Epilepsy and psychiatric disorder in the mentally retarded adult.

302 mentally retarded adults, sampled by epidemiological criteria, were examined with regard to epilepsy and psychiatric disorder. Each of the complications was frequent and related to degree and origin of mental retardation. In 55 (18.2%) epilepsy had occurred at some time during their lives, in 25 (8.3%) of these in the past year. In 52% of persons with seizures in the past year a present state psychiatric diagnosis was established, compared to 26% in those without seizures. The nature of the combination of epilepsy and psychiatric disorder is complex, but in the mentally retarded most often reflecting underlying brain pathology in the form of widespread cortical and subcortical cerebral damage causing epilepsy of generalised or mixed type, and predominantly interictal psychiatric disorders unrelated in time to seizures and dominated by behaviour problems.