The effects of torsion on horizontal motor fusion and stereopsis

To investigate the effects of ocular torsion on horizontal motor fusion and stereopsis in normal adults and to probe the effects of torsion on peripheral fusion, macular fusion and foveal fusion. Twenty-five normal adults aged 30-38 were enrolled in this study. During the synoptophore assessment, the break points (BP) and recovery points (RP) of convergent fusion (CF) and divergent fusion (DF) and random-dots stereopsis were measured and analyzed at intorsion and extorsion of 3°, 5°, 7°, and 9°. According to the different sizes of the retinal areas stimulated by the synoptophore slides, fusion was classified into three categories: peripheral fusion (p-F), macular fusion (m-F) and foveal fusion (f-F). The p-F, m-F and f-F were analyzed and compared at the same torsional angle. There were significant differences in BPCF, RPCF, BPDF and RPDF among different torsion angles (ANOVA, P < 0.05). The Tukey's multiple comparison test showed that BPCF and RPCF of p-F, m-F and f-F decreased significantly at extorsion and intorsion ≥ 5°, compared with baseline (0° torsion) (P < 0.05). Compared with the baseline, BP of DF decreased significantly at torsion angles ≥ 3°, ≥ 5° and ≥ 7° for p-F, m-F, and f-F, respectively (P < 0.05), and RP of DF decreased significantly at torsion angles ≥ 5°, ≥ 9° and ≥ 7° for p-F, m-F, and f-F, respectively (P < 0.05). Comparison among p-F, m-F and f-F revealed significant differences only in BPCF at an intorsion of 3°, extorsion of 3° and the baseline (ANOVA, P < 0.05). There was a significant difference in the proportion of subjects with different sizes of RDS at different torsional angles (Fisher's exact test, P = 0.000). Fine stereopsis was damaged with increasing torsion. Torsion within the normal range of cyclofusion affects the horizontal motor fusion of convergent and divergent fusion and stereopsis. Torsion ≥ 5° should be considered during strabismus surgery for regaining fine binocular vision.