Epilepsy in a population of mentally retarded children and adults.

All mentally retarded (MR) subjects in a northern Swedish county were assessed for the occurrence of active epilepsy on a prevalence day. Active epilepsy was found in 299 subjects (20.2% of those with MR) corresponding to a crude prevalence rate of 1.2/1000 inhabitants. The age-specific prevalence for 0-9 years was higher for females than for males, while in other age groups it was slightly higher for males or showed no difference between the sexes. Epilepsy and MR were the only disorders in 129 subjects (43.1%). Cerebral palsy was the most common associated disorder and occurred in 100 (33.4%). A presumable etiology for epilepsy and MR was identified in 73.2% and 71.9%, respectively. The presumable etiological factors which caused MR occurred prenatally in 35%, perinatally in 10% and postnatally in 9%. The pathogenetic period was unknown in 31%. In 15%, the etiological events occurred during more than one of the above periods. The presumable causes were responsible for both epilepsy and MR in all except 7 cases. MR individuals with epilepsy were significantly more retarded than those without epilepsy. The first seizure occurred during the neonatal period in 11.6% and before 1 year of age in 27.7%. Generalized tonic-clonic seizures were the most common type and occurred in 204 subjects (68.2%). Seventy-one of these also had partial seizure manifestations. Daily to weekly seizures occurred in 26.8% and 32.0% had been seizure-free for the past year.