Rasmussen syndrome (or Rasmussen encephalitis) is a rare neurodegenerative disease that leads to brain inflammation and atrophy, usually affecting 1 cerebral hemisphere. It is characterized by intractable epilepsy, progressive hemiplegia, and cognitive decline. The cause of inflammation is unknown, making targeted treatments difficult.

The age of onset ranges from infancy to adulthood, but onset is most commonly during childhood. Prior to the acute phase of disease, there may be a prodromal phase of mild hemiparesis or well-controlled focal epilepsy for up to several years. The acute phase is characterized by rapidly progressive seizures, hemiparesis, hemianopia, aphasia (if the dominant hemisphere is involved), and cognitive decline over the first year. The final stage consists of permanent neurological deficits and relapsing epilepsy. Adolescent- or young adulthood-onset occurs in about 10% of patients.

Adolescent-onset disease has a slower rate of progression and less severe fixed neurological deficits compared to the childhood-onset disease. These patients may present with unilateral movement disorders such as hemiathetosis and hemidystonia. Although early seizures are common in Rasmussen syndrome, cases with delayed seizure onset or even absence of seizures have been reported.