Intellectual disability is significantly below average intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily living.
- Intellectual disability can be genetic or the result of a disorder that interferes with brain development.
- Most children with intellectual disability do not develop noticeable symptoms until they are in preschool.
- The diagnosis is based on the results of formal testing.
- Proper prenatal care lowers the risk of having a child with intellectual disability.
- Support from many specialists, therapy, and special education help children achieve the highest level of functioning possible.
Intellectual disability is a neurodevelopmental disorder.
The previously used term mental retardation has acquired an undesirable social stigma, so health care practitioners have replaced it with the term intellectual disability.
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 is severe enough to limit 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 may be categorized into several areas including
- Conceptual area: Competence in memory, reading, writing, and math
- Social area: Awareness of others' thoughts and feelings, interpersonal skills, and social judgment
- Practical area: Personal care, task organization (for work or school), money management, and health and safety
People with intellectual disability 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 impact on the person's life 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: Occasional support needed
- Limited: Support such as a day program in a sheltered workshop
- Extensive: Daily, ongoing support
- Pervasive: High level of support for all activities of daily living, possibly including extensive nursing care
Based only on IQ test scores, about 3% of the population has intellectual disability (an IQ of less than 70). If classification is based on the need for support, only about 1% of the population has severe intellectual disability.
Levels of Intellectual Disability
Ability at Preschool Age (Birth to 6 Years)
Adaptive Skills at School Age (6 to 20 Years)
Support Required at Adult Age (21 Years and Older)
Often presents as speech–language delay
Often not diagnosed until later age
Can develop social and communication skills
Some difficulty learning reading, writing, and math, but can learn up to about the 6th-grade level by late adolescence
Challenges making plans and managing money
Socially immature but can be expected to learn appropriate social skills
Some limitation of judgment and understanding of risk—more easily manipulated by others
Needs guidance and assistance in complex tasks (such as health care and legal decisions) and during times of unusual social or economic stress
Can usually achieve enough social and vocational skills for self-support
Poor social awareness
Can profit from training in self-help
Can talk or learn to communicate
With support, can progress to elementary school level in schoolwork
May learn to travel alone in familiar places
Social judgment and understanding limited but can learn some social and occupational skills
May have successful friendships and romantic relationships
Cares for simple personal and household needs after extended guidance
Needs supervision and guidance managing money, scheduling, and all but simplest daily tasks
May achieve self-support by doing unskilled or semiskilled work in a supportive environment
Able to learn some self-help skills
Has limited speech skills
Can say a few words
Can talk or learn to communicate about simple, everyday events and learn simple health habits
Little understanding of written language, numbers, time, or money
Benefits from habit training
Usually successful relationships with family members and familiar others
Sometimes maladaptive behavior (including self-injury)
Can develop some useful self-protection skills in controlled environments
Requires support for all daily tasks but may contribute partially to self-care under complete supervision
IQ 19 or below
May need nursing care due to limited self-care skills
Extreme cognitive limitation
Often sensory and/or physical impairments
Limited understanding of speech or gestures; communicates mainly nonverbally
Enjoys company of well-known family and caretakers, but sensory and physical impairments often limit social activities
Often needs nursing care
May have very limited participation in self-care
Causes of Intellectual Disability
A wide variety of medical and environmental conditions can cause intellectual disability. Some conditions 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. Even with recent advances in genetics, especially techniques of chromosome analysis, a specific cause of ID often cannot be identified.
Some causes that can occur before or at conception include
- Inherited disorders (such as phenylketonuria, Tay-Sachs disease, neurofibromatosis, hypothyroidism, and Fragile X syndrome)
- Chromosomal abnormalities (such as Down syndrome)
Some causes that can occur during pregnancy include
- Severe maternal undernutrition
- Infections with human immunodeficiency virus, cytomegalovirus, herpes simplex virus, toxoplasmosis, rubella, or Zika virus
- Toxins (such as lead and methylmercury)
- Alcohol (fetal alcohol spectrum disorder)
- Drugs (such as phenytoin, valproate, isotretinoin, and cancer chemotherapy drugs)
- Abnormal brain development (such as porencephalic cyst, gray matter heterotopia, and encephalocele)
- Preeclampsia and multiple births (such as twins or triplets)
Some causes that can occur during birth include
- Insufficient oxygen (hypoxia)
- Extreme prematurity
Some causes that can occur after birth include
Symptoms of Intellectual Disability
Some children with intellectual disability may 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 20 to 35% of people with ID also have a mental health disorder. In particular, anxiety and depression are common, especially in children who are aware that they are different from their peers or who are bullied and mistreated because of their disability.
