Near addition lenses as a tool to investigate vergence adaptation in myopic children
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Accommodation and vergence are two interacting ocular motor systems that function to maintain clear and single vision across a wide range of distances. Sustained fixation results in the adaptation of these ocular motor systems and has been widely investigated in adults but not in children. Moreover, limited reports have measured adaptation to disparities induced by ophthalmic lenses. This thesis used near addition lenses as a means to investigate binocular adaptation in children. The specific aims of this thesis were three-fold. First, the thesis aimed to gain insight into the mechanism of changes to accommodation and vergence during binocular adaptation in children. The second objective was to determine the role of vergence-bias category (eso/exo/normals) on adaptation. Lastly, this thesis evaluated the influence of myopia on binocular adaptation. Thirty- eight myopic and 38 emmetropic children between 7-14 years of age were examined for the purpose of this thesis. A series of studies were performed to evaluate adaptation using varying demands for accommodation and vergence, stimulated by binocular fixation at near (33 cm), through the addition of +2D and -2D over corrective lenses (closed loop accommodation) and using 10 base-out prisms (open-loop accommodation at 4M). In each closed-loop condition, measures of binocular and monocular accommodation (PowerRefractor, Multichannel systems) and near phoria (modified Thorington technique) were recorded at frequent intervals when children binocularly fixated a high contrast near target (33 cm) for 20 min. For the open-loop condition (obtained using 0.5 mm pinhole pupils), binocular accommodation and tonic vergence (distance heterophoria through pinhole pupils) were determined at frequent intervals when binocular fixation was sustained at 4M for 20 min. For all conditions, tonic accommodation was measured before and after the near task to measure accommodative adaptation. The results of this thesis make three major contributions to the literature. First, it outlines that the addition of +2D and -2D lenses alters both accommodation and near phoria during sustained binocular fixation, which can be explained based on the models of accommodation and vergence. Second, it shows that the direction of phoria influences the pattern of binocular vs. monocular accommodation in closed-loop conditions and alters the degree of vergence adaptation in both closed and open-loop accommodation. These changes have been primarily attributed to the varying demands on fusional vergence. Lastly, this thesis demonstrates that myopic children show reduced vergence adaptation when fusional convergence was initiated through plus adds or base-out prisms but not when fusional divergence was initiated through minus addition lenses. Further, myopic children also showed variations in other ocular motor parameters such as higher accommodative lags, greater variability of accommodative response, larger accommodative after-effects, and higher AV/A ratios compared to emmetropes. Consistent with the models of accommodation and vergence, the thesis highlights that it is necessary to measure changes to both accommodation and vergence when evaluating the response of the ocular motor system. The direction of phoria and type of refractive error play a significant role in determining binocular adaptation in children. Future studies should differentiate these parameters when evaluating adaptation of the ocular motor system.