The effect of aging on the ciliary muscle and its potential relationship with presbyopia: a literature review.

Background: The ciliary muscle is known to play a part in presbyopia, but the mechanism has not received a comprehensive review, which this study aims to achieve. We examined relevant articles published from 1975 through 2022 that explored various properties of the muscle and related tissues in humans and rhesus monkeys. These properties include geometry, elasticity, rigidity, and composition, and were studied using a range of imaging technologies, computer models, and surgical methods. We identified a notable age-related displacement of the ciliary muscle apex that is characterized by anterior and medial shifts, and hypothesized to be primarily attributed to the accrual of connective tissue and tension exerted by the thickening lens. Other factors could also contribute to the movement, particularly the "inward bowing" of the sclera. Another noteworthy observation is that while the ciliary muscle experiences increasing constraint with advancing age due to adjacent anatomical structures, its contractile capacity remains unaltered, alongside the sustained constancy in both the concentration of muscarinic receptors and their binding affinity. Overall, more studies on human ciliary muscle are needed, as it ages differently from that of monkeys' ciliary muscle. These studies should explore other perspectives, including those regarding changes in the physical properties of the tissue and its relationship with other connected tissues. Methodology: This literature review utilized a systematic methodology to identify and analyze pertinent studies of the presbyopia and ciliary muscles. The approach encompassed a thorough examination of available literature across different academic databases, such as PubMed, Embase, and Cochrane Library. Results: Many studies have identified age-related thickening in the ciliary muscle and its potential causes, including the heightened deposition of connective tissues and traction exerted by the thickening lens. Notably, these inquiries culminated in the formulation of a geometric theory positing that the morphology of the ciliary muscle and its spatial relationship with adjacent structures exert pivotal influence over the tension exerted on zonular fibers. Conclusion: The decline in the accommodative response of the muscle is prevalent in advanced age, with reduced mobility likely attributable to the increased stiffness of the Bruch's Membrane-Choroid Complex (BMCC), where the tendons of the ciliary muscle insert, as well as the thickening and stiffening of the lens. Importantly, the ciliary muscle forces do not change with age.