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 involves the detection of light by the retina and transmission of that information to the brain by way of the optic nerves (cranial nerve (CN) 2) (Fig. 10–1). Each CN 2 transmits light information to the pretectal nuclei of the dorsal midbrain, which communicate with the Edinger-Westphal nuclei of the dorsal midbrain, which in turn transmit pupillary constrictor fibers to the oculomotor nerves (CN 3). Each CN 3 synapses with the ciliary ganglion in each orbit, and short ciliary neurons arising from each ciliary ganglion innervate each iris. Each pretectal nucleus projects bilaterally to both Edinger-Westphal nuclei so that both pupils constrict 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: McGraw-Hill Education; 2015.
Impaired Pupillary Constriction
Pupillary constriction to light can be impaired due to CN 2 (afferent) dysfunction, CN 3 (efferent) dysfunction, or lesions involving the pretectal or Edinger-Westphal nuclei in the midbrain.
Impaired Pupillary Constriction Due to a Lesion of Cranial Nerve 2
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.
For example, if there is a lesion of CN 2 on the right (and the right CN 3, left CN 2, and left CN 3 are all intact), there will be no (or minimal) pupillary constriction in either eye when light is shined in the right eye, but both pupils will constrict when light is shined in the left eye (Fig. 10–2). In ...