The ciliary muscle initiates the crystalline lens’ adjustments to keep the retinal image in focus at various viewing distances. Previous studies found that both the muscle’s morphology and the accommodation behavior differ in subjects with different refractive errors.
Furthermore, there is evidence for a possible relationship between near vision and myopia development. Using a newly developed segmentation tool for optical coherence tomography (OCT) images, we assessed the ciliary muscle biometry and additionally the accommodation dynamics as well as their possible relationship in young adults with emmetropia and myopia.
Eighteen emmetropic and 20 myopic students in the age from 19 to 25 years with a mean refractive error in the spherical equivalent of 0.03 ± 0.30 D and -2.44 ± 1.04 D were included in the trial. Refractive power changes of the crystalline lens for the stimuli pattern far-near-far with a presentation duration of 15 s each (near distances 2.5 D, 3 D, 4 D; random order) were recorded via eccentric infrared photorefraction.
Subsequently, the right eye’s temporal ciliary muscle was imaged using anterior segment OCT while subjects first fixated a target at far, then at near distance (2.5 D, 3 D, 4 D; random order). The OCT images were analyzed with a custom-developed software allowing for selective anterior ciliary muscle thickness (CMT) readings, determination of the positions of scleral spur and ciliary muscle apex, and the plotting of continuous thickness profiles along the muscle’s boundary.
Myopic ciliary muscles were thinner in the anterior region, until 1.4 mm posterior to the scleral spur, and then became thicker than emmetropic until about 4.5 mm from the scleral spur. The muscle’s anterior thickness increased continuously with accommodation demand in myopes, but not in emmetropes. While the anterior thickness changes during accommodation were smaller in the myopic group, the ciliary muscle apex’ movement relative to the scleral spur was increased compared to their emmetropic counterparts.
The analysis of the accommodation dynamics measurements, which included the accommodation changes from far to near vision and vice versa, velocity, microfluctuations, power spectra, and lag of accommodation, revealed no significant differences between the refractive groups. Refractive error and anterior muscle thickness were significantly correlated with increased values in less myopic eyes. For the closest target distance of 4 D, larger anterior thickness changes were associated with lower lens power changes in the disaccommodation phase of the step pulse.
Novel continuous CMT profiles along the entire muscle boundary revealed remarkable anatomical differences between emmetropic and myopic eyes as for the ciliary muscle’s shape, thickness, as well as its movement and the CMT changes during accommodation. It might be possible that the larger muscle movement found in myopic eyes compensates for the lower anterior CMT changes. In contrast to previous reports, in our study, accommodation dynamics were not influenced by the subject’s refractive error.
This outcome raises the question whether the ciliary muscle structure changes prior to or as a consequence of myopia development and whether a specific fiber composition of the muscle affects this process. To further assess a possible influence of the ciliary muscle on eye growth, longitudinal investigations including children with different refractive errors are required.
Relevance to clinical myopia management:
If the ciliary muscle was found to be a causative factor in the development or progression of myopia, the assessment of the muscle’s morphology and its changes during accommodation might be instrumental in screening procedures with the anterior segment OCT being a non-invasive and fast examination tool. Moreover, it might be possible to use the morphologic changes of the ciliary muscle as an additional parameter for monitoring the treatment effects of myopia control interventions.
I completed a Bachelor of Science in Optometry and Audiology and a Master of Science in Optometry and Psychophysics both at Aalen University, Germany, and I am currently a PhD candidate at the Institute for Ophthalmic Research, University of Tuebingen, Germany.
1 Institute for Ophthalmic Research, Eberhard Karls University Tuebingen, Elfriede-Aulhorn-Str. 7, 72076 Tuebingen, Germany
2 Werner Reichardt Centre for Integrative Neuroscience (CIN) Tuebingen, Otfried-Mueller-Str. 25, 72076 Tuebingen, Germany
Published: Wagner, S., Zrenner, E., & Strasser, T. (2019). Emmetropes and myopes differ little in their accommodation dynamics but strongly in their ciliary muscle morphology. Vision research, 163, 42-51. Doi: 10.1016/j.visres.2019.08.002