A 23-year-old woman has an acute attack of optic neuritis. She has a history of relapsing-remitting multiple sclerosis. An MRI of her brain reveals an area of demyelination involving the right optic nerve. Which of the following is most likely to be found on examination of both eyes?

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11ec160b_35d3_63b8_9249_29c9ece816c9_MD0001_00 This patient with unilateral optic neuritis is most likely to demonstrate an afferent pupillary defect due to demyelination of the right optic nerve (CNII). The optic nerve carries the afferent limb of the pupillary light reflex, transmitting information about light entering the eye to the pretectal area in the midbrain. The pretectal area activates the Edinger-Westphal nuclei bilaterally which send parasympathetic efferent (motor) fibers down the oculomotor nerve (CN III) to innervate the pupillary sphincter muscle of each iris. The crossovers in this pathway allow light entering one eye to cause both direct pupillary constriction in the ipsilateral eye and consensual pupillary constriction in the contralateral eye. Patients with an afferent pupillary defect (eg, optic neuritis, retinal detachment) have sluggish or absent bilateral pupillary constriction when shining a light into the affected eye (in this case, a right afferent pupillary defect). However, signals from the eye on the unaffected side (the left eye in this patient) bypass the damaged optic nerve, resulting in normal pupillary constriction of both eyes (Choice E). (Choices B and D) An absent pupillary response in one eye when light enters either eye could occur with a defect in the efferent portion of the pupillary light reflex pathway carried by the oculomotor nerve (CNIII). Pupillary constriction in the eye ipsilateral to the lesion is impaired when light is shone in either eye; however, pupillary constriction in the contralateral eye occurs normally due to preserved afferent input bilaterally. (Choice C) This constellation of findings is not consistent with any specific condition affecting the pupillary light reflex pathway. Educational objective: The optic nerve carries the afferent limb of the pupillary light reflex pathway, which activates the efferent limb bilaterally and causes both direct and consensual pupillary constriction. Unilateral optic nerve lesions (eg, demyelination due to optic neuritis) can result in impaired pupillary constriction bilaterally when light enters the eye ipsilateral to the lesion. When light enters the contralateral eye, pupillary constriction occurs normally in both eyes.

A 23-Year-Old Woman Has an Acute Attack of Optic Neuritis