Loss of fusional control in intermittent exotropia may be due to binocular inhibition

This prospective study demonstrated that binocular interaction of visual acuity (VA) is associated with accommodative response during binocular vision in patients with intermittent exotropia. Furthermore, accommodative amplitudes significantly correlated with the angle of exodeviation at distance, suggesting that accommodative convergence may affect binocular interaction of VA in patients with intermittent exotropia.

Subjects were 63 patients with intermittent exotropia between 8 and 15 years of age. Accommodative responses of both eyes were measured using the WAM-5500 autorefractor/keratometer (GrandSeiko) during binocular and monocular viewing conditions at 6 m.

More than 60 percent of patients showed binocular equivalency. Binocular summation and inhibition were noted in 30 percent and 9.5 percent, respectively. Linear regression analysis revealed a significant correlation between binocular interaction and accommodative responses during binocular vision (P < 0.001) and significant correlation between accommodative responses and size of exodeviation at distance (P = 0.010).

This study demonstrated the usefulness of binocular VA in patients with intermittent exotropia. The authors write that the pattern of binocular interaction, in addition to stereoacuity, can be used as a measure of fusional control. The study also suggests that surgery may have a significant effect on a patient's quality of life by improving distance binocular VA, something that should be investigated in future studies.

They write that the mechanisms of ocular alignment in intermittent exotropia are thought to be fusional and accommodative convergence. If fusional convergence is insufficient for maintenance of ocular alignment, increased accommodative convergence may work to maintain alignment, resulting in decreased distance binocular VA. This is the plausible mechanism for the increased prevalence of binocular inhibition among patients with intermittent exotropia. The significant correlation between the size of exodeviation and accommodative response also supports the idea that the angle of exodeviation determines the amount of accommodation required to maintain ocular alignment.

The authors conclude that further studies with larger sample sizes are warranted to determine the prevalence of binocular inhibition in patients with intermittent exotropia and to investigate the possible cause of the variability.