A POPULATION-BASED STUDY ON THE CAUSES OF MILD AND SEVERE MENTAL-RETARDATION

The causes of mental retardation (MR) were studied as part of a multid isciplinary epidemiological case-control study in 151 mentally retarde d patients identified by screening four age cohorts (12882 children) a t 8-9 years of age in the province of Kuopio, Finland. The causes of M R in 77 severely retarded (SD less than or equal to -3 SD) and 74 mild ly retarded (-2 > SD > -3) children were divided into pre-, peri-, pos tnatal and unknown groups according to the probable time of onset. The causes were pre-, peri-, postnatal and unknown in 60%, 9%, 8% and 23% , and 22%, 1%, 3% and 74%, in the two populations, respectively. Genet ic causes were found in 28% of all 151 cases; the three most common su bgroups were trisomy 21, fragile X syndrome and aspartylglycosaminuria (13%, 4% and 2% respectively). The study design used provided reliabl e information on the causes of MR and also demonstrated those forms of genetic metabolic diseases typical of Finnish inheritance.