A middle aged woman comes to her physician's office with complaints of visual difficulties. A review of systems and physical examination are unremarkable except for her eye exam. When a light is shined in her right eye, there is no pupillary response in either eye. However, upon shining a light in her left eye, both ipsilateral and contralateral pupillary responses are apparent. Her extraocular movements are intact. What is the most likely location of her lesion?

Know your cranial nerves! This woman has a "Marcus-Gunn pupil," with a defect in the afferent pathway of the optic nerve (in this case on the right side). Recall that the afferent limb of the pupillary light reflex is the optic nerve (CN II); the efferent limb is the oculomotor nerve (CN III; parasympathetic fibers). When light is shined into her right eye, because her right optic nerve is not functioning properly, the light signal is not transmitted to the CNS, resulting in no pupillary response. As light is shined into her left eye, the left optic nerve transmits the signal to the CNS, which then sends an outbound signal through both the right and left oculomotor nerves to cause pupillary constriction in both eyes. The oculomotor nerve innervates all extraocular muscles except the lateral rectus (innervated by the abducens nerve), and the superior oblique (innervated by the trochlear nerve. The oculomotor nerve also mediates pupillary constriction (parasympathetic fibers), eyelid opening (levator palpebrae), and innervates the ciliary muscle (allowing accommodation). A patient with a lesion of the left optic nerve would have no pupillary responses in either eye when shining a light in the left eye; pupillary responses would be present in both eyes when shining a light in the right eye. Ref: Chang D.F. (2011). Chapter 2. Ophthalmologic Examination. In P. Riordan-Eva, E.T. Cunningham, Jr. (Eds), Vaughan & Asbury's General Ophthalmology, 18e.