PURPOSE: To evaluate the adaptive vertical vergence aftereffect and determine whether it contributes to a difference of vertical deviation with respect to gaze distance in patients with vertical strabismus. DESIGN: Prospective noncomparative studies. METHODS: Eighty-four patients with unilateral superior oblique palsies were enrolled and classified into three types - A, B, and C - based on the difference in vertical deviation between distant and near viewing. The prism adaptation test was performed for 2 to 3 hours to correct vertical deviation and the response of vertical deviation to the prism adaptation test was compared among the three types. RESULTS: Adaptive vertical vergence aftereffect, defined as an increase of deviation by 5 prism diopters or more with the prism adaptation test, was identified in 13 patients (16%) at distance and in 23 patients (27%) at near viewing. Among the three types, the adaptive vergence aftereffect contributed mostly to the type B, in which distance deviation exceeds near deviation. Nine patients (39%) of type B changed to type A category with the prism adaptation test; and of these, 7 increased near deviation so that the deviation difference between distant and near viewing decreased. CONCLUSION: The adaptive vertical vergence aftereffect contributes to a difference in vertical deviation between distant and near viewing. The vertical prism adaptation test is specifically useful to determine the extent of surgery by breaking fusional vergence in patients with hypertropia in whom deviation differs with respect to viewing distance.
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