The iris is a smooth muscular ring forming the pupil of the eye (Fig. 2B). Contraction and dilation of the pupil during the light reflex changes the amount of light entering the eye and is also important in the accommodation reflex. In the elderly, the iris appears paler in the middle, mainly due to loss of pigmentation in the radial dilator muscles. With aging, there is a mild but constant increase in the density of collagen fibers in the iris stroma and noncellular perivascular zones.
A characteristic ocular impairment in the elderly is a persistent reduction in the pupil size (diameter), the so-called "senile miosis" (13). Senile miosis is particularly notable in the fully dark-adapted eye; the reduction in diameter occurs gradually with aging, decreasing from a mean of 8 mm in the third decade to 6 mm in the seventh decade and 5 mm in the tenth decade of life (Fig. 4) (6). Senile miosis results from a relatively higher rate of aging atrophy in the radial dilator muscles, which dilate the pupil, compared to the sphincter constrictor muscles, which constrict it. As a result, the sphincter is constantly dominant in the elderly, causing persistent constriction. Compared to youth, the reduced pupil aperture of the elderly results in a one-third reduction in the amount of light entering the eye (13).
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