Effects of experimentally induced anisometropia on visual functions

Abstract

Anisometropia relates to the differences of refractive power between both eyes which is expected to have deleterious effects on visual functions. In this study, visual functions comprise of visual acuity, stereoacuity, contrast sensitivity, aniseikonia and crowding. Hence, this study aimed to evaluate the outcome of mimicked anisometropia on visual functions among different anisometropia groups. Specifically, on each visual function, current study seeks for i) magnitude of anisometropia in which visual functions started to deteriorate, ii) types of anisometropia (myopia, hyperopia and astigmatism) which have maximal effect on visual functions. In this study, 20 healthy emmetropic adults volunteered with the mean age 23.85 ± 1.67 years old and refractive error of -0.06 ± 0.11 DS. Anisometropia (unilateral myopia, hyperopia and astigmatism) were mimicked on each participant by placing spherical soft contact lenses over the dominant eye in 1-diopter (D) increments ranging from +1 to +4DS and -1 to -4DS and -1DCx90 to -4DCx90. Visual acuity, stereoacuity, contrast sensitivity, aniseikonia and crowding were assessed at baseline and each magnitude of anisometropia. Repeated Measures ANOVA analyses showed statistically significant differences of visual acuity, stereoacuity, contrast sensitivity, aniseikonia and crowding at all magnitudes of anisometropia for each myopic, hyperopic and astigmatic anisometropia (p<0.05). With a maximal of 4D anisometropia magnitude, visual acuity in myopic and hyperopic anisometropia reduced by 10 lines and 6 lines for astigmatic anisometropia. The stereoacuity and contrast sensitivity were absent among 4D magnitude of myopic and hyperopic anisometropia while astigmatic anisometropia retained the both visual functions. Horizontal aniseikonia demonstrated significantly higher percentage of aniseikonia at 3D and 4D of all anisometropia groups (p>0.05), while vertical aniseikonia was highly significant at 3D and above for myopic anisometropia, 4D of hyperopic anisometropia and no significant difference in astigmatic anisometropia. In conclusion, mimicked myopic anisometropia had maximal effect on visual functions followed by hyperopic and astigmatic anisometropia. Stereoacuity was affected the most as compared to visual acuity and contrast sensitivity.