Ataxia and weakness in a young woman.

A 28-year-old woman presented with slowly progressive ataxia and signs of upper and lower motor neuron dysfunction. She had no family history of neurologic disease. Neuroimaging study results were unremarkable, and the diagnosis remained obscure for several years. The differential diagnosis of motor system degeneration in a young adult is broad but focuses on inherited disorders that may have late onset. In this case, close analysis of the clinical features in the history and neurologic examination and interpretation of the laboratory study results led to the diagnosis.