EFFECTS OF AGING ON LENS AND CILIARY MUSCLE FUNCTION AND AQUEOUS HUMOR DYNAMICS





M.A. Croft*1, B. Gabelt*1, A. Glasser2, G.A. Heatley1, P.L. Kaufman1

1Department of Ophthalmology and Visual Sciences, University of




Wisconsin-Madison, Madison, WI 53792; 2College of Optometry, Univ. of Houston Aging has a major impact on the function of the ciliary muscle (CM) of the eye which is involved in regulating fluid (aqueous humor) flow and therefore intraocular pressure (IOP), and in changing focus of the eye for distance and near objects by its action on the eye lens (accommodation). Age-related changes in the CM may affect IOP and therefore may lead to glaucoma. Presbyopia, the age-related loss of the ability to focus the eye, is the most common ocular affliction in both humans and rhesus monkeys We have undertaken in vivo studies in rhesus monkeys to: (1) document age-related changes in the functional interaction between the CM and lens in relation to presbyopia; (2) investigate age-related changes in aqueous humor formation (AHF) and drainage.

(1) The iris was removed in 16 rhesus (Macaca mulatta) aged 6 to 24 yr to observe CM and lens accommodative movements. An electrode permanently implanted in the Edinger-Westphal nucleus of the brain was used to stimulate accommodation, which was measured optically. The accommodative response was measured before and after various lens surgeries to study the function of each component. (2) Radiolabelled monkey albumin was circulated through the eyes of 24 rhesus, ages 3-29 yr. Dilution of radioactivity in the eye with newly formed aqueous and recovery of radioactivity in the blood gave a measure of AHF and outflow through the trabecular pathways (FTB). Outflow through the CM (Fu) was calculated from the difference.

(1) Accommodative amplitude stimulated by central electrical stimulation declined age dependently (-0.74±0.08 diopters/yr; p=0.001). In young monkeys, the lens and its elastic sheath, the capsule, facilitate accommodative movements of the CM. In older monkeys the extent and velocity of the accommodative movements of the CM/lens/capsular complex are reduced. In old monkeys, removal of lens substance improves, but does not fully restore CM accommodative movements. In young, but not old monkeys, accommodative contraction of the CM causes a forward movement past its anterior insertion, the scleral spur. This movement of the CM was restored in an old monkey in which, the posterior suspensory elastic zonule was almost completely removed. (2) AHF remained unchanged with age. Fu and the ratio Fu/AHF were significantly decreased in monkeys age 25-29 yr compared to those 3-23 yr (0.24±0.05 vs 0.53±0.09 respectively, p=0.012). Conversely, FTB tended to increase in the oldest monkeys.

(1) Collectively, these data suggest age-related loss of capsule elasticity/tone and/or stiffening of posterior attachments of the CM, without excluding a lenticular hardening / sclerotic component to presbyopia pathophysiology. Surgically disturbing the lens-zonule complex can be used to probe normal accommodative function and age-related changes. (2) The decline in Fu in very old monkeys is comparable to findings in humans, suggesting that rhesus monkeys can serve as a model for investigating outflow through this pathway. Supported by NIH grant numbers EY10213, EY02698, RR00167 and RPB.







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