A 78-year-old man is evaluated for vertigo that began abruptly 2 days ago. The vertigo waxes and wanes in severity, with each episode lasting several hours, and is accompanied by visual blurring, nausea, low-grade headache, and unsteadiness on his feet. He reports no chest pain, shortness of breath, or recent head trauma. He does not smoke cigarettes and drinks one 4-ounce glass of wine daily. Medical history is remarkable for myocardial infarction 5 years ago, type 1 diabetes mellitus, hypertension, and hyperlipidemia. Medications are insulin glargine, lisinopril, atorvastatin, and aspirin.
On physical examination, temperature is 37.5 °C (99.5 °F), blood pressure is 162/80 mm Hg, pulse rate is 78/min, and respiration rate is 15/min. BMI is 24. The neurologic examination is notable for impaired near vision, a Dix-Hallpike maneuver that reveals 8 beats of lateral nystagmus without delay, and an unsteady gait with leaning to his right side. The remainder of the physical examination is normal.
Laboratory studies show a plasma glucose level of 244 mg/dL (13.5 mmol/L).
Electrocardiogram reveals a normal sinus rhythm, evidence of an old inferior myocardial infarction, and no acute ST- or T-wave changes.
Which of the following is the most appropriate diagnostic test to perform next?