Mental retardation

Definition:

Below-average general intellectual function with associated deficits in adaptive behavior that occurs before age 18.

Causes, incidence, and risk factors:

Causes of mental retardation are numerous, but a specific reason for mental retardation is determined in only 25% of the cases.

Failure to adapt normally and grow intellectually may become apparent early in life or, in the case of mild retardation, not become recognizable until school age or later. An assessment of age-appropriate adaptive behaviors can be made by the use of developmental screening tests. The failure to achieve developmental milestones is suggestive of mental retardation.

A family may suspect mental retardation if motor skills, language skills, and self-help skills do not seem to be developing in a child or are developing at a far slower rate than the child's peers. The degree of impairment from mental retardation has a wide range from profoundly impaired to borderline retardation. Risk factors are related to the causes. Mental retardation affects an estimated 1 to 3% of the population.

Causes of mental retardation can be roughly broken down into several categories:

• unexplained (This category is the largest and a catchall for undiagnosed incidences of mental retardation.)
• trauma (prenatal and postnatal)
  • intracranial hemorrhage before or after birth (for example, periventricular hemorrhage)
  • hypoxic injury before or after birth (for example, that associated with cerebral palsy)
  • severe head injury

• infectious (congenital and postnatal)
  • congenital rubella
  • meningitis
  • congenital CMV
  • encephalitis
  • congenital toxoplasmosis
  • listeriosis
  • HIV infection

• chromosomal abnormalities
  • errors of chromosome numbers (Down's syndrome, and so on)
  • defects in the chromosome (fragile X syndrome)
  • chromosomal translocations (a gene is located in an unusual spot on a chromosome, or location on a different chromosome than usual)
  • Klinefelter's syndrome
  • Prader-Willi syndrome
  • cri du chat syndrome

• genetic abnormalities and inherited metabolic disorders
  • galactosemia
  • Tay-Sachs disease
  • phenylketonuria
  • Hunter syndrome
  • Hurler syndrome
  • Sanfilippo syndrome
  • metachromatic leukodystrophy
  • adrenoleukodystrophy
  • Lesch-Nyhan's syndrome
  • Rett's disorder
  • tuberous sclerosis

• metabolic
  • Reye's syndrome
  • hypernatremic dehydration
  • congenital hypothyroid
  • hypoglycemia (poorly regulated diabetes mellitus)

• toxic
  • intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs
  • methylmercury poisoning
  • lead poisoning

• nutritional
  • Kwashiorkor
  • Marasmus
  • malnutrition

• environmental
  • poverty
  • low socioeconomic status