Mental retardation

Definition

Mental retardation is a condition diagnosed before age 18 that includes below-average general intellectual function, and a lack of the skills necessary for daily living.

Alternative Names

Intellectual and developmental disability

Causes, incidence, and risk factors

Mental retardation affects about 1 - 3% of the population. There are many causes of mental retardation, but doctors find a specific reason in only 25% of cases.

A family may suspect mental retardation if the child's motor skills, language skills, and self-help skills do not seem to be developing, or are developing at a far slower rate than the child's peers. Failure to adapt (adjust to new situations) normally and grow intellectually may become apparent early in a child's life. In the case of mild retardation, these failures may not become recognizable until school age or later.

The degree of impairment from mental retardation varies widely, from profoundly impaired to mild or borderline retardation. Less emphasis is now placed on the degree of retardation and more on the amount of intervention and care needed for daily life.

Risk factors are related to the causes. Causes of mental retardation can be roughly broken down into several categories:

• Infections (present at birth or occurring after birth)
  • Congenital CMV
  • Congenital rubella
  • Congenital toxoplasmosis
  • Encephalitis
  • HIV infection
  • Listeriosis
  • Meningitis
• Chromosomal abnormalities
  • Chromosome deletions (cri du chat syndrome)
  • Chromosomal translocations (a gene is located in an unusual spot on a chromosome, or located on a different chromosome than usual)
  • Defects in the chromosome or chromosomal inheritance (for example, fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)
  • Errors of chromosome numbers (such as Down syndrome)
• Environmental
  • Deprivation syndrome
• Genetic abnormalities and inherited metabolic disorders
  • Adrenoleukodystrophy
  • Galactosemia
  • Hunter syndrome
  • Hurler syndrome
  • Lesch-Nyhan syndrome
  • Phenylketonuria
  • Rett syndrome
  • Sanfilippo syndrome
  • Tay-Sachs disease
  • Tuberous sclerosis
• Metabolic
  • Congenital hypothyroid
  • Hypoglycemia (poorly regulated diabetes)
  • Reye syndrome
  • Hyperbilirubinemia (very high bilirubin levels in babies)
• Nutritional
  • Malnutrition
• Toxic
  • Intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs
  • Lead poisoning
  • Methylmercury poisoning
• Trauma (before and after birth)
  • Intracranial hemorrhage before or after birth
  • Lack of oxygen to the brain before, during, or after birth
  • Severe head injury
• Unexplained (this largest category is for unexplained occurrences of mental retardation)

Symptoms

• Continued infant-like behavior
• Decreased learning ability
• Failure to meet the markers of intellectual development
• Inability to meet educational demands at school
• Lack of curiosity

Note: Changes to normal behaviors depend on the severity of the condition. Mild retardation may be associated with a lack of curiosity and quiet behavior. Severe mental retardation is associated with infant-like behavior throughout life.

Signs and tests

An assessment of age-appropriate adaptive behaviors can be made using developmental screening tests. The failure to achieve developmental milestones suggests mental retardation.

The following may be signs of mental retardation:

• Abnormal Denver developmental screening test
• Adaptive behavior score below average
• Development way below that of peers
• Intelligence quotient (IQ) score below 70 on a standardized IQ test

Treatment

The primary goal of treatment is to develop the person's potential to the fullest. Special education and training may begin as early as infancy. This includes social skills to help the person function as normally as possible.

It is important for a specialist to evaluate the person for other affective disorders and treat those disorders. Behavioral approaches are important for people with mental retardation.

Expectations (prognosis)

The outcome depends on:

• Opportunities
• Other conditions
• Personal motivation
• Treatment

Many people lead productive lives and function on their own; others need a structured environment to be most successful.

Complications

Complications vary. They may include:

• Inability to care for self
• Inability to interact with others appropriately
• Social isolation

Calling your health care provider

Call your health care provider if:

• You have any concerns about your child's development
• You notice that your child's motor or language skills are not developing normally
• Your child has other disorders that need treatment

Prevention

Genetic: Prenatal screening for genetic defects and genetic counseling for families at risk for known inherited disorders can decrease the risk of inherited mental retardation.

Social: Government nutrition programs are available to poor children in the first and most critical years of life. These programs can reduce retardation associated with malnutrition. Early intervention in situations involving abuse and poverty will also help.

Toxic: Environmental programs to reduce exposure to lead, mercury, and other toxins will reduce toxin-associated retardation. However, the benefits may take years to become apparent. Increased public awareness of the risks of alcohol and drugs during pregnancy can help reduce the incidence of retardation.

Infectious: The prevention of congenital rubella syndrome is probably one of the best examples of a successful program to prevent one form of mental retardation. Constant vigilance, such as limiting exposure to cat litter that can cause toxoplasmosis during pregnancy, helps reduce retardation that results from this infection.

References

Shapiro BK, Batshaw ML. Mental retardation (intellectual disability). In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 38.