Intellectual disability (previously called mental retardation) is significantly subaverage intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily living.
The previously used term mental retardation has acquired an undesirable social stigma, so health care practitioners have replaced it with the term intellectual disability.
ID is not a specific medical disorder like pneumonia or strep throat, and it is not a mental health disorder. People with ID have significantly below average intellectual functioning that limits their ability to cope with one or more areas of normal daily living (adaptive skills) to such a degree that they require ongoing support. Adaptive skills are divided into 3 areas:
People with ID have varying degrees of impairment, classified from mild to profound. Although fundamentally, impairment is caused by the decreased intellectual functioning (typically measured by standardized intelligence tests), the degree of impairment depends more on the amount of support the person requires. For example, a person who has only mild impairment on an intelligence test may have such poor adaptive skills that extensive support is required. Support is categorized as intermittent, limited, extensive, or pervasive. Intermittent means occasional support. Limited means support such as a day program in a sheltered workshop. Extensive means daily, ongoing support. Pervasive means a high level of support for all activities of daily living, possibly including full-time nursing care.
Based only on IQ test scores, about 3% of the total population are considered to have ID. However, if classification is based on the need for support, only about 1% of people are classified as having significant mental (cognitive) limitation.
|PrintOpen table in new window
A wide variety of medical and environmental conditions can cause ID. Some are genetic. Some are present before or at the time of conception, and others occur during pregnancy, during birth, or after birth. The common factor is that something interferes with the growth and development of the brain. However, doctors can identify a specific cause in only about one third of people with mild ID and in two thirds of people with moderate to profound ID.
Some children with ID have abnormalities apparent at birth or shortly thereafter. These abnormalities may be physical as well as neurologic and may include unusual facial features, a head that is too large or too small, malformations of the hands or feet, and various other abnormalities. Sometimes children have an outwardly normal appearance but have other signs of serious illness, such as seizures, lethargy, vomiting, abnormal urine odor, and failure to feed and grow normally. During their first year, many children with more severe ID have delayed development of motor skills, and are slow to roll, sit, and stand.
However, most children with ID do not develop symptoms that are noticeable until the preschool period. Symptoms become apparent at a younger age in those more severely affected. Usually, the first problem parents notice is a delay in language development. Children with ID are slower to use words, put words together, and speak in complete sentences. Their social development is sometimes slow, because of cognitive impairment and language deficiencies. Children with ID may be slow to learn to dress and feed themselves. Some parents may not consider the possibility of cognitive impairment until the child is in school or preschool and is unable to keep up with age-appropriate expectations.
Children with ID are somewhat more likely than other children to have behavioral problems, such as explosive outbursts, temper tantrums, and physically aggressive or self-injurious behavior. These behaviors are often related to specific frustrating situations compounded by an impaired ability to communicate and control impulses. Older children may be gullible and easily taken advantage of or led into minor misbehavior.
About 10 to 40% of people with ID also have a mental health disorder (dual diagnosis). In particular, anxiety and depression are common, especially in children who are aware that they are different from their peers or who are maligned and mistreated because of their disability.
Many children are evaluated by teams of professionals, including a pediatric neurologist or developmental pediatrician, a psychologist, speech pathologist, occupational or physical therapist, special educator, social worker, or nurse.
Doctors evaluate a child suspected of having ID by testing intellectual functioning and looking for a cause. Even though the cause of the child's ID may be irreversible, identifying a disorder that caused the disability may allow doctors to predict the child's future course, prevent further loss of skills, plan any interventions that can increase the child's level of functioning, and counsel parents on the risk of having another child with that disorder.
Newborns with physical abnormalities or other symptoms suggestive of a condition associated with ID often need laboratory tests to help detect metabolic and genetic disorders. Imaging tests, such as magnetic resonance imaging (MRI), may be done to look for structural problems within the brain. An electroencephalogram (EEG) records the brain's electrical activity and is used to evaluate a child for possible seizures. Chromosome analysis, including the new technology of chromosome microarray, urine and blood tests, and x-rays of bones can also help rule out suspected causes of ID.
Some children who are delayed in learning language and mastering social skills have conditions other than ID. Because hearing problems interfere with language and social development, a hearing evaluation is typically done. Emotional problems and learning disorders also can be mistaken for ID. Children who have been severely deprived of normal love and attention (see Physical Neglect) for long periods of time may seem to have ID. A child with delays in sitting or walking (gross motor skills) or in manipulating objects (fine motor skills) may have a neurologic disorder not associated with ID.
Because mild developmental problems are not always noticed by parents, doctors routinely do developmental screening tests during well-child visits. Doctors use simple questionnaires, such as the Ages and Stages Questionnaires or Child Development Inventories, to quickly evaluate the child's cognitive, verbal, and motor skills. Parents can help the doctor determine the child's level of functioning by completing a Parents' Evaluation of Developmental Status (PEDS) test. Children who perform significantly below their age level on these screening tests are referred for formal testing.
