Clinical Reasoning: A 20-year-old man with headache and double vision.

SECTION 1 A 20-year-old man presented to the emergency department with 1 week of headaches and double vision following 2 days of fever (1028F), nausea, and vomiting. His headache was progressively worsening, throbbing behind his right eye, nonpositional, and associated with photophobia, blurry vision, and pain with eye movement. Occasionally, it was severe enough to wake him up from sleep. Horizontal double vision ensued soon after the headache. His diplopia was worse looking at a distance, improved on leftward gaze, worsened on rightward gaze, and resolved with closing one eye. He denied neck stiffness, focal weakness, numbness, or other neurologic symptoms. He denied recent rashes, infections, or tick bites. He lives on a farm in central Brazil. He arrived in Massachusetts 2 months before his presentation to visit family members. He had no significant medical history and took no medications. He did not smoke or use drugs.