Characteristics of seizures in a population-based series of mentally retarded children with active epilepsy.

Purpose: The characteristics of seizures were analysed in a population-based study of active epilepsy in 6- to 13-year-old mentally retarded children.
Methods: The search procedure included diagnostic registers, EEG registers, and registers of the Education of the Subnormal. Medical files were scrutinized, and clinical examinations and interviews with parents or caretakers or both were performed.
Results: The median age of seizure onset was 1.3 years, 3.1 for children with mild retardation and 0.8 for children with severe retardation. Among the 98 children identified, current seizure groups were partial in 20, generalized in 59, and mixed in 19. The prevailing seizure types were tonic-clonic, myoclonic, atypical absences, and partial complex seizures, present in 42, 33, 23, and 23 children, respectively. A total of 46 children had more than one seizure type. Seizures every day/week occurred in 44 children. There was a constancy between seizure type at onset and later seizure type. Neonatal seizures (n = 25), infantile spasms (n = 12), and status epilepticus (n = 37) occurred independent of one another. Prognostic factors for poor neurologic outcome were early onset of epilepsy, infantile spasms as onset type, and prior neonatal seizures. Children with only partial seizures less frequently had severe mental retardation, cerebral palsy, and visual impairment than those with only generalized seizures.
Conclusions: Epilepsies in children with mental retardation are characterized by severe seizure manifestations. The brain damage giving rise to mental retardation and epilepsy is probably the main factor in terms of seizure outcome.