An Epidemiological Study on the Cause of Mental Retardation in Yokohama City

We investigated the cause of mental retardation in 337 individuals seeking healthcare services at the Yokohama City Social Welfare Center for Disabilities during a period of a year and a half, from October 1987 to March 1989. The total number of participants in the study was 337 and consisted of 207 males and 130 females. Age ranged from 14 to 58 years with an average age of 22 years. IQ was evaluated using the Tanaka-Binet test. Individuals with an IQ under 50 were classified as having severe mental retardation and those with IQs between 51 and 70 were classified as having mild mental retardation, in accordance with international standards. The number of cases with severe mental retardation was 262 and 75 cases presented with mild mental retardation. Physical and neurological examination was performed in all cases in an attempt to ascertain the causes of mental retardation. Only when deemed necessary by the attending physician, and if parental consent was granted, electroencephalogram, brain CT scan, and chromosome analysis were carried out. In cases where the diagnosis had already been established, past medical records were consulted. The results of the study were compared to results of a very similar study published by the University of Kuopio in eastern Finland. The time of insult was categorized into four groups: prenatal, perinatal, postnatal and uncertain. In the severely mentally retarded group, the time of insult was considered to have occurred in the prenatal period in 25.6% of cases, in the perinatal period in 9.2% of cases, in the postnatal period in 8.0% of cases and was uncertain in 57.2% of cases. In the mildly mentally retarded group the insult occurred in the prenatal period in 14.7% of cases, in the perinatal period in 8.0% of cases, in the postnatal period in 12.0% of cases, and was uncertain in 65.3% of cases. In the mildly mentally retarded group, familial association was recognized in 27% of cases. The Finland study revealed similar patterns. Both studies demonstrated that most individuals with severe mental retardation and a determinable cause have genetic and chromosomal abnormalities. The cause of mental retardation was harder to determine in individuals with mild mental retardation, and most of those with an uncertain cause had a positive family history. The above findings point out the need for