Clinical Reasoning: A 60-Year-Old Man With Rapidly Progressive Left Hemibody Weakness and Vision Loss.

A 60-year-old man presented with 2 weeks of progressive left-sided weakness, dysarthria, and an incongruous left homonymous hemianopsia, with MRI brain showing a lesion involving the right cerebral peduncle, optic radiation, and dorsal midbrain. Serum and CSF testing was largely unremarkable. His weakness worsened rapidly, and short-interval repeat imaging demonstrated expansion of the brainstem abnormality. A methodical investigation of inflammatory, infectious, neoplastic, and paraneoplastic causes ultimately yielded the diagnosis. This case provides a comprehensive differential for atypical brainstem lesions, which can pose a diagnostic dilemma, particularly when CSF studies are unrevealing and the location is not amenable to biopsy. In such situations, a systematic approach, including whole-body imaging, can aid in diagnosis.