The effect of convergence and divergence stress on near stereoacuity.

The aim of this study was to evaluate the effect of convergence and divergence stress on near stereoacuity in healthy individuals. In this experimental study, 38 subjects with fine stereoacuity (≤30 arcsec in TNO test, 17th edition) were enrolled. Near fusional convergence and divergence amplitudes were measured. Near stereoacuity was evaluated at different levels of convergence and divergence stress induced with base-in and base-out prisms. The amount of base-in and base-out prisms that cause a decreased stereoacuity level of >240 and >60 arcsec were noted for each subject. There were 21 female and 17 male visually normal adults (aged 25-32 years) in the study. The mean near convergence amplitude of the subjects was 37.7 ± 6.9 (25-54) prism dioptres (PD), and the mean near divergence amplitude was 16.5 ± 2.8 (10-25) PD. The mean convergence stress that decreased the stereoacuity level to >240 and >60 arcsec were 20.1 ± 7.9 (4-35) PD and 13.9 ± 5.4 (4-30) PD, respectively. The mean divergence stress that decreased the stereoacuity levels to >240 arcsec was 11.5 ± 2.9 (4-18) PD, and to >60 arcsec was 8.7 ± 2.9 (4-18) PD. Decreased stereoacuity is an important criterion for judging deterioration for patients with intermittent deviations. However, it is not clear what might be occurring in these deteriorating patients. We specifically determined that short periods of prism-induced convergence and divergence are accompanied by a decrease in fine near stereoacuity in visually normal adults. Controlling intermittent deviations by using vergence system could be the simple cause of decreased stereoacuity in these patients.