Clinical evaluation of vertical fixation disparity. Part IV. Slope and adaptation to vertical prism of vertical heterophoria patients.

The purpose of this study was to determine the characteristics of the clinically measured vertical fixation disparity (VFD) curve and the ability to adapt to vertical prism for patients who have a vertical heterophoria and are comfortably wearing vertical prism, and to relate these data to similar findings for nonvertical heterophoria patients. The correlation between the amount of vertical prism that was being worn comfortably and the amount of vertical prism that reduced the VFD to zero was also evaluated. The results indicate that patients with vertical heterophoria who are wearing vertical prismatic corrections comfortably have VFD curves with shapes and slopes similar to nonvertical heterophoria patients; have lower coefficients of adaptability to vertical prism than nonvertical heterophoria patients; and the amount of the vertical prismatic correction to produce visual comfort can be determined by the vertical prism that reduces the VFD to zero.