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Epidemiology of mental retardation--a Swedish survey
Insights
Severe and mild mental retardation affect approximately 0.7% of Swedish school children. Prenatal factors, particularly Down syndrome, are major causes of severe intellectual disability, while unknown causes dominate mild cases.
Area of Science:
- Pediatrics
- Genetics
- Epidemiology
Background:
- Epidemiological studies in Swedish school children indicate a prevalence of severe mental retardation (SMR) at 0.3% and mild mental retardation (MMR) at 0.4%.
- Understanding the etiological factors behind intellectual disability is crucial for developing effective prevention strategies.
Purpose of the Study:
- To investigate the prevalence and etiological factors of severe mental retardation (SMR) and mild mental retardation (MMR) in Swedish school-aged children.
- To identify the contribution of prenatal, perinatal, and other factors to different categories of intellectual disability.
Main Methods:
- Analysis of epidemiological data from Swedish school-aged children.
- Categorization of intellectual disability into SMR (IQ < 50) and MMR (IQ 50-70).
- Examination of etiological factors including chromosomal abnormalities, genetic syndromes, metabolic disorders, and perinatal brain damage.
Main Results:
- Prenatal causes accounted for 55% of SMR, with Down syndrome being the largest single cause. Perinatal factors contributed 15-20% to SMR.
- For MMR, only 23% were attributed to prenatal causes, while 55% had no traceable brain pathology. Polygenic subcapacity is considered a cause of MMR.
- Chromosomal errors were found in 29% of SMR and 4% of MMR. Fragile X syndrome affected 4% of SMR boys and 10% of MMR boys. Fetal alcohol syndrome was 8% of urban MMR. Inborn errors of metabolism contributed 4-5% to SMR and less than 1% to MMR.
- Perinatal brain damage implicated 15% of SMR and 18% of MMR.
Conclusions:
- Etiology of severe mental retardation is largely prenatal, with specific genetic syndromes like Down syndrome being significant contributors.
- Mild mental retardation has a more diverse etiology, with a substantial proportion attributed to unknown or polygenic factors.
- Identifying specific etiological pathways, such as chromosomal errors and fetal alcohol syndrome, provides targets for public health interventions and preventive measures.
Abstract:
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.