Acute lethal encephalopathy in systemic juvenile rheumatoid arthritis.

Macrophage activation syndrome is the most common cause of death in children with systemic juvenile rheumatoid arthritis. We present a first patient with systemic juvenile rheumatoid arthritis in which acute necrotizing encephalopathy developed as a complication of macrophage activation syndrome but not of Reye's syndrome. The suspected mechanism of this lethal complication is discussed.