The relationship between fixation disparity and heterophoria.

Twenty-five asymptomatic subjects with normal binocular vision underwent determination of the magnitude and direction of their heterophoria and their fixation disparity. Fixation disparity was measured at a 2-m fixation distance with the phase-difference haploscope under conditions simulating those of casual seeing. When combined with careful attention to symmetrical control of accommodation, this method eliminates several known technical artifacts. Examples of the normal fluctuation of the magnitude and direction of fixation disparity for individual subjects were demonstrated. Whereas only a slight preponderance of subjects were esophoric, many subjects had convergent fixation disparity (esodisparity). Linear regression analysis of the data revealed a coefficient of correlation between fixation disparity and heterophoria at 6 m of 0.61, and at 33 cm of 0.50. The results suggest that normal, small magnitude heterophoria does not necessarily produce fixation disparity and that fixation disparity does not necessarily sustain heterophoria.