Purpose: The purpose of this study was to investigate the relationships between ciliary muscle thickness, refractive error, axial length, and accommodative lag in a sample of anisometropic adults.
Methods: The right and left eyes of 30 adult subjects were measured. All subjects possessed a difference of at least one diopter of refractive error between the two eyes. Accommodative lag to a 4.00 D target was measured. Then, cycloplegic measurements of the nasal ciliary muscle thickness (CMT), spherical equivalent refractive error, and axial length were made. Ciliary muscle thickness measurements were made at the maximum ciliary muscle thickness (CMTMAX) and at 1.0 mm (CMT1), 2.0 mm (CMT2), and 3.0 mm (CMT3) posterior to the scleral spur using the Zeiss Visante¿¿¿¿¿¿¿ Anterior Segment OCT. Simple linear regression and multilevel regression models were used to compare ciliary muscle thickness, refractive error, axial length, and accommodative lag both across and within subjects.
Results: Across subjects, CMT was significantly negatively associated with mean refractive error and significantly positively associated with mean axial length when refractive error and axial length were first averaged between the two eyes of each subject. This relationship was most pronounced for CMT2 (p < 0.0001). When CMT2 was controlled for, CMTMAX and CMT1, which represent the presumed radial/circular fibers of the ciliary muscle, are significantly positively associated with mean refractive error (p < 0.0001) and significantly negatively associated with mean axial length (p = 0.0002) across subjects. Within subjects, there was no significant difference in ciliary muscle thickness. Accommodative lag was significantly negatively associated with the thickness of the ciliary muscle, but only at CMT2 (p = 0.02) and CMT3 (p = 0.01). Within subjects, no difference in accommodative lag between the two eyes was observed.
Conclusions: Across subjects, this sample of anisometropic subjects behaves in accordance with the literature: thicker ciliary muscles are associated with increased myopic refractive error and axial length. Within subjects, however, this relationship ceases to exist, indicating that an eye can grow longer and more myopic than its fellow eye without resulting in an increase in ciliary muscle thickness. Across subjects, the longer, more myopic eyes tended to have thicker posterior portions of their ciliary muscle (posterior to CMT2) and lesser amounts of accommodative lag, while the shorter, more hyperopic eyes tended to have thicker anterior portions of their ciliary muscle and larger amounts of accommodative lag. This suggests a division in both structure and function at the position of CMT2. Specifically, the portion of the ciliary muscle posterior to CMT2 may be more implicated in sustaining accommodation and reducing lag, while the portion of the ciliary muscle anterior to CMT2 may play a greater role in accommodative workload.