Accommodation and Presbyopia
Accommodation, the mechanism by which the eye changes focus from distant to near images, occurs when the action of the ciliary muscle on the zonular fibers changes the lens shape. The lens substance is most malleable during childhood and the young-adult years, progressively losing its ability to change shape with increasing age.
According to the Helmholtz theory of accommodation, most of the accommodative change in lens shape occurs at the central anterior lens surface. The central anterior capsule is thinner than the peripheral capsule (see Chapter 2, Fig 2-3), and the anterior zonular fibers insert slightly closer to the visual axis than do the posterior zonular fibers, resulting in a central anterior bulge during accommodation. The curvature of the posterior lens surface changes minimally with accommodation. The central posterior capsule, which is the thinnest area of the capsule, maintains the same curvature regardless of zonular tension.
The ciliary muscle is a ring-shaped muscle that, on contraction, has the opposite effect from what one intuitively expects of a sphincter. When a sphincter muscle contracts, it usually tightens its grip. However, when the ciliary muscle contracts, the diameter of the muscle ring is reduced, thereby relaxing the tension on the zonular fibers and allowing the lens to become more spherical. Thus, when the ciliary muscle contracts, the axial thickness of the lens increases, the equatorial diameter of the lens decreases, and the dioptric power of the lens increases, resulting in accommodation. When the ciliary muscle relaxes, the zonular tension increases, the lens flattens, and the dioptric power of the eye decreases (Table 3-1; Video 3-1).
Table 3-1 Changes With Accommodation
Changes with accommodation. Go to www.aao.org/bcscvideo_section11
to access all videos in Section 11.
The accommodative response may be stimulated by the known or apparent size and distance of an object or by blur, chromatic aberration, or a continual oscillation of ciliary tone. Accommodation is mediated by the parasympathetic fibers of cranial nerve III (the oculomotor nerve). Parasympathomimetic drugs (eg, pilocarpine) induce accommodation, whereas parasympatholytic medications (eg, atropine) block accommodation. Drugs that relax the ciliary muscle are called cycloplegics.
The amplitude of accommodation is the amount of change in the eye’s refractive power that is produced by accommodation. It diminishes with age and may be affected by some medications and diseases.
Presbyopia is the gradual loss of accommodative response, resulting from reduced elasticity of the crystalline lens. Once an individual is approximately 40 years of age or older, the rigidity of the lens nucleus reduces accommodation, as contraction of the ciliary muscle no longer results in increased convexity and dioptric power of the anterior surface of the lens. This decreased accommodation then becomes clinically significant. Studies have shown that, throughout life, the hardness or stiffness of the human lens increases more than 1000-fold. (See also BCSC Section 3, Clinical Optics.)
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.