A 54-year-old woman with an unremarkable medical history was in a minor motor vehicle collision resulting in airbag deployment. She reported loss of consciousness lasting a few seconds. She had no bruises or lacerations, but she immediately noted drooping of her left eyelid and double vision when she elevated the lid. She was taken to a local hospital, where she had an isolated, complete left oculomotor nerve palsy with a nonreactive pupil (figure). CT, MRI, and MR angiography were all normal (see figure E-1 on the Neurology Web site; go to www.neurology.org). Figure. Appearance of patient on initial presentation. Note complete third nerve palsy with pupil involvement. Over the next 6 weeks, the patient experienced mild improvement in ptosis, pupillary function, and ocular motility, but because of persistent diplopia and ptosis, she presented to the Wilmer Eye Clinic. On examination, the patient had normal visual sensory function, except for reduced accommodation on the left. The pupils were equal in size, but the left pupil was …