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Aetiology in severe and mild mental retardation: a population-based study of Norwegian children
1Department of Paediatrics, The National Hospital, Rikshospitalet, University of Oslo, Norway. petter.stromme@rh.uio.no
Insights
Biopathological factors explain most cases of severe and mild mental retardation (MR). However, a significant portion of mild MR cases remain unspecified, possibly due to normal IQ variation.
Area of Science:
- Neuroscience
- Genetics
- Pediatrics
Background:
- Mental retardation (MR) is a significant developmental disorder with diverse etiologies.
- Understanding the causes of MR is crucial for early intervention and prevention strategies.
Purpose of the Study:
- To investigate the aetiology of mental retardation in a population-based cohort of Norwegian children.
- To differentiate between biopathological and unspecified causes of severe and mild MR.
Main Methods:
- A population-based study of 178 children with MR (79 severe, 99 mild) born between 1980-1985.
- Aetiological classification into biopathological (prenatal, perinatal, postnatal, undetermined) and unspecified groups.
- Analysis of risk factors including gestational age, birthweight, and Apgar scores.
Main Results:
- Biopathological causes accounted for 96% of severe MR and 68% of mild MR.
- Prenatal factors were the most common biopathological cause.
- Single-gene disorders were identified in 15 of 63 genetic cases.
- Low gestational age and birthweight, and low Apgar scores were associated with increased MR risk.
- Aetiological diagnoses were revised for 15% of children.
- 32% of mild MR cases remained unspecified, potentially representing the lower end of normal IQ.
Conclusions:
- The majority of severe and mild mental retardation cases have identifiable biopathological causes, with prenatal factors being predominant.
- A significant proportion of mild MR cases lack clear biological markers, suggesting a potential overlap with normal cognitive variation.
- While certain perinatal factors increase MR risk, they account for a small percentage of overall cases.
Abstract:
The aetiology of mental retardation (MR) was studied in a population-based series of Norwegian children derived from 30 037 children born between 1980 and 1985. The study included 178 children, 79 with severe MR (SMR) (IQ<50) and 99 with mild MR (MMR) (IQ 50 to 70). Aetiology was divided into two main groups: biopathological and unspecified. The biopathological group comprised 96% of SMR and 68% of MMR, and was subdivided into prenatal (70% and 51%), perinatal (4% and 5%), and postnatal damage (5% and 1%), and a group of undetermined timing of the damaging event (18% and 11%). Single-gene disorders accounted for 15 of the 63 children with genetic disorders, including X-linked recessive in six. During the course of the study, at least 27 (15%) children had their aetiological diagnosis revised. Gestational age <32 weeks, birthweight <1500 g, and Apgar scores 0 to 2 at 1 and 5 minutes implied a significantly increased risk of MR, but contributed to only 4% of the children in the study. Decreased birthweight (1500 to 2499 g) and Apgar scores 3 to 6 at 1 and 5 minutes showed increased probability of MR. Despite extensive investigations, 4% of SMR and 32% of MMR were not identified with any biological markers and were considered as unspecified MR, several most probably representing the lower end of the normal IQ distribution in the population.