In vivo assessment of ciliary muscle morphology using swept source optical coherence tomography

Abstract

Purpose : There is growing interest in understanding the role of ciliary muscle (CM) morphology in accommodation and myopigensis. Much of the current research has been conducted using time-domain optical coherence tomography (TD-OCT). More recently, swept source OCT (SS-OCT) has provided faster scan times and for new methods of assessing the CM, namely real-time viewing of the conjunctival area over-which the scan is acquired, and cross-sectional area capture of the CM. The study examines the utility of SS-OCT for assessing the CM.

Methods : Thirty phakic subjects aged 19 - 48 years were imaged using SS-OCT (Casia SS-1000, Tomey). The temporal (T) and nasal (N) CM cross-sections were evaluated in the relaxed state and at stimulus vergence levels of 4D and 8D. Using Casia software, manual measures of T and N ciliary muscle thickness (CMT) and area (CMA) were taken at CMT_max, and in successive posterior steps from the scleral spur at 1 mm (CMT₁, CMA₁), 2 mm (CMT₂, CMA₂) and 3 mm (CMT₃, CMA₃). Repeated assessment of CM morphology was conducted along a specific scan line and at varying positions to determine the effect of scan alignment using conjunctival images. Non-cycloplegic refractive error measurements were taken with an open field autorefractor (Grand Seiko Co. Ltd) using distance and 4D stimulae. Statistical analyses were conducted using mixed repeated measures ANOVAs. Repeatability was assessed using coefficient of variation (CoV).

Results : With increasing accommodative demand CMT_max (T: p<0.001, N: p=0.002), CMA₁ (T: p<0.001, N: p<0.001) and CMT₁ (T: p<0.001, N: p<0.001) all increased, whereas CMT₂ (T: p<0.001, N: p<0.001) and CMT₃ (T: p<0.001, N: p<0.001) decreased. Two of the area metrics did not change with increasing accommodative demand: CMA₂ (T: p=1.000, N: p=0.563) and CMA₃ (T: p=1.000, N: p=1.000). The use of a conjunctival reference point improved the CoV repeatability of T CMT on average from 9.40% to 1.76%.

Conclusions : The high resolution cross-sectional images of the CM provided by the Casia enables measurement of cross-sectional area; this additional parameter of CM morphology may assist with modeling the accommodative process and understanding the potential biomechanical role of the CM. Future studies employing repeated OCT scanning should, where possible utilize a conjunctival reference point to maximize repeatability.

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