Accommodation, the mechanism by which the eye changes focus from distant to near images, is produced by a change in lens shape resulting from the action of the ciliary muscle on the zonular fibers. The lens substance is most malleable during childhood and the young adult years, progressively losing its ability to change shape with age. After approximately 40 years, the rigidity of the lens nucleus clinically reduces accommodation because the sclerotic nucleus cannot bulge anteriorly and change its anterior curvature as it could before. Recent studies have shown that, throughout life, the hardness or stiffness of the human lens increases more than 1000-fold.
According to the classic theory of von Helmholtz, 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 Fig 1-3 in Chapter 1), and the anterior zonular fibers insert slightly closer to the visual axis than do the posterior zonular fibers, resulting in a central anterior bulge with accommodation. The posterior lens surface curvature changes minimally with accommodation. The central posterior capsule, which is the thinnest area of the capsule, tends to bulge posteriorly to the same extent regardless of zonular tension.
The ciliary muscle is a ring that, upon contraction, has the opposite effect from that intuitively expected 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, its diameter decreases, and its dioptric power increases, producing accommodation. When the ciliary muscle relaxes, the zonular tension increases, the lens flattens, and the dioptric power of the lens decreases (Table 3-1).
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 (oculomotor). 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. Adolescents generally have 12–16 D of accommodation, whereas adults at age 40 have 4–8 D. After age 50, accommodation decreases to less than 2 D. It is thought that hardening of the lens with age is the principal cause of this loss of accommodation, which is called presbyopia. Research is under way into other possible contributing factors in presbyopia, such as changes in lens dimensions, in the elasticity of the lens capsule, and in the geometry of zonular attachments with age.
, KaufmanPL. Accommodation and presbyopia. In: Kaufman PL, Alm A, eds. 10th ed. St Louis: Mosby;2003:197–233.