Infantile spasms (West syndrome) is an epilepsy syndrome that starts in infancy, characterized by epileptic spasms and associated with hypsarrhythmia on electroencephalogram (EEG) and developmental regression. Onset of disease is typically between 3 and 12 months of age.

The seizures are characterized by sudden, typically bilateral, spasms of the neck, trunk, and/or extremities with associated brief loss of consciousness. Spasms can be flexor, extensor, or mixed. Most seizures last seconds and occur in clusters with up to hundreds of spasms in a day.

Infantile spasms can be associated with many neurologic conditions including tuberous sclerosis, neuronal migration disorders, hypoxic-ischemic injury, cerebral palsy, or Down syndrome. Up to one-third of cases have no obvious underlying abnormality. Prognosis is typically poor (although somewhat better in the cryptogenic group); while spasms usually remit by age 3, most children go on to develop other seizure types, with up to 50% developing Lennox-Gastaut syndrome. Developmental delay and cognitive impairment is common.