Aqueous humor dynamics and trabecular meshwork and anterior ciliary muscle morphologic changes with age in rhesus monkeys.

PURPOSE To determine in rhesus monkeys the age-dependence of uveoscleral outflow (Fu) and morphology of the trabecular meshwork (TM) and anterior ciliary muscle (CM). METHODS Intraocular pressure (IOP) was measured by Goldmann applanation tonometry in monkeys under ketamine anesthesia. After anterior chamber cannulation under pentobarbital anesthesia, aqueous humor formation (AHF), anterior chamber volume, trabecular outflow, and Fu were determined isotopically. The CM and TM were examined by light and electron microscopy. RESULTS IOP increased significantly with age in monkeys aged 3 to 29 years. AHF and anterior chamber volume were unchanged. Fu was decreased, and trabecular outflow increased in monkeys aged 25 to 29 years compared with the remaining monkeys. Morphologically, there was a significant increase in the thickness of the elastic fibers of the trabeculum ciliare covering the anterior tips of the CM, and an increase in extracellular material between the muscle tips. The number of TM cells decreased with age, whereas the amount of fibrillar material and sheath-derived plaques increased. This increase was less pronounced in the middle filtering portion of the cribriform region than in the anterior and posterior portions. CONCLUSIONS The decline in Fu in very old rhesus monkeys with normal IOP parallels that seen in normotensive aging humans. This may be correlated with thickening of the elastic fiber sheath in the CM tips in addition to other morphologic changes. The TM findings are analogous to those in the aging human eye and are consistent with the age-related decrease in outflow facility reported in both humans and monkeys.