The course of Rasmussen’s encephalitis (RE) was studied by a correlation of serial MRI and histopathology of surgical specimens in 10 patients (7 children, 1 adolescent, 2 young adults) followed at the University of Bonn, Germany, and University of Vienna, Austria. All developed the typical progressive hemispheric signs and symptoms of RE. MRI abnormalities had a focal onset and spread across one hemisphere with late progressive atrophy. The earliest abnormal MRI finding was cortical swelling with hyperintense T2/FLAIR signal. Histopathology showed a higher number of T cells and reactive astrocytes in the earlier MRI focal stages compared to the late atrophic stage. The early stages are characterized by the highest inflammatory changes and late stages are mainly atrophic. 
COMMENT. The course of RE is characterized by an early inflammatory stage, as judged by density of T cells and microglial nodules, followed by a decrease in inflammation and an atrophic stage. These inflammatory changes have a temporal or frontocentral onset and spread across a hemisphere, as judged by MRI. Deterioration with atrophy occurs within the first 2 years after onset. If a diagnostic brain biopsy is planned, early inflammatory changes should be found in areas of increased MRI signal. The data suggest that immunosuppressive treatment may be most effective during stages of active inflammation, usually in the first 15 months after onset. Hemispherectomy is usually reserved for patients with a residual hemiplegia, at the late atrophic stage.