Stereoacuity and binocular visual acuity in prism-induced exodeviation

Background: Intermittent exotropia may be controlled by accommodative or motor convergence. Previous studies have reported that reduced binocular visual acuity in intermittent exotropia is due to the accommodation required to control the deviation. To test this hypothesis, we simulated convergence stress by inducing exodeviations in normal volunteers to investigate whether the transition from nonfused to fused is associated with a gradual or immediate transition in stereoacuity and binocular visual acuity. Methods: Convergence stress was induced in 25 visually normal adults with 40^Δ base-out prism and reduced stepwise in increments of 5^Δ until 20^Δ, and 2^Δ thereafter. Stereoacuity (Frisby Davis Distance [FD2] and the Distance Randot [DR]) and binocular visual acuity were measured at each step. For each subject, the recovery of fine stereoacuity (≤60 arcsec) was categorized as immediate (nil to fine) or gradual (nil to moderate to fine). Results: Twenty-four of 25 (96%) showed gradual recovery of fine stereoacuity on either FD2 or DR. Median binocular visual acuity at baseline, first level of fusion, and subsequent levels was 20/15. Conclusions: Under convergence stress, the transition from nonfused to fused is accompanied by a gradual recovery of fine stereoacuity in most individuals, consistent with some studies of patients with intermittent exotropia. Nevertheless, this degradation of stereoacuity was not associated with decreased binocular visual acuity, suggesting that accommodative convergence may not be recruited to restore and maintain binocularity under conditions of convergence stress.