Clinical Reasoning: A 73-year-old man with diplopia and ataxia.

SECTION 1 A 73-year-old right-handed man with a history of hypertension and hyperlipidemia presented with an 18-month history of diplopia and unsteady gait. He also noted oscillating vision when turning his head to the left. The diplopia was horizontal and was worse looking at objects on his right or at a distance. Over a year, his balance worsened to the point where he required a walker due to recurrent falls. Six months prior to presentation, he developed dysphagia (liquids more than the solids). He was also noted to have short-term memory problems in the last 3 months. On examination, he had moderate cognitive impairment with a Montreal Cognitive Assessment (MoCA) score of 20/30 with predominant deficits in visuospatial functions and language. Glabellar, bilateral palmomental, and snout reflexes were present. The left pupil was surgical and nonreactive to light, and there was mechanical ptosis on the left. He had left exotropia with full eye movements, bilateral horizontal gaze-evoked nystagmus, and upbeat nystagmus on upgaze. Motor strength and tone were normal. He was diffusely hyperreflexic with bilateral extensor plantar responses. Detailed sensory examination was normal. Dysmetria was present bilaterally, more prominent on the left compared to right, with finger to nose testing and finger chase. His gait was ataxic with wide base and a tendency to fall toward the left side. He was unable to tandem walk (video on the Neurology® Web site at Neurology.org). Systemic examination was unremarkable.