Effect of Cycloplegia on the Microfluctuations of Steady-State Ocular Accommodation

Abstract

Purpose:: Under steady-state viewing, ocular accommodation exhibits microfluctuations with a magnitude typically less than 0.3 D. These are thought to be generated both by the accommodative neural control system & by mechanical fluctuations resulting from cardiopulmonary pulsations. The relative contributions of these two factors were assessed by experimentally blocking neural innervation to ciliary muscle using cycloplegia.

Methods:: 6 emmetropic adults (22 - 28 yrs) accommodated monocularly (right eye) to a broadband target while their accommodation was measured binocularly using a PowerRefractor (25 Hz sampling rate). 6 experimental repetitions were conducted on each subject - 1^st repetition, before instilling the drug (1 drop of 1.0% Cyclopentolate in right eye) & 2^nd - 6^th repetitions, were conducted once every 8 min for 40 min after the drug instillation. Each repetition consisted of two parts. First, the steady-state accommodative response to a 2 D stimulus was measured for 2.0 min. The amplitude spectrum of the right eye response was compared to the amplitude spectrum obtained from a model eye (PowerRefractor noise spectrum). Second, the accommodative gain (gain = response / stimulus) to defocus stimuli (0.5 - 1.75 D) was measured in right eye & left eye (uncyclopleged & occluded with IR filter) & the mean gain was assessed as a function of time after the drop in right eye.

Results:: The amplitude spectrum obtained before cycloplegia showed typical fluctuations at frequencies less than 1.5 Hz. The mean summed amplitude at < 0.5 Hz was 0.27 +/- 0.08 D, which was larger than that obtained from a model eye (0.09 +/- 0.02 D) (p-value: <0.01). The fluctuations in the right eye at low frequencies progressively decreased following drug instillation & by the third repetition (16 min after the drop), the mean amplitude was not significantly different (0.15 +/- 0.05 D) from the model eye (p-value: >0.5). The summed amplitude at higher frequencies (1.0 - 2.2 Hz) showed large variability & was not significantly different from that of model eye after the drop was instilled. Accommodative gain in the right eye also decreased significantly with cycloplegia (pre-cycloplegia: 0.83 +/- 0.2; 3^rd repetition: 0.1 +/- 0.2; p-value: <0.01) while the gain in the left eye increased significantly indicating an increased neural effort to accommodate.

Conclusions:: The rapid decrease in amplitude of low frequency microfluctuations with cycloplegia & increased neural effort after cycloplegia confirm earlier reports & suggests that neural input to the ciliary muscle predominantly determines the characteristics of low frequency fluctuations.

Keywords: ciliary muscle • refraction • ocular motor control