The Repeatability Of Closed–loop Accommodative Adaptation

Abstract

Abstract: : Purpose: Sustained periods of nearwork will stimulate accommodative adaptation, resulting from a failure to relax the within–task accommodative response fully. This adaptation has been demonstrated under both open– and closed–loop conditions. Indeed, a number of workers have suggested that accommodative adaptation may be a precursor to permanent axial myopia. However, the repeatability of this sustained accommodative response, particularly when measured under naturalistic, closed–loop conditions has not been examined to date. Methods: Accommodative adaptation was measured on 3 different occasions, each of which was separated by a period of at least 24 hours, in 10 young, visually–normal individuals. Adaptation was stimulated by subjects performing a detailed, cognitively–demanding task at a viewing distance of 25cm for a continuous 10 min period. The refractive state of the eye was measured both before and immediately following the sustained near–task using an objective, infra–red optometer. In addition, the regression back to the base–line value was assessed during the 60s following completion of the near task. Results: The initial magnitude of accommodative adaptation was determined by calculating the post–task shift in the refractive state, with respect to the pre–task value, during the 10s period immediately following completion of the near–vision task. The mean value of this initial adaptation, averaged across the 3 trials, was +0.07D (SEM=0.03D; range +0.21 to –0.10D). Furthermore, the 95% limits of repeatability of this parameter were ±0.21D. Conclusions: These findings demonstrate the variability of closed–loop accommodative adaptation, and indicate that the magnitude of change required to exceed test–retest variability should be at least 0.25D. Many previous investigations of this closed–loop parameter, as well as the present study, have observed mean values of adaptation that are smaller than these 95% limits of agreement. The larger magnitude of adaptation observed under open–loop conditions, while being less naturalistic due to the elimination of blur–feedback, may provide a more repeatable assessment of changes in the accommodative response following sustained nearwork.

Keywords: adaptation: blur • myopia • refraction