Studies in vertical fixation disparity.

We studied vertical fixation disparity (VFD) with the purpose of assisting the clinician in the evaluation of patients having vertical heterophoria and/or vertical anisometropia and in the prescription of an appropriate vertical prismatic correction. Our research shows that VFD represents a relatively stable and reliable measurement of the vertical deviation; provides the clinician with a mechanism to study a patient's ability to adapt to an induced vertical fusional demand; and provides a process to determine the amount of the vertical prismatic correction.