The iris contains two sets of muscles that regulate pupillary size and reaction to light. With age, the pupil becomes smaller, reacts more sluggishly to light, and dilates more slowly in the dark. Thus, elderly persons may complain that objects are not as bright (a smaller pupil allows less light to enter the eye), that they are dazzled when going outdoors (slow pupillary constriction), and that they experience difficulty when going from a brightly lit environment to a darker one (slow pupillary dilation). If visual acuity is normal, only reassurance is needed.
Relative pupillary size and reaction to light can be evaluated in a dimly lit room by shining a penlight obliquely into each eye and observing constriction of the pupil in the illuminated eye and the contralateral eye. Because pupillary diameter decreases with age, the direct and consensual reactions to light tend to be reduced. If the pupillary response is sluggish or absent, the patient may be taking medication that causes pupillary constriction or dilation.