Epidemiology of mental retardation

Mental retardation (MR) is a significant deficit in intelligence and adaptive skills necessary for communication, self-care, social interaction, leisure, and work. This discussion will be limited to deficits presumed to be present at birth, although in many cases the intellectual deficit may not be identified for several years.

Classification

MR may be classified into four grades of severity, defined in terms of intelligence quotient (IQ):

  • mild (IQ, 50–70)
  • moderate (IQ, 35–49)
  • severe (IQ, 20–34)
  • profound (IQ, <20)

Most epidemiologic studies collapse the last three categories into the single category of severe MR, merely distinguishing it from the mild form.

Prevalence

The overall prevalence of MR varies according to the severity of the disability and the country in which it has been evaluated. In developed countries, the prevalence of severe MR in children has consistently been found to range from 3 to 5 per 1,000, whereas wider ranges of prevalence rates have been reported for mild MR (2.5–40.0 per 1,000).6–8 Few estimates are available from countries with emerging economies, although prevalence rates generally are higher than those in developed countries (5.0–25.0 per 1,000), especially for mild MR, which may be more influenced by social and environmental factors than is severe MR.9–12

For both severe and mild MR, prevalence increases with increasing age in childhood and tends to decline with advancing age throughout adulthood. Mild MR commonly is recognized when children enter school, whereas severe MR is much more commonly diagnosed during infancy and early childhood.

Causes

Chromosomal abnormalities are the leading known cause of severe MR in developed countries. These abnormalities include:

  • chromosomal duplication, such as trisomy 21 in Down syndrome
  • abnormalities of chromosome structure, such as X-linked trinucleotide repeats (amplification of CGG triplet in the fragile X syndrome)
  • autosomal-dominant–inherited deletions (e.g., chromosome 9/16 in tuberous sclerosis, chromosome 17 or 22 in neurofibromatosis types 1 and 2)
  • autosomal-recessive substitutions (e.g., chromosome 12 in phenylketonuria)

In developed countries, genetic causes are responsible for at least 30% of the cases of severe MR.

In less developed countries, MR more often is attributed to nutritional deficiencies (folate or iodine), traumatic injury, intrauterine disorders, and postnatally acquired infectious diseases.13

Whether such perinatal etiologies events as preterm birth, low birth weight, and birth asphyxia are associated with increased risk of impaired mental development in infancy or childhood remains unclear. These factors seem to be associated with MR only when MR is accompanied by cerebral palsy.14–16

Adapted from: D'Amelio M, Shinnar S, and Hauser WA. Epilepsy in children with mental retardation and cerebral palsy. In: Devinsky O and Westbrook LE, eds. Epilepsy and Developmental Disabilities. Boston: Butterworth-Heinemann; 2001;3–16. 
With permission from Elsevier (www.elsevier.com). 

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