Correlation Between Stereoacuity and Experimentally Induced Graded Monocular and Binocular Astigmatism

Stereopsis, the highest grade of binocular single vision, is affected by various factors, such as mis-alignment of visual axes, refractive errors especially anisometropia and astigmatism, both of which may result in amblyopia. There are very few studies in literature regarding the relationship between stereoacuity and refractive errors, especially astigmatism.

The present study was conducted to determine the correlation between stereoacuity and experimentally induced graded astigmatism in emmetropes.

A randomized study was conducted on 2000 individuals of either gender, between the ages of 8-35 years, at tertiary care centre attached to a medical college during the period of August 2012 to August 2014, All subjects were emmetropic with normal binocular single vision. Participants were randomly divided into four groups of 500 individuals each. Two groups were subjected to induced myopic astigmatism, either uni-ocularly or binocularly, using + 1.0 D and + 2.0 D cylinders at varying axes i.e., 45(0), 90(0) and 180(0). Similarily, the remaining two groups were subjected to induced hypermetropic astigmatism, using - 1.0 D and - 2.0D cylinders at varying axes i.e. 45(0), 90(0) and 180(0). Near stereoacuity was determined by the Titmus Fly Stereo Test, both before and after induction of astigmatism. Statistical analysis was done using paired t-test and ANOVA.

The mean stereoacuity in emmetropes was 28.81±4.97 seconds of arc. There was a decrease in stereoacuity with increase in dioptric power of astigmatism (p<0.001). Oblique astigmatism reduced the stereoacuity maximally, while stereoacuity was least affected at 180(o) axis. Hypermetropic astigmatism caused more deterioration in stereoacuity than myopic astigmatism. A gross reduction in stereoacuity was noted in induced monocular astigmatism as against binocular astigmatism.

This study suggests that stereoacuity is significantly affected by even minor degrees of monocular or binocular astigmatism.