Epidemiology of mental retardation--a Swedish survey.

ecent epidemiological studies in Swedish school age children revealed a prevalence of severe mental retardation (SMR = IQ less than 50) of 0.3% and of mild mental retardation (MMR = IQ 50-70) around 0.4%. In SMR prenatal causes were found in 55%, perinatal in 15-20%, no traceable brain pathology in 18%. Corresponding figures for MMR were 23%, 18% and 55%, respectively. Down syndrome was the largest single cause of SMR and polygenic subcapacity considered to be that of MMR. Chromosomal errors were detected among 29% SMR and 4% MMR school children. Fragile X accounted for 4% SMR and 10% MMR in boys. Fetal alcohol syndromes constituted 8% of urban MMR. The contribution of inborn errors of metabolism was 4-5% and less than 1%, in SMR and MMR, respectively. Perinatal (28th prenatal week-28th postnatal day) brain damage was implicated in 15% of SMR and 18% of MMR. Pathogenetic data are considered for potential preventive measures.