The goal of this article is to review the anatomy and physiology of pupillary function and then employ that information to develop a comprehensive framework for understanding and diagnosing pupillary disorders.
The contribution of rods and cones to the pupillary light reflex has long been known. A third photosensitive cell type, the intrinsically photosensitive retinal ganglion cell, has recently been discovered. This cell type employs melanopsin to mediate a portion of the pupillary light reflex independent of rods and cones (the postillumination pupillary response) and photic regulation of circadian rhythm.
The autonomic nervous system regulates pupil size in response to stimuli. The parasympathetic nervous system causes miosis in response to light and near visual stimuli. These stimuli activate supranuclear pathways that project to the Edinger-Westphal nuclei. The sympathetic nervous system causes mydriasis in response to a variety of arousing factors, both physiologic (wakefulness) and pathologic (pain). Abnormalities of physiologic function cause disturbances of pupil size, shape, and response to stimuli. The clinical approach to pupillary abnormalities should focus on the clinical and pharmacologic assessment of the pupil’s expected response to diverse stimuli.