Refractive group differences in accommodation microfluctuations with changing accommodation stimulus.

Abstract

PURPOSE: Microfluctuations of accommodation are known to increase in magnitude with increasing accommodation stimulus. Reduced sensitivity to blur in myopic subjects could also lead to increases in the magnitude of the microfluctuations. The aim of this study is to examine the effect of variations in accommodation stimulus upon the microfluctuations in different refractive groups. METHODS: Thirty-six subjects were divided into three groups depending upon their refractive error and age of onset of their myopia; 12 emmetropes (EMMs), 12 early onset myopes (EOMs) and 12 late-onset myopes (LOMs). Steady-state accommodation responses were recorded continuously for 2 min using the Shin-Nippon SRW-5000 autorefractor at a sampling rate of 52 Hz while viewing targets at accommodation stimuli levels of 0, 1, 2, 3 and 4 D in a Badal (+5 D) optical system. RESULTS: The EMMs and EOMs showed systematic increases in the root mean square (r.m.s.) value of the microfluctuations with increasing accommodation stimulus. In contrast, no systematic variation with accommodation stimulus was found for the LOMs. Power spectrum analysis demonstrated that increases in the size of the microfluctuations were mediated by increases in the power of the low frequency components of the accommodation response. CONCLUSIONS: The magnitude of the microfluctuations in the EMMs and EOMs may be influenced primarily by accommodation response-induced zonular relaxation effects or to changes in the physical properties of the accommodation plant with increasing accommodation response. The LOMs may have an increased baseline neural blur threshold, which appears to modulate the magnitude of the accommodative microfluctuations for low accommodation levels. At higher accommodation demands, the changes in the physical properties of the accommodation plant or the zonular relaxation effects appear to exceed the blur threshold, and the known association between microfluctuations and accommodation stimulus level is restored.