Clinical Reasoning: A 79-Year-Old Man With Subacute Onset Involuntary Facial Movements and Discoordination.

A 79-year-old man with metastatic Merkel cell carcinoma undergoing cancer-directed treatment was admitted to the hospital with 1 week of bilateral upper and lower extremity tremor and nearly continuous involuntary facial movements. These abnormal movements of his face, head, and limbs were exacerbated by purposeful movement. Examination revealed dysarthria with tremulous speech, intention tremors of the head and all extremities, distally diminished vibration sense with hyporeflexia, bilateral upper extremity dysdiadochokinesis, bilateral lower extremity dysmetria on heel-to-shin testing, and truncal ataxia. MRI of the brain showed no acute abnormalities. CSF and serum testing showed evidence of CNS inflammation with a CSF lymphocytic pleocytosis and elevated total protein. Diagnosis was ultimately made using serum and CSF studies. Here, we discuss the differential diagnosis of 2 weeks of diffuse intention tremor, involuntary facial movements, and ataxia. We discuss the workup, diagnosis, acute and nonacute management of this condition, as well as important clinical points regarding the broader category into which this diagnosis falls.