An adult case of Leigh disease.

Leigh's disease is a mitochondrial disease of infancy and early childhood, and is rare in adults. Following a febrile illness, a 21-year-old woman developed ataxic paraparesis and was originally diagnosed as multiple sclerosis. Her illness progressed to somnolence and quadriparesis. The unusual MR images, the discovery of elevated blood lactate and pyruvate levels, the results of muscle biopsy and the lack of response to corticosteroid treatment, led to the correct diagnosis of Leigh disease. Initiation of a ketogenic diet resulted in a rapid partial response. She recovered sufficiently to be able to walk after 6 months.