Diagnosis of Intellectual Disability
- Prenatal screening
- Developmental screening
- Formal intellectual and skills testing
- Imaging tests
- Genetic and other laboratory tests
Screening before birth (prenatal screening) can be done to determine whether the fetus has certain abnormalities, including certain genetic disorders, that may cause intellectual disability.
From birth on, growth and development, including cognitive ability, are routinely assessed at well-child visits.
When doctors suspect intellectual disability, children are evaluated by teams of professionals, including early intervention or school staff, a primary care doctor, a pediatric neurologist or developmental pediatrician, a psychologist, speech pathologist, occupational or physical therapist, special educator, social worker, or nurse. These professionals evaluate a child suspected of having intellectual disability 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.
Certain tests, such as ultrasonography, amniocentesis, chorionic villus sampling, and various blood tests such as quad screening, can be done during pregnancy to identify conditions that often result in ID. Amniocentesis or chorionic villus sampling is often done for pregnant women over 35 years of age because they have an increased risk of having a baby with Down syndrome and for pregnant women who have a family history of metabolic disorders. The quad screen is a test done in most pregnant women. It is done to measure levels of four substances in a woman's blood. The results of this test help doctors evaluate whether the fetus has an increased risk of having certain conditions, such as Down syndrome, trisomy 18, or neural tube defects.
Measuring the mother's blood level of alpha-fetoprotein is a helpful screening test for neural tube defects, Down syndrome, and other abnormalities. Noninvasive prenatal screening (NIPS) detects small amounts of DNA from the fetus in the mother's blood and uses that to diagnose genetic disorders in the fetus such as trisomy 21 (Down syndrome), trisomy 13, or trisomy 18 and certain other chromosome disorders.
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 intellectual and skills testing
Formal testing has three components:
- Interviews with parents
- Observations of the child
- Tests in which the child's performance is compared with scores of many children of the same age
Some tests, such as the Stanford-Binet Intelligence Test and the Wechsler Intelligence Scale for Children, are done to measure intellectual ability. Other tests, such as the Vineland Adaptive Behavior Scales, are done to 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 (called norm-referenced tests). 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.
Identifying the cause
Newborns with physical abnormalities or other symptoms suggestive of a condition associated with intellectual disability often need certain tests.
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. X-rays of bones can also help rule out suspected causes of ID.
Genetic tests, such as chromosomal microarray analysis, may help identify disorders. 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.
Other urine and blood tests are done depending on what doctors suspect is the cause.
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 Overview of Child Neglect and Abuse) 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.
Prognosis of Intellectual Disability
A person with mild ID has a relatively normal life expectancy, and health care is improving long-term health outcomes for people with all types of intellectual disabilities. Many people with ID can support themselves, can live independently, and can be successfully employed with appropriate support.
Because intellectual disability sometimes coexists with serious physical problems, the life expectancy of people with ID may be shortened, depending on the specific condition. People with more severe intellectual disability are likely to require support for life. In general, the more severe the cognitive disability and the more physical problems the person has, the shorter the life expectancy.
Prevention of Intellectual Disability
Fetal alcohol spectrum disorders is a highly common and totally preventable cause of intellectual disability. 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.
Women who plan to get pregnant should receive necessary vaccinations, particularly against rubella. Women who are at risk of infectious diseases 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, especially neural tube defects.
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.
A few conditions, such as hydrocephalus and severe 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.
Treatment of Intellectual Disability
- Multidisciplinary support
A child with ID is best cared for by a multidisciplinary team consisting of the following:
- Primary care doctor
- Social workers
- Speech pathologists
- Occupational therapists
- Physical therapists
- Neurologists or developmental pediatricians
Other professionals may also be part of the team if necessary. Together with the family, these people develop a comprehensive, individualized program for the child that 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 person'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. People with ID and coexisting mental health disorders such as depression may be given appropriate drugs in dosages similar to those given to those without ID. However, giving a child drugs without doing behavioral therapy and making 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—that is, a setting where the children have every opportunity to interact with nondisabled peers and have equal access to community resources. The Americans with Disability Act and Section 504 of the Rehabilitation Act also provide for accommodations in schools and other public settings.
Did You Know...
A child with intellectual disability usually does best living at home. However, some families cannot provide care at home, especially for children with severe, complex disabilities or behavior concerns. This decision is difficult and requires extensive discussion between the family and their entire support team. 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 care givers, 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.
- Individuals with Disabilities Education Act (IDEA)
- March of Dimes
- Americans with Disability Act
- Section 504 of the Rehabilitation Act
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