Lennox-Gastaut syndrome is a severe epilepsy syndrome characterized by frequent seizures, intellectual impairment, and slow spike and wave activity on interictal electroencephalogram (EEG). About 70% of cases are due to underlying pathology such as hypoxic ischemic injury or other birth injuries, brain malformations, metabolic disorders, tuberous sclerosis, meningitis, or encephalitis. Some cases are idiopathic with no identifiable cause. There may be many types of seizures including tonic, myoclonic, atonic, tonic-clonic, and atypical absence. Seizures are often difficult to treat. Slowed psychomotor development, intellectual disability, and behavior problems are also common.

Typical age of onset is 2-6 years old. Patients may have progressed from prior diagnoses of infantile spasms (West syndrome). Prognosis is variable, although unfavorable overall, with many patients suffering from intractable seizures, intellectual disability, and psychiatric disorders.