Rasmussen syndrome (RS) / Rasmussen's encephalitis (RE) is a rare but severe inflammatory brain disease that leads to the progressive destruction of one cerebral hemisphere over periods ranging from months to ten years or more. RS/RE typically affects previously normal children aged two to ten years. However, some cases begin in adolescence and adulthood. The first sign of RS/RE is typically a seizure. Other early symptoms include mild weakness in arms or legs. Seizures typically become frequent, and about half of people with RS/RE have seizures that happen every few seconds or minutes. Within a few months to years of the first seizure, other symptoms may follow, including increasing issues with thinking, intellect, and memory; progressive loss of movement skills on one side of the body, up to paralysis; and progressive loss of speech and language. RS/RE is diagnosed based on symptoms and test results from brain imaging. The outlook for RS/RE varies, but the deficits ultimately caused by RS/RE are usually severe: they include hemiparesis, ataxia, intellectual decline, personality and behavioral changes, visual problems, sensory deficits, and speech problems.