Accommodation and presbyopia.

Accommodation is a dioptric change in power of the eye that occurs to allow near objects to be focused on the retina. The ability to accommodate is lost with increasing age in humans and monkeys. This phenomenon, called presbyopia, is the most common human ocular affliction, and its pathophysiology remains uncertain. The progressive loss of human accommodative amplitude begins early in life and results in a complete loss of accommodation by age 50 to 55 years. Presbyopia is correctable by various optical means and, although not a blinding condition, its cost in devices, lost productivity, and (more recently), for surgical interventions is considerable. The classic theory of accommodation in humans proposes that the ciliary muscle moves forward and axially in the eye during contraction, releasing tension on the anterior zonular fibers and allowing the lens to become more spherical and thicken axially. During disaccommodation, the ciliary muscle relaxes, allowing the elastic choroid to pull the ciliary muscle posteriorly, increasing the tension on the anterior zonules to flatten the lens. Alteration of every component of the accommodative apparatus has been proposed to explain presbyopia. Rhesus monkeys and humans exhibit a similar accommodative mechanism and lens growth throughout life and develop presbyopia with a similar relative age course. Theories to explain the pathophysiology of presbyopia fall into two main categories, involving dysfunction of either the lens or the ciliary muscle. Another theory is based on a proposed mechanism of accommodation different from that which is classically accepted. We summarize what is known about the anatomy and aging of the accommodative apparatus and how such changes might contribute to the loss of accommodative amplitude.