Clinical findings before the onset of myopia in youth .3. Heterophoria

Background. Some contemporary theories of the etiology of youth onset myopia suggest differences in accommodation and convergence function. Previous studies have related esophoria to various aspects of myopia development. This paper examines whether differences in phorias exist before the onset of myopia. Methods. A cohort of initially emmetropic children had eye and vision examinations at 6-month intervals for a period of 3 years. Dissociated phorias were measured by the von Graefe method. These prospective data were analyzed by: (1) comparing phorias in children who became myopic to those in children who remained emmetropic; (2) determination of sensitivity and specificity of phorias to distinguish between children who became myopic and those who did not; and (3) determining the change in phorias with time. These same analyses were also performed as additional analyses of some previously published retrospective private practice data. Results. Distance phorias were not significantly different in the became-myopic and remained-emmetropic group by the Mann Whitney U test in either the private practice data or the prospective data. The near phoria was more convergent in the became-myopic group than in the remained-emmetropic group in the private practice data (p < 0.02 by Mann Whitney U test). In the prospective data there were proportionately more children who became myopic outside the near phoria range of 3 Delta exo to 1 Delta eso (p < 0.05 by chi(2) test). In both data sets the near phoria showed an eso shift with time in the children who became myopic, beginning before and continuing after the onset of myopia. The children in the remained-emmetropic groups did not show eso shifts in near phoria. Conclusions. A near phoria which is not close to orthophoria appears to be a risk factor for youth onset myopia, but sensitivity and specificity were not as high as those for other variables such as corneal power, axial length to corneal radius ratio, positive relative accommodation (PRA), and midpoint of the near fusional vergence range. The onset of myopia is associated with an eso (convergent) shift in the near phoria.