Use of horizontal prism to correct vertical fixation disparity

By noting the direction of gaze in which the vertical phoria is largest, one can determine where binocular discomfort is likely to be greatest. Treatment of the discomfort is then based upon prescribing horizontal prism for the horizontal phoria. The recommended amount of horizontal prism is that amount which reduces the vertical fixation disparity to zero. Although the fixation disparity can change in different directions of gaze, in most instances, the prescribed amount of horizontal prism is determined in the primary position of gaze.