Formal testing has three components: interviews with parents, observations of the child, and tests in which the child's performance is compared with scores of many children of the same age (norm-referenced test). Some tests, such as the Stanford-Binet Intelligence Test and the Wechsler Intelligence Scale for Children, measure intellectual ability. Other tests, such as the Vineland Adaptive Behavior Scales, assess areas such as communication, daily living skills, social abilities, and motor skills. Generally, these formal tests accurately compare a child's intellectual and social abilities with those of others in the same age group. However, children of different cultural backgrounds, non–English-speaking families, and very low socioeconomic status are more likely to do poorly on these tests. For these reasons, a diagnosis of ID requires that the doctor integrate the test data with information obtained from parents and direct observations of the child. A diagnosis of ID is appropriate only when both intellectual and adaptive skills are significantly below average.
Prognosis and Prevention
Because ID sometimes coexists with serious physical problems, the life expectancy of children with ID may be shortened, depending on the specific condition. In general, the more severe the cognitive disability and the more physical problems the child has, the shorter the life expectancy. However, in the absence of physical problems, a child with mild ID has a relatively normal life expectancy, and health care is improving long-term health outcomes for people with all types of developmental disabilities. Many people with mild to moderate ID can support themselves, can live independently, and can be successful at jobs that require basic intellectual skills.
Prevention applies to environmental, genetic, and infectious disorders as well as to accidental injuries. Fetal alcohol syndrome is a highly common and totally preventable cause of ID. The March of Dimes and other groups concerned about the prevention of ID focus much of their efforts on alerting women to the seriously damaging effects of drinking alcohol during pregnancy. Doctors may recommend genetic testing for people who have a family member or other child with a known inherited disorder, particularly ones related to ID, such as phenylketonuria, Tay-Sachs disease, or Fragile X syndrome. Identification of a gene for an inherited disorder allows genetic counselors to help parents evaluate the risk of having an affected child. Women who plan to get pregnant should receive necessary vaccinations, particularly against rubella. Women who are at risk of infectious disorders that may be harmful to a fetus, such as rubella and human immunodeficiency virus (HIV), should be tested before getting pregnant.
Proper prenatal care lowers the risk of having a child with ID. Folate (folic acid), a vitamin supplement, taken before conception and early in pregnancy can help prevent certain kinds of brain abnormalities. Advances in the practices of labor and delivery and in the care of premature infants have helped to reduce the rate of ID related to prematurity.
Certain tests, such as ultrasonography, amniocentesis, chorionic villus sampling, and various blood tests, can be done during pregnancy to identify conditions that often result in ID. Amniocentesis or chorionic villus sampling is often used for women at high risk of having a baby with Down syndrome, especially those aged 35 and older, and for women with family histories of metabolic disorders. Measuring maternal serum alpha-fetoprotein is a helpful screening test for neural tube defects, Down syndrome, and other abnormalities. A few conditions, such as hydrocephalus and severe Rh incompatibility (see Rh Incompatibility), may be treated during pregnancy. Most conditions, however, cannot be treated, and early recognition can serve only to prepare the parents and allow them to consider the option of abortion.
A child with ID is best cared for by a multidisciplinary team consisting of the primary care doctor, social workers, speech therapists, occupational therapists, physical therapists, neurologists or developmental pediatricians, psychologists, nutritionists, educators; and others. Together with the family, these people develop a comprehensive, individualized program for the child, which is begun as soon as the diagnosis of ID is suspected. The parents and siblings of the child also need emotional support and sometimes counseling. The whole family should be an integral part of the program.
The full array of a child's strengths and weaknesses must be considered in determining what kind of support is needed. Factors such as physical disabilities, personality problems, mental illness, and interpersonal skills are all taken into consideration. Affected children with coexisting mental health disorders such as depression may be given appropriate drugs in dosages similar to those given to children without ID. However, giving a child drugs without also instituting behavioral therapy and environmental changes is usually not helpful.
All children with ID benefit from special education. The federal Individuals with Disabilities Education Act (IDEA) requires public schools to provide free and appropriate education to children and adolescents with ID or other developmental disorders. Education must be provided in the least restrictive, most inclusive setting possible—where the children have every opportunity to interact with non-disabled peers and have equal access to community resources.
A child with ID usually does best living at home. However, some families cannot provide care at home, especially for children with severe, complex disabilities. This decision is difficult and requires extensive discussion between the family and their entire support team. Having a child with severe disabilities at home requires dedicated care that some parents may not be able to provide. The family may need psychologic support. A social worker can organize services to assist the family. Help can be provided by day care centers, housekeepers, child caregivers, and respite care facilities. Most adults with ID live in community-based residences that provide services appropriate to the person's needs, as well as work and recreational opportunities.
Last full review/revision November 2013 by Stephen Brian Sulkes, MD