Encephalopathy that occurs in Lennox Gastaut Syndrome. Onset is typically between 3 and 5 years of age and is characterized by TONIC SEIZURES during sleep, ABSENCE SEIZURES, or nonconvulsive STATUS EPILEPTICUS, as well as intellectual disabilities. May be idiopathic; germline deletions in the MAPK10 gene have been identified but not confirmed (OMIM: 602897).
