Characteristics of ciliary muscle profile in high myopes measured by swept-source anterior segment optical coherence tomography

Abstract

Objective: To characterize and compare the ciliary muscle thickness (CMT) between low and high myopes using swept-source anterior segment optical coherence tomography (AS-OCT).

Methods: Forty visually healthy young Chinese adults aged 18-25 years were divided into two groups based on refractive errors: low myopia (n = 20, spherical-equivalent refractive error (SER) between -0.50 D to -3.00 D) and high myopia (n = 20, SER ≤ -6.00 D). Cycloplegic refractions were performed before axial length (AL) and CMT were measured using a partial coherence laser interferometer and an AS-OCT respectively. CMT was measured perpendicularly to the sclera-ciliary muscle interface at 1 mm (CMT_1), 2 mm (CMT_2), and 3 mm (CMT_3) posterior to the scleral spur, and at the location with maximal thickness (CMT_MAX).

Results: High myopes demonstrated thicker CMT at 2 mm (CMT_2, p = 0.035) and 3 mm (CMT_3, p = 0.003) posterior to the scleral spur, but thinner maximal CMT (CMT_MAX, p = 0.005) than low myopes. The apical CMT_1 and CMT_MAX were also thinner in high myopes than in low myopes (both p< 0.001). CMT_MAX, apical CMT_1, and apical CMT_MAX correlated directly with SER and inversely with AL; in contrast, CMT_2 and CMT_3 showed inverse correlations with SER but direct correlations with AL.

Conclusion: Our findings revealed significant differences in CMT between low and high myopes, with high myopes showing thicker CMT at 2 mm and 3 mm posterior to the scleral spur, but thinner maximal CMT. These results provide new evidence of the potential structural differences in ciliary muscles during myopia development and progression.

Fig 1. Flow chart depicting the experimental design.

Fig 2. A representative image of nasal ciliary muscle captured by the AS-OCT.

Ciliary muscle thicknesses at the nasal side were measured at the location (red circle) 1 mm below the pupil center (yellow star) and 3 mm nasally from the limbus.

Fig 3. Representative images of the nasal ciliary muscle captured by AS-OCT.

(A) The raw AS-OCT image. (B) The processed AS-OCT image. The dashed line encloses the region of interest, while the solid lines indicate the CMT measured at different locations: 1 mm (CMT_1), 2 mm (CMT_2) and 3 mm (CMT_3) posterior to the scleral spur or at the point of maximal thickness (CMT_MAX). Color-shaded areas indicate individual CMT regions: blue (CMT_3), yellow (CMT_2), and pink (CMT_1 and CMT_MAX).

Fig 4. Bland-Altman plots showing the agreement between the two sets of regional CMTs.

In each plot, the solid blue line and dotted red lines represent the mean difference and the 95% limits of agreement, respectively.

Fig 5. Correlations between CMTs and biometric parameters at different locations.

CMTs (A&B) and apical CMTs (C&D) were plotted against SER (Left) and AL (Right). Circle and cross symbols represent low and high myopes, respectively. Red: CMT_1, blue: CMT_2, green: CMT_3, black: CMT_MAX. Pearson’s r is presented. *p<0.05, **p<0.01, ***p<0.001.