Prolonged nearwork affects the ciliary muscle morphology

Abstract

There is evidence for a possible link between myopia development and near vision. We investigated the effect of prolonged nearwork on ciliary muscle (CM) morphology and accommodation in 18 myopic and 17 emmetropic subjects (age 19 to 25). The CM was imaged during far (0.25 D) and near vision (4 D) using optical coherence tomography (OCT), and accommodation to a step pulse (0.25 D - 4 D - 0.25 D, 15 s each) was assessed by eccentric infrared photorefraction before and after a 30-min reading task at 25 cm. OCT images were analyzed using a custom-developed semi-automatic segmentation algorithm to determine CM thickness (CMT) profiles and selective CMT readings. Accommodation was assessed using a non-linear model. On average, the CM got thinner after nearwork, predominantly at 0.0–1.4 mm posterior to the scleral spur in emmetropes, and at 1.0–1.9 mm in myopes. Selective CMT readings confirmed a significant thinning after nearwork (univariate ANOVA F1,66 = 26.313, p < 0.001), without any influence of the subjects’ refractive state (F1,66 = 1.887, p = 0.174) or the target distance (F1,66 = 0.014, p = 0.907). The mean accommodation response for targets at infinity was significantly increased after nearwork (F1,32 = 7.775, p = 0.009), with a larger myopic shift in myopes (F1,32 = 11.310, p = 0.002). No change in velocity of accommodation was found. Sharing properties of striated muscles, the CM was expected to increase its thickness, but the opposite was found. Previous studies suggesting sustained nearwork to result in a CM spasm cannot be confirmed by the data presented here. Further research exploring the possible impact of sympathetic innervation is necessary as it is activated during intense nearwork.