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P. W. Laird, S. R. Hatt, D. A. Leske, J. M. Holmes; Distance Stereoacuity in Prism-Induced Convergence Stress. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5534.
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In an initial study of induced convergence stress in normal volunteers we found that degradation of stereoacuity was not associated with decreased binocular visual acuity (BVA). This suggests that accommodative convergence was not recruited to restore and maintain binocularity. We also found that stereoacuity degraded earlier when measured with a real world stereo test (the Frisby Davis Distance [FD2]), than when measured with a random dot polaroid vectograph (the Distance Randot [DR]). In order to better understand these findings, we conducted a follow-up study with a refined testing protocol that would enable us to address potential biases of testing order and different measurable levels of stereoacuity in the original study.
Convergence stress was induced in 20 adults with normal baseline visual acuity and stereoacuity. Convergence stress was increased stepwise by increasing base out prism power from 2 prism dioptres (pd) up to 40pd. Stereoacuity (DR and FD2) and BVA were measured at each step. The disparities tested for the DR and FD2 were identical (200, 100, and 60 seconds of arc). To determine whether adaptation or fatigue occurred, we tested stereoacuity four times at each prism level in the following sequence: DR, FD2, DR, FD2.
Almost all subjects showed decreased distance stereoacuity at some point during convergence stress when measured with the FD2 in contrast to the DR (95% vs. 50%, P=0.003, McNemar’s). In subjects whose stereoacuity degraded on either test, reduction of stereoacuity occurred earlier on the FD2 than on the DR (median 14pd vs. 40pd, P=0.0001, Signed-rank). Degradation of stereoacuity was associated with minimal change in BVA from baseline to maximal convergence stress (20/15 to 20/20). We found no evidence of adaptation or fatigue based on the results of repeat testing.
Increasing convergence stress is associated with decreased distance stereoacuity, but this degradation does not appear to be due to excessive accommodative convergence. Convergence stress affects performance on real world stereotests more than random dot tests, in contrast to our study of intermittent exotropia where random dot performance was preferentially affected. There appear to be different mechanisms for decreased stereoacuity under conditions of convergence stress in normals and in intermittent exotropia.
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