A rare, slowly progressive encephalitis caused by chronic infection with the measles virus. The condition occurs primarily in children and young adults, approximately 2-8 years after the initial infection. A gradual decline in intellectual abilities and behavioral alterations are followed by progressive myoclonus; muscle spasticity; seizures; dementia; autonomic dysfunction; and ataxia. death usually occurs 1-3 years after disease onset. Pathologic features include perivascular cuffing, eosinophilic cytoplasmic inclusions, neurophagia, and fibrous gliosis. It is caused by the sspe virus, which is a defective variant of measles virus. (From Adams et al., Principles of neurology, 6th ed, pp767-8)