We measured stereoacuity using TNO test on 25 patients without fixation disparity (FD) and compared the result with other 25 patients exhibited FD related to visual stress. All patients were presbyopes of ages ranging from 40 to 80 years, with visual acuity 6/6 or better in each eye, free from ocular diseases and generally healthy. The results showed statistically significant difference in the stereoacuities measured between the groups (Mann-Whitney U = 181.0, p < 0.01), suggesting that FD significantly reduced stereoacuity. Although the correlation between the magnitudes of FD and stereothreshold is statistically not significant (Spearman's r(s) = 0.33, p>0.01), elimination of FD using prisms correction statistically improved stereoacuity (Wilcoxon's Z = 2.43, p<0.01). The findings conclude that visual stress manifested as FD causes deficit in stereoperformance measurable with the TNO test and can be improved by prism correction.
The characteristics of foveal suppression (FS) in fixation disparity (FD) due to visual stress were investigated and their relationship's between, age, symptoms, and the effect of temporary elimination of FD using prisms on the degree of the FS were analysed. Forty-five presbyopic subjects (15 without FD and 30 with stress related FD) participated in the study. The subjects underwent comprehensive optometric examination prior to the study. Their FS and FD were measured. The FD was later corrected with ophthalmic prisms, the power of which was equally divided between the eyes, and the FS was later verified. Age and FS had no significant correlation for subjects without FD (Spearman's rs = 0.17, p = 0.55, NS) and in subjects with FD (rs = 2.49, p = 0.19, NS), respectively. Correlation between the degree of FS and FD was weak (rs=0.38, p=0.07), however the magnitude of FD significantly increased with age (r=0.27, p=0.04). Subjects with FD had significantly larger degree of FS compared with subjects without FD (Wilcoxon's Z =-0.25, p=0.01). There was no significant difference in the magnitudes of FD (t = -0.38, p=0.07) and in their degrees of FS (Mann-Whitney U = 1.5, p=0.71) between subjects with and without symptoms. Correcting the FD with prisms generally reduced the degree of FS (Wilcoxon's Z =1.96, p=0.04), however, significant change in FS only occured in subjects with symptoms (Z=-1.97, p=0.03), but was not significant in subjects without symptoms (Z=-0.70, p=0.48).