Effect of Induced Heterophoria on Distance Stereoacuity by using the Howard-Dolman Test

Purpose: To evaluate the effect of induced esophoria and exophoria on distance stereoacuity at 6 meters Methods: Fifty subjects with distance corrected or uncorrected visual acuity of 6/6 and orthophoria without history of amblyopia, strabismus or any ocular pathology were recruited and distance stereoacuity was measured by Howard-dolman test before and after introducing 2, 4, 6 and 8Δ base in and base out to induce esophoria and exophoria, respectively. Results: Although the decrease in stereoacuity was not significant by introducing 2Δ base in prism (P=0.062), but it was significant statistically by introducing 4 and 6Δ base in (P=0.002, P=0.026, respectively) and 2, 4, 6, 8Δ base out (P=0.042, P<0.001, P<0.001, P<0.001, respectively) prisms compared with orthophoric position. There was no significant difference between stereoacuity in 2Δ and 4Δ induced esophoria with 2Δ and 4Δ induced exophoria (P=0.696, P=0.677, respectively), but 6Δ induced esophoria decreased stereoacuity more than induced exophoria of the same amount (P=0.007). Conclusion: Induced heterophoria at distance of 6 meters reduced stereoacuity measured by modified Howard-Dolman test. Greater than 4Δ of induced esophoria had statically and clinically adverse effect on distance stereoacuity.