Relationships between lateral heterophoria, prism vergence, and the near point of convergence.

Numerous physiologic functions of the binocular neuromuscular mechanism have been measured for years with various kinds of testing equipment. Relatively few studies have been made to determine to what extent these various testing devices measure the same or different characteristics. This study was designed to investigate the relationships between lateral heterophoria, prism vergence, and the near point of convergence, as well as the relationship of these variables to age, sex, and refractive error. Other partially similar studies have been made previously. Haessler considered the relationship of heterophoria at 20 feet to refractive error and to the convergence angle. His sample consisted of 1,000 cases and was preselected to the extent that the following subjects were eliminated: (a) those with more than 0.5 prism diopters of hyperphoria; (b) those with more than 4.0 diopters of ametropia; and (c) those who had not worn correcting lenses for their ametropia for at least several months. Haessler employed the Maddox-rod test for heterophoria and justified his use of it on the basis of a study by Weymouth, who, with data from 12 subjects, concluded that the screen and parallax test, von Graefe's prism diplopia test, and the Maddox-rod test were essentially equivalent at a testing distance of 20 feet. Weymouth's interpretatation has since been confirmed. Haessler