The impact of degraded binocularity on functional depth vision in keratoconus

Abstract

Purpose : To investigate, 1) the impact of degraded stereopsis in keratoconus on a depth-related vision task that emulates complex activities of daily living; 2) the binocular advantage in this task performance over monocular viewing, vis-à-vis, controls with intact binocularity; 3) the change in task performance with contact lens (CL) wear, commensurate with their stereoacuity.

Methods : 24 keratoconic cases (17–34yrs) and 26 age-similar controls moved a 1cm diameter metal hoop through three variants of a 33cm long buzz wire convoluted three times in the z-axis, requiring accurate depth perception for task performance. Subjects performed the task under binocular and monocular (better eye occluded) viewing conditions, with their habitual spectacles and CLs. Task accuracy and speed were calculated from the frequency of contact between the hoop and wire (signaled by audio buzzes) and the total task duration (adjusting for the time spent in error), respectively. Random-dot stereoacuity was determined using standard 3-down, 1-up psychophysical staircase paradigm.

Results : Binocular task accuracy of keratoconic cases with their habitual spectacle correction (Mean±1SEM: 0.25±0.03errors/sec) was poorer than in controls (0.12±0.02errors/sec; p<0.01). A binocular advantage in task accuracy was observed in this viewing condition (fold change from monocular to binocular viewing: 2.20±0.07), but its magnitude was lesser than in controls (4.15±0.13; p=0.02). Task accuracy improved with CL wear (0.18±0.03errors/sec; p<0.01) but it did not quite reach the performance level of controls. Unlike controls who slowed down when inaccurate (Bino: 1.63±0.13cm/sec; Mono: 1.31±0.13cm/sec; p<0.01), cases moved at similar speeds irrespective of their accuracy (1.97±0.12cm/sec; p=0.71). Task performance of cases with spectacles and contact lenses was independent of their stereoacuity (12.9-1906arc sec; r=0.26; p=0.10).

Conclusions : Functional depth vision is deficient in keratoconus, with the degraded stereopsis offering lesser binocular advantage to task performance than in controls with intact binocularity. Task performance overall improves with the status of binocularity, but this improvement may not be predicted from psychophysical measures of stereoacuity.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.