Age-related loss of morphologic responses to pilocarpine in rhesus monkey ciliary muscle.

Ciliary muscle topography and connective tissue distribution were studied by light microscopy in atropinized, pilocarpinized, or untreated eyes from rhesus monkeys of various ages. With age, the connective tissue ground plate between ciliary muscle and ciliary processes thickens, while there is very little increase in connective tissue within the ciliary muscle. With age, the atropinized muscle becomes shorter and smaller in area while it remains unchanged in width and position. In pilocarpinized eyes, the ciliary muscle is shorter, narrower, smaller in longitudinal and total area (ie, more circular and compact), and positioned more anteriorly than in contralateral atropinized eyes. These contractile responses to pilocarpine diminish with age at a rate similar to that for accommodative decline. According to these topographic findings, physicians seeking the pathophysiologic characteristics of presbyopia, which occurs in humans and rhesus monkeys on a comparable relative time scale, should redirect their attention toward the ciliary muscle.