Characteristics and variability of vertical phoria adaptation in normal adults

We evaluated the characteristics of phoria adaptation for vertically induced retinal disparity. An adaptive change in the fusion-free ocular alignment, phoria adaptation, was measured with a computer-aided mirror haploscope at 10, 30, and 60 minutes after the start of wearing of a 3-prism-diopter base up prism by 35 normal subjects ranging in age from 21 to 67 years (mean: 37 years). The relationships between phoria adaptation and the subjects' age, the vertical fusional amplitude, the amount of heterophoria, and the starting time of the examination were evaluated. All subjects showed phoria adaptation, with the mean (+/- SD) degree of 0.78 +/- 0.28 degree, 0.96 +/- 0.26 degree and 1.02 +/- 0.30 degrees at 10, 30, and 60 minutes, respectively, after wearing the prism. The repeatability (95% confidence interval) for the measurements was less than +/- 0.24 degree. There was a significant correlation between the vertical fusional amplitude and the gain of phoria adaptation (at 10 and 60 minutes, P < 0.05). The gain of phoria adaptation measured at 60 minutes showed a significant decrease with age (P < 0.01). The results indicate that the time course of phoria adaptation in the vertical direction is similar to that in the horizontal direction and its gain differs considerably among subjects.