Distance stereoacuity in prism-induced convergence stress

Purpose: In an initial study of prism-induced exodeviation, degraded stereoacuity was not associated with decreased binocular visual acuity, suggesting that accommodative convergence was not recruited. Distance stereoacuity degraded earlier when measured with the Frisby-Davis Distance (FD2) than when measured with the Distance Randot (DR). We now describe a follow-up study in which we reversed the prism order and also addressed potential biases of testing order and different measurable levels of stereoacuity to clarify the relationship between exodeviation, distance stereoacuity, and binocular visual acuity. Methods: Convergence stress was induced with base-out prism in 20 adults and increased stepwise. Stereoacuity and binocular visual acuity were measured at each step. Disparities tested were 200, 100, and 60 arcsec, and testing was repeated at each prism step. Results: Most subjects showed degraded stereoacuity with the FD2 in contrast to the DR (80% vs 45%, p = 0.02). Reduction of stereoacuity occurred earlier on the FD2 than on the DR (median, 18^Δ vs 40^Δ, p = 0.001). Degradation of stereoacuity was associated with minimal change in binocular visual acuity from baseline to maximal convergence stress (median, 20/15 to 20/20). Conclusions: Convergence stress is associated with decreased distance stereoacuity that does not appear to be due to accommodative convergence. Performance on real depth stereotests (FD2) is affected more than random dot tests (DR), in contrast to previous findings in intermittent exotropia. There appear to be different mechanisms for decreased stereoacuity in intermittent exotropia and under conditions of convergence stress in nonstrabismic subjects.