Mental retardation is described as 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 mild or borderline retardation. Less emphasis is now placed on degree of retardation and more on the amount of intervention and care required for daily life. Risk factors are related to the causes. Mental retardation affects about 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)

lack of oxygen to the brain before, during, or after birth

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 )

defects in the chromosome or chromosomal inheritance (

fragile X syndrome , Angelman syndrome, Prader-Willi syndrome )

chromosomal translocations (a

gene is located in an unusual spot on a chromosome, or location on a different chromosome than usual) and deletions ( 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 syndrome

tuberous sclerosis

metabolic

Reye's syndrome

congenital hypothyroid

very high bilirubin levels in babies

hypoglycemia

(poorly regulated

diabetes mellitus )

toxic

intrauterine

exposure to alcohol,

cocaine , amphetamines, and other drugs

methylmercury poisoning

lead poisoning

nutritional

malnutrition

environmental

poverty

low socioeconomic status

deprivation syndrome

Signs and tests

development significantly less than that of peers

an intelligence quotient (IQ) less than two standard deviations below the mean (This is frequently classified as a score below 70 on a standardized IQ test where 100 is the mean.)

adaptive behavior score

abnormal Denver developmental screening 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 coexisting affective disorders and treat those disorders. Behavioral approaches are important in understanding and working with mentally retarded individuals.

Expectations (prognosis)

The outcome is related to the aggressiveness of treatment, personal motivation, opportunity, and associated conditions. Many people lead productive lives while functioning independently; others require a structured environment to be most successful.

Complications

Complications vary. They may include:

social isolation

inability to care for self

inability to interact with others appropriately

Calling your health care provider

Call your health care provider if you have any concerns about your child's development, if you notice a lack of normal development with motor or language skills in a child, or if there are associated disorders that need treatment.

Prevention

Genetic: Prenatal screening for genetic defects and genetic counseling for families at risk for known heritable disorders can decrease the incidence of genetically caused mental retardation. Social: Government programs to insure adequate nutrition are available to the underprivileged in the first and most critical years of life, which can reduce retardation associated with malnutrition. Social programs to reduce poverty and provide good education can impact the mild "retardation" associated with impoverished and lower socioeconomic status. Early intervention in abuse and deprivation will also help. Toxic: Environmental programs to reduce lead and mercury exposure and other toxins will reduce toxin-associated retardation. However, the benefits may take years to become apparent. Increased awareness by the public of the effects of alcohol and drugs during pregnancy can 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 in the relationship of cats, toxoplasmosis, and pregnancy, helps to reduce retardation that results from this infection.