Preventable and unpreventable causes of childhood-onset epilepsy associated with mental retardation were determined in 692 patients with epilepsy onset between 1977 and 1985 in a Nova Scotia population-based cohort studied in the Department of Pediatrics, Dalhousie University, Halifax, Canada. Causes and family history obtained by chart review and caregiver interview after 18.8 years of follow-up found 147 patients (77 boys, 70 girls) had mental retardation and epilepsy, involving 21% of all childhood epilepsy cases. Average age of onset of epilepsy was 38 months (range, 1-195 months); 44% had the first seizure in the first year. Mental retardation assessed by psychological testing occurred in 57% of patients; retardation was too profound for testing in 38.5%, and was assessed clinically in 4%. Mental retardation was mild in 24%, moderate in 23%, and severe in 53%. Severe neurological deficits, usually associated with severe mental retardation, were present in 59%. Twenty-nine patients died at an average age of 12 years (range 1.4-30 years). Epilepsy syndromes were symptomatic generalized in 50%, partial in 39%, and miscellaneous in 11%. CT scan was performed in 91% and MRI in 12%. The cause of epilepsy was defined in 63% and unknown in 37%. A defined cause was more frequent in patients with severe mental retardation (77% of 78 vs 48% of 65 with mild/moderate MR; P<.001), and in those with coexistent severe neurological disability (64% vs 18%;P<.0001). Causes identified were prenatal or genetic in 65%, perinatal (8%), and complications of prematurity in 13%; 14 (15%) had a postnatally acquired cause. Prenatal or genetic causes included tuberous sclerosis (3.4%), focal heterotopia (0.7%), neural tube defects (7.5%), and chromosome abnormality (4.1%). Perinatal asphyxia accounted for 4.8%. Acquired causes were potentially preventable in only 11 (7%) patients. A history of epilepsy in a first or second-degree relative occurred in 36%, especially in idiopathic cases (54% vs 30%), and in those without neurological deficits (57% vs 26%). 
COMMENT. One in 5 children with epilepsy in Canada is mentally retarded. Two thirds have a prenatal or genetic cause, and only 7% have an acquired preventable cause. Epilepsy and mental retardation appear to have a common cause, and mental retardation is not the result of the epilepsy. Genetic factors are important in etiology, 50% of those with no clear cause having a positive family history. The authors comment that the prevalence of a defined cause would probably be greater if the study had involved a later time period when MRI became more readily available.