Pupillary constriction is a parasympathetic function and pupillary dilation is a sympathetic function (“wide eyed with fear”). The pupils constrict in response to light and accommodation, and dilate in response to darkness and adrenergic states. Pupillary asymmetry is referred to as anisocoria, and can be caused by a variety of neurologic and ophthalmologic conditions. Changes in pupil size can also be caused by medications. Miosis refers to an abnormally constricted pupil, and mydriasis refers to an abnormally dilated pupil (mnemonic: mydriasis is a longer word than miosis, and mydriasis refers to the larger pupil size [i.e., dilated]).
PUPILLARY CONSTRICTION: THE PARASYMPATHETIC PATHWAY
Pupillary constriction in response to light requires transmission of light information from the retina to the brain (afferent pathway), and signals from the brain to constrict the pupils (efferent pathway) (Fig. 10–1). The afferent pupillary light reflex fibers travel through the optic nerves, optic chiasm, and optic tracts, and then separate from the optic tracts to proceed to the pretectal nuclei of the dorsal midbrain (note the separation of light reflex afferents from visual pathway fibers at this point—the visual pathway fibers in the optic tracts proceed to the lateral geniculate nuclei; Fig. 6–1). In the dorsal midbrain, the pretectal nuclei communicate with the Edinger-Westphal nuclei, which give rise to the efferent pupillary constrictor fibers that travel in the oculomotor nerves (CN 3). These parasympathetic pupillomotor fibers of CN 3 synapse in the ciliary ganglion in the orbit, and short ciliary neurons arising from the ciliary ganglion innervate the pupillary constrictor muscles of the iris. Each pretectal nucleus projects bilaterally to both Edinger-Westphal nuclei so that both pupils constrict equally in response to light input from either side. For example, light shined in the left eye causes constriction of both the left pupil (direct response) and the right pupil (consensual response) and vice versa.
The parasympathetic pathway for pupillary constriction. Reproduced with permission from Aminoff M, Greenberg D, Simon R: Clinical Neurology, 9th ed. New York, NY: McGraw Hill; 2015.
Impaired Pupillary Constriction
Impaired pupillary constriction to light can be caused by dysfunction in the afferent pathway (most commonly CN 2 dysfunction) or the efferent pathway (CN 3 dysfunction).
Impaired Pupillary Constriction due to a Lesion of the Afferent Pathway
If CN 2 is not functioning properly on one side (e.g., optic neuritis), no light will enter on that side, and so there will be neither a direct (ipsilateral) nor a consensual (contralateral) response to light shined on the affected pupil. However, since CN 2 is functioning on the unaffected side and both CN 3s are functioning, both pupils will constrict in response to light shined in the unaffected eye.