The effect of anisometropia on stereopsis

For good stereopsis, the two retinal images should have similar clarity, shape, and size. Anisometropia is one of the factors which has affect on stereopsis. The purpose of this study was investigation of induced anisometropia effect on binocular function and stereopsis. In this semi experimental study, a total of 135 students from Zahedan University of Medical Sciences who had inclusion criteria were selected randomly. At the beginning of the work, the refractive errors of the subjects were determined and corrected with retinoscop and their stereopsis measured using T.N.O test. Subsequently, this hyperopic and myopic anisometropia induced by use of negative and positive spherical lenses (power 1, 2, 3 diopter) in trial frame in front of one eye and in each state stereopsis measured again. In agreement and against the rule astigmatic anisometropia induced by positive cylindrical lenses in 180 and 90 degree axes and stereopsis measured again. Data were analyzed by SPPS.14 software using within subjects factorial and repeated measurement ANOVA test. This study showed that both type and amount of anisometropia had significant effect on stereopsis (P<0.001). The mean of stereopsis before intervention was (36.42±12.65). The most and least reduction of stereopsis were related to 3D hyperopic anisometropia (339.42±125.1) and 1D with the rule astigmatic anisometropia (36.57±12.6), respectively. The mean of stereopsis before and after different type and amount of induced anisometropia were significantly different (P<0.001). The result of this study showed that with increasing of anisometropia, stereoacuity decreases. Therefore, low degree of anisometropia, even 1D, can decrease both stereopsis and binocular function and thus, the amendment of stereopsis is necessary.