Ciliary muscle thickness profiles in far and near accommodation

Abstract

Purpose : Interest in ciliary muscle (CM) imaging has lately increased, especially in myopia and presbyopia research, however, no standardized method for biometry exists. We performed a pilot study testing a new approach to analyze the CM’s morphologic changes during accommodation using optical coherence tomography (OCT).

Methods : Right eye’s temporal CM of 13 near-emmetropic subjects (age 29±5yr; refractive error spherical equivalent -0.04±0.34D) was imaged using Zeiss Visante AS-OCT while subjects fixated a text with their fellow eye, presented in random order at 0D or 3D distance. 6 images were taken in each accommodative state, with subject repositioning after 3 images. An in-house semi-automatic algorithm was applied for CM segmentation in raw DICOM images. Refraction distortion was corrected using Snell’s law (Fig.1). Intra-session repeatability was tested by comparing CM area (CMA) before vs. after repositioning. Intra-observer repeatability was determined by independently positioning segmentation landmarks twice and calculating CMA. CM thickness profiles for far and near accommodation were averaged, respectively.

Results : Intra-session repeatability was found to be good (ICC=0.879), however, repeated landmark positioning revealed a significant effect on CMA (repeated measures ANOVA F_1,24=7.715, p=.010). Averaged thickness profiles (Fig.2) indicated the CM to exhibit thickness changes for near compared to far vision predominantly within 0.02 to 0.20mm from scleral spur (SP) (median -39.4µm) and within 0.40 to 1.60mm (median +31.8µm). Evaluating the displacement of ciliary apex (CA) vs. SP, the CM’s forward and inward movement during accommodation was confirmed: CA was positioned 0.53±0.25mm in front of SP during far vision, and shifted to 0.17±0.36mm behind SP during near vision (paired t-test, t₁₂=-6.133, p<.001, two-tailed). While the distance between CA and SP was 0.92±0.14mm for far vision, it decreased to 0.85±0.10mm for 3D accommodation.

Conclusions : We present a novel, semi-automatic approach for analyzing the CM’s anatomy using OCT that facilitates formation of CM thickness profiles and definition of areas of largest change during accommodation, rather than taking punctual or proportional readings. Repeatability analysis disclosed consistency within one cycle, but a possible examiner’s training effect in segmenting over time. We aim at applying this tool for an in-depth comparison of CM morphology in emmetropes vs. myopes.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.