Sympathetic Control of Accommodation in Young Adults

Abstract

Abstract: : Purpose: Deficiency in sympathetic control of accommodation has been suggested as a possible risk factor in the development of late onset myopia. This work examines the proportion of young adult subjects demonstrating sympathetic inhibitory control of accommodation with respect to their refractive category. Methods: Fifty-eight healthy young adult volunteers were screened for refractive error and questioned on refractive history to determine refractive group. Subjects were designated as emmetropes (EMM, mean refractive error (MRE) <-0.50 D, N=30), early onset myopes (EOM, myopia onset prior to 15 years of age, MRE >-0.50 D, N=14), or late onset myopes (LOM, myopia onset after 15 years of age, MRE >-0.50 D, N=14). A controlled, double blind experimental protocol was conducted to measure post-task open-loop accommodative regression following distance (0 D) or near (3 D above baseline tonic accommodation level) closed-loop tasks of short (10 second) or long (3 minute) duration. Closed-loop tasks consisted of observation of a high contrast Maltese cross target; open-loop conditions were imposed by observation of a 0.2 c/deg Difference of Gaussian function. All targets were viewed via a +5 D Badal system. Accommodation responses were recorded continuously at 42 Hz using a modified Shin-Nippon SRW-5000 open-view infra-red optometer, following topical instillation of 30 µl of 0.5% timolol maleate (non-selective ß-adrenoceptor antagonist), 60 µl of 0.5% betaxolol (selective ß₁ antagonist) or 30 µl of 0.9% saline (inert control). Retarded open-loop accommodative regression under ß₂ blockade following a sustained near task indicates the presence of sympathetic facility. Results: Repeated measures ANOVA was applied to 20s of open-loop accommodative regression data (averaged to 1s intervals) to identify subjects exhibiting increased accommodative hysteresis under the timolol condition following the long duration near task. Sympathetic inhibitory facility was identified in 8 out of 30 EMMs (27%), 3 out of 14 EOMs (21%), and 4 out of 14 LOMs (29%). Overall, 26% of subjects exhibited sympathetic facility. Conclusions: Sympathetic inhibition of accommodation mediated via ß₂ adrenoceptors is present in similar proportions in EMM, EOM and LOM refractive groups. The results demonstrate therefore, that sympathetic facility is not a specific attribute of early- or late-onset myopia, and its relatively equal distribution across myopia and emmetropia suggests that it is a constant feature of accommodative control across refractive groups.

Keywords: myopia • accommodation • neurotransmitters/neurotransmitter systems