Clinical implications of vergence adaptation.

Placement of a prism in front of an eye results in a change in the tonic position of the eyes, a shift in the forced fixation disparity curve, and a shift in fusional amplitudes. These changes remain in effect as long as motor fusion is maintained. Elimination of fusion by occlusion or by removal of the prism results in a slow movement of the eyes back to the preprism position. This phenomenon, known as prism adaptation or slow fusional vergence, has important clinical implications in maintaining binocular vision with anisometropic prescriptions, age-related physiological changes in the positions of the eyes, blinking, high phorias, etc. Vergence adaptation is useful in explaining previous discrepancies between alternate and unilateral cover test, pre- and postorthoptic ACA ratios, stimulus and response ACA ratios, changes in phorias after orthoptics, and the observation of patients "eating up prism." Vergence adaptation anomalies have been implicated in causing asthenopia. Adaptation has been shown to change after orthoptic therapy. This paper reviews the clinical findings associated with vergence adaptation.