Adaptation of Vergence and Accommodation in Convergence Insufficiency

Abstract

Abstract: : Purpose: Patients with binocular instability are thought to have an imbalance of their vergence and accommodation adaptation mechanism. Though individual case studies have been published showing high CA/C and low AC/A ratios₁ there has been no systematic study of the adaptive abilities of such patients. The purpose of this study was to compare the vergence and accommodative adaptation in a group of convergence insufficiency (CI) patients with a group of matched controls. Methods: Subjects: 10 CI patients (mean age 23.7 3.4 years) and 17 matched controls (mean age 24.4 3.1 years) were included in the study. Vergence adaptation to prism induced heterophoria (6Δ; BI and 6Δ; BO at 6m and 0.4m) and spherical lens induced heterophoria (-2.0D at 6m and -2.0D and +2.0D at 0.4m) was measured with a Maddox rod and tangent scale technique₂. A total of 7 minutes of binocular vision was allowed through each prism and lens and the heterophoria was measured 15 times over this 7 minutes period. The time taken to reach half the prismatically or spherically induced change in heterophoria was taken as a measure of vergence adaptability. Accommodative adaptation: Accommodation was stimulated by fixation of a transparent acuity terget (7.5' angular subtense) superimposed on a 0.2 cpd DOG target at 40cm. Monocular stimulation prevented convergence accommodation from contributing to the accommodative response. Once adaptation was complete (after 30s) the transparent visual acuity target was removed and continued fixatiion of the 0.2 cpd DOG target opened the accommodative loop₃. A PowerRefractor (Multichannel Systems, Germany) measured the accommodative adaptation and the time to return to the baseline value. Results: The vergence adaptation to both prism and lens induced heterophoria was less complete in the CI group as compared to the matched controls. Statistically significant differences in vergence adaptation was found for both BO and BI prisms at both distance and near and for the minus and plus lenses at distance and near (p<0.05). The CI group had a greater amplitude of accommodative adaptation and a longer duratiion of accommodative after-effect as compared to the matched controls (p<0.05). Conclusions: Patients with CI have reduced vergence and enhanced accommodative adaptation. This adaptive imbalance is the probable cause of the high CA/C and low AC/A ratios that have been found in CI patients. ₁Schor CM, Horner D. Ophthal Physiol Opt 1989; 9: 264-268 ₂Henson DB, North R. Am J Optom Physiol Opt 1980; 57: 129-137 ₃Tsuetaki TK, Schor CM. Am J Optom Physiol Opt 1987; 64: 437-449

Keywords: 617 vergence • 304 accommodation