Approach to differentiating lesions (brainstem) Quiz: Ace Your Exams

A 72-year-old man presents to the emergency department with a progressive, constant headache over the past week, which is associated with frequent vomiting. The patient has also had abdominal pain for the past three months. He has been evaluated by his primary care physician and a gastroenterologist, and his symptoms have persisted despite treatment with an H2 receptor antagonist and proton pump inhibitor. On review of systems, he endorses fatigue and an unintentional 13-pound weight loss. His past medical history is significant for hypertension, for which he takes lisinopril. His temperature is 37.0°C (98.6°F), pulse is 84/min, respirations are 12/min, blood pressure is 130/82 mmHg, and oxygen saturation is 98% on room air. On physical examination, he appears frail and uncomfortable, but he is alert and converses easily without confusion. Neurologic examination reveals impaired extraocular movements, right facial droop, rightward deviation of the tongue, and horizontal diplopia. Which of the following diagnostic tests would be most helpful in confirming the diagnosis?