A 32-year-old woman comes to the office to establish medical care. The patient reports no symptoms. Medical history is significant for migraine headaches for which she takes acetaminophen and naproxen as needed. Vital signs are within normal limits. Physical examination is unremarkable except for anisocoria. Pupillary examination findings in normal, bright, and dim light are shown in the image below:
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This patient's pupillary abnormality is likely due to a defect in which of the following pathways?
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This patient has incidental anisocoria (asymmetric pupils). Pupil size is governed by the balance between ocular sympathetic (dilation) and parasympathetic (constriction) efferents to the iris. These responses are tested by examining the pupils in bright and dim light:
In bright light, the parasympathetic pathway is dominant. With parasympathetic defects, pupillary constriction occurs in the "good" eye but not the "bad," causing the difference between pupils to widen (ie, anisocoria increases) in bright light. The larger pupil is abnormal (Choice C).
In dim light, the sympathetic pathway becomes dominant. With sympathetic defects, pupillary dilation occurs in the "good" eye but not the "bad," causing anisocoria to increase in the dark. The smaller pupil is abnormal.
This patient's anisocoria increases in bright light, consistent with a parasympathetic defect causing her right pupil to not constrict properly in response to light; the unaffected left pupil constricts normally. In the dark, both pupils dilate because sympathetic input is intact bilaterally; a small degree of anisocoria will still be present because basal parasympathetic activity is reduced in the left eye.
(Choices A and B) The visual pathway carries afferent signals from the retina to the visual cortex. Lesions preceding the optic chiasm (eg, retina, optic nerve [CN II]) cause monocular vision loss; however, pupils remain symmetric (no anisocoria) with ambient lighting because the efferent sympathetic or parasympathetic tracts are not involved. Instead, prechiasmatic visual pathway lesions cause a relative afferent pupillary defect: When light is shone in the affected eye, no signal is transmitted, so pupillary constriction is equally impaired bilaterally.
(Choices E and F) With sympathetic defects (eg, damage to cervical sympathetic chain or ganglia), the affected pupil cannot properly dilate, so anisocoria increases in dim light. In response to bright light, both pupils constrict because parasympathetic innervation is intact bilaterally, and the anisocoria is reduced.
Educational objective:
Anisocoria (asymmetric pupils) is caused by a lesion in the ocular parasympathetic (constriction) or sympathetic (dilation) pathways. In this patient, anisocoria increases in bright light, indicating that the larger right pupil is unable to constrict due to a defect in the right parasympathetic pathway.