Mild mental retardation in children in a northern Swedish county.

A series of children with mild mental retardation (171 children, IQ 50-69) born in 1959-1970 in a northern Swedish county and registered by the Board for Provisions and Services to the Mentally Retarded were analysed retrospectively. The mean annual incidence of mild mental retardation in registered children alive at the age of one year decreased during the period 1959-1970 and for the whole period was 4.2 per 1000, compared with 3.9 per 1000 for severe mental retardation in the same period of time and area. Ten children once classified as mildly mentally retarded made considerable intellectual progress and after some time no longer needed special help from the authorities. There was a pronounced male excess in the total series (1.8:1) and in all the main aetiological groups. Only small differences were found in mean birth weights, gestational age and maternal age in the main aetiological groups. In the series as a whole, however, there was an increased frequency of children small for gestational age. Prenatal causes were considered relevant in 43 per cent, perinatal in 7 per cent, and postnatal in 5 per cent. In 2 per cent there was a psychosis. In 43 per cent the aetiology could not be traced. Genetic causes were found in 31 per cent; in 8 per cent there was a chromosomal aetiology, in a further 8 per cent a mutant gene disorder and in 15 per cent the aetiology were considered multifactorial, i.e. at least one first-degree relative also had mental retardation and in both cases the cause of the retardation was unknown. Thirty per cent of the children had associated CNS handicaps, epilepsy (16 per cent) being the most frequent.