Comparison of Monocular to Effective Binocular Central Scotomas in Low Vision Patients

Abstract

Purpose: : Determine the proportion of low vision patients with central scotomas who have reduced effective scotoma sizes with binocular viewing.

Methods: : Central scotomas were measured with a video haploscope tangent screen in 700 low vision patients. The haploscope consisted of 2 flat-panel displays, one for each eye, imaged at infinity through 9mm diameter exit pupils centered in each eye’s entrance pupil. The subject’s refractive error correction was incorporated in the optical path for each eye. Infrared-sensitive video cameras imaged the subject’s pupils on axis for alignment and monitoring fixation during testing. The display images were 50^o horizontal by 40^o vertical with 100% binocular overlap. The rest of the visual field was occluded and all ambient illumination was blocked. A black fixation cross was presented to both eyes and the subject fixated binocularly while test stimuli were presented to each eye separately. Test stimuli were presented at each of 28 locations in a 15^o square grid centered on fixation. In the central 5^o the stimuli were spaced at 1^o intervals; they were spaced at 5^o intervals elsewhere. Stimuli were presented in random order to the 28 corresponding points in each eye. The stimulus was scored as "seen" if the subject detected it in two out of three presentations.

Results: : The test produced a scotoma map for each eye centered on the binocular fixation point. The scotoma maps of the two eyes were combined to produce a map of binocular scotomas, showing where the stimuli could not be seen in either eye when presented to corresponding points in the visual field. To compare monocular to binocular scotomas, the total number of unseen points were counted for each eye (N_R and N_L) and for the binocular scotoma map (N_B). A summary variable was constructed by calculating the reduction in the number of unseen points with binocular relative to monocular viewing (1-N_B/N_M where N_M=N_R if N_R<N_L else N_M=N_L). No change in the number of unseen points (0% reduction) was observed in 16% of the patients and no binocular scotomas (100% reduction) was observed in 12% of patients. The median reduction in the number of unseen points with binocular viewing was 24% (Q1=7%, Q3=57%).

Conclusions: : Binocular viewing can reduce the effective size of central scotomas in the majority of low vision patients.

Keywords: low vision • perimetry • binocular vision/stereopsis