researchers observed some trends: Patients with subtypes intermittent exotropia, acquired hypertropia, acquired exotropia, and congenital superior oblique palsies tended to benefit more. Patients with childhood esotropia tended to benefit the least and have the most deterioration.

In addition, patients with infantile or childhood esotropia were less likely than patients in the other subgroups to benefit postoperatively.

Although BiS has been studied in laboratories for more than 50 years, "it is not well studied in patients with strabismus in clinical settings," the researchers write.

They conclude, "This study provides further evidence that there may be functional benefits to strabismus surgery beyond improved stereopsis, visual field normalization, and psychosocial concerns."

Foundational Research

"Dr Pineles and her extensive work have made an important contribution to our field by highlighting the adverse impact of strabismus on binocular summation," Gena Heidary, MD, PhD, a strabismus expert at Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, told Medscape Medical News.

"We're in the formative stages in terms of the clinical application of this metric [BiS]," she continued. "This work, however, provides the basis for examining the relationship between binocular summation and other aspects of visual function such as stereoacuity, and for improving our understanding of how best to serve our patients with strabismus."

As to what comes next, Dr Pineles said, "we are following these patients out for a full year after surgery to see if the benefits persist."

This research was supported by the National Institutes of Health, the Knights Templar Eye Foundation, and the Oppenheimer Family Foundation. The authors and Dr Heidary have disclosed no relevant financial relationships.

JAMA Ophthalmol. Published online January 2, 2015. Abstract