Accommodation and Presbyopia
Accommodation is the mechanism by which the eye changes refractive power by altering the shape and position of its crystalline lens. The changes in lens geometry that create this alteration were first described by Helmholtz. The posterior focal point is moved forward in the eye during accommodation (Fig 3-13A). Correspondingly, the far point moves closer to the eye (Fig 3-13B). Accommodative effort occurs when the ciliary muscle contracts in response to parasympathetic stimulation, thus allowing the zonular fibers to relax. The outward-directed tension on the lens capsule is decreased, and the lens becomes more convex, possibly in response to a pressure gradient pressing the lens forward against a “sling” formed by the zonular fibers and the anterior lens capsule (“catenary suspension”). Accommodative response results from the increase in lens convexity (primarily the anterior surface) and the net forward displacement of the lens. It may be expressed as the amplitude of accommodation (in diopters) or as the range of accommodation, the distance between the far point of the eye and the nearest point at which the eye can maintain focus (near point). It is evident that as the lens loses elasticity from the aging process, the accommodative response wanes, a condition called presbyopia (Greek, “old eyes”) even though the amount of ciliary muscle contraction (or accommodative effort) is virtually unchanged. For an eye with presbyopia, the amplitude is a more useful measurement for calculating the power requirement of the additional eyeglass lens. For appraising an individual’s ability to perform a specific visual task, the range is more informative.
Figure 3-13 Emmetropia with accommodation stimulated. A, Parallel light rays now come to a point focus in front of the retina, forming a blurred image on the retina. B, Light rays emanating from a point on the retina focus to a near point in front of the eye, between optical infinity and the cornea.
(Illustration by C. H. Wooley.)
Coleman DJ, Silverman RH, Lloyd H, Physiology of accommodation and role of the vitreous body. In: Sebag J, ed. Vitreous: in Health and Disease. New York: Springer; 2014.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.