A rare complication of MEASLES due to infection of the brain with the measles virus. It develops 2–18 years after the onset of the measles, and is characterised by mental deterioration leading on to CONVULSIONS, COMA and death. The annual incidence in Britain is about one per million of the childhood population. The risk of its developing is 5–25 times greater after measles than after measles vaccination (see MMR VACCINE; IMMUNISATION).