Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient 70 to 75) combined with limitations of adaptive functioning (ie, communication, self-direction, social skills, self-care, use of community resources, and maintenance of personal safety), along with demonstrated need for support. Management consists of education, family counseling, and social support.
Intellectual disability is considered a neurodevelopmental disorder. Neurodevelopmental disorders are neurologically based conditions that appear early in childhood, typically before school entry, and impair development of personal, social, academic, and/or occupational functioning. They typically involve difficulties with the acquisition, retention, or application of specific skills or sets of information. Neurodevelopmental disorders may involve dysfunction in one or more of the following: attention, memory, perception, language, problem-solving, or social interaction. Other common neurodevelopmental disorders include attention-deficit/hyperactivity disorder, autism spectrum disorders, and learning disorders (eg, dyslexia).
Intellectual disability must involve early-childhood onset of deficits in both of the following:
Intellectual functioning (eg, in reasoning, planning and problem solving, abstract thinking, learning at school or from experience)
Adaptive functioning (ie, ability to meet age- and socioculturally appropriate standards for independent functioning in activities of daily life)
Basing severity on IQ alone (eg, mild, 52 to 70 or 75; moderate, 36 to 51; severe, 20 to 35; and profound, < 20) is inadequate. Classification must also account for the level of support needed, ranging from intermittent to ongoing high-level support for all activities. Such an approach focuses on a person’s strengths and needs, relating them to the demands of the person’s environment and the expectations and attitudes of the family and community.
About 3% of the population functions at an IQ of < 70, which is at least 2 standard deviations below the mean IQ of the general population (IQ of 100); if the need for support is considered, only about 1% of the population has severe intellectual disability. Severe intellectual disability occurs in families from all socioeconomic groups and educational levels. Less severe intellectual disability (requiring intermittent or limited support) occurs most often in lower socioeconomic groups, paralleling with observations that IQ correlates best with success in school and socioeconomic status rather than specific organic factors. Nevertheless, recent studies suggest that genetic factors play roles even in milder cognitive disabilities.
Etiology of Intellectual Disability
Intelligence is both genetically and environmentally determined. Children born to parents with intellectual disability are at increased risk of a range of developmental disabilities, but clear genetic transmission of intellectual disability is unusual. Although advances in genetics, such as chromosomal microarray analysis and whole genome sequencing of the coding regions (exome), have increased the likelihood of identifying some causes of intellectual disability, a cause of intellectual disability in a specific person often cannot be identified. A cause is most likely to be identified in severe cases. Deficits in language and personal–social skills also may be due to emotional problems, environmental deprivation, learning disorders, or deafness rather than intellectual disability.
Prenatal factors
A number of chromosomal anomalies and genetic metabolic and neurologic disorders can cause intellectual disability ( see Table: Some Chromosomal and Genetic Causes of Intellectual Disability*).
Congenital infections that can cause intellectual disability include rubella and those due to cytomegalovirus, Toxoplasma gondii, Treponema pallidum, herpes simplex virus, or HIV. Prenatal Zika virus infection causes congenital microcephaly and associated severe intellectual disability.
Prenatal drug and toxin exposure can cause intellectual disability. Fetal alcohol syndrome
Severe undernutrition during pregnancy may affect fetal brain development, resulting in intellectual disability.
Perinatal factors
Complications related to prematurity, central nervous system bleeding, periventricular leukomalacia, breech or high forceps delivery, a multifetal pregnancy, placenta previa, preeclampsia, and perinatal asphyxia may increase the risk of intellectual disability. The risk is increased in small-for-gestational-age infants; intellectual impairment and decreased weight share similar causes. Very low- and extremely low-birth-weight infants have variably increased chances of having intellectual disability, depending on gestational age, perinatal events, and quality of care.
Postnatal factors
Undernutrition and environmental deprivation (lack of physical, emotional, and cognitive support required for growth, development, and social adaptation) during infancy and early childhood may be the most common causes of intellectual disability worldwide. Viral and bacterial encephalitides (including AIDS-associated neuroencephalopathy) and meningitides (eg, pneumococcal infections, Haemophilus influenzae infection), poisoning (eg, lead, mercury), and accidents that cause severe head injuries or asphyxia may result in intellectual disability.
Symptoms and Signs of Intellectual Disability
The primary manifestations of intellectual disability are
Slowed acquisition of new knowledge and skills
Immature behavior
Limited self-care skills
Some children with mild intellectual disability may not develop recognizable symptoms until preschool age. However, early identification is common among children with moderate to severe intellectual disability and among children in whom intellectual disability is accompanied by physical abnormalities or signs of a condition (eg, cerebral palsy) that may be associated with a particular cause of intellectual disability (eg, perinatal asphyxia). Delayed development is usually apparent by preschool age, often manifesting more as delay in communication than in motor skills. Among older children, hallmark features are a low IQ combined with limitations in adaptive behavior skills (eg, communication, self-direction, social skills, self-care, use of community resources, maintenance of personal safety). Although developmental patterns may vary, it is much more common for children with intellectual disability to experience slow progress than developmental arrest.
Behavioral problems are the reason for most psychiatric referrals and out-of-home placements for people with intellectual disability. Behavioral problems are often situational, and precipitating factors can usually be identified. Factors that predispose to unacceptable behavior include
Lack of training in socially responsible behavior
Inconsistent limit setting
Reinforcement of faulty behavior
Impaired ability to communicate
Discomfort due to coexisting physical problems and mental health disorders such as depression or anxiety
In institutional settings (now uncommon in the US), overcrowding, understaffing, and lack of activities contribute to both behavior challenges and to limited functional progress. Avoidance of long-term placement in large congregate care settings is extremely important in maximizing the individual's success.
Comorbid disorders
Comorbid disorders are common, particularly attention-deficit/hyperactivity disorder, mood disorders (depression, bipolar disorder), autism spectrum disorders, anxiety disorder, and others.
Some children have comorbid motor or sensory impairments, such as cerebral palsy or other motor deficits, language delays, or hearing loss. Such motor or sensory impairments can mimic cognitive impairment but are not in themselves causes of it. As children mature, some develop anxiety or depression if they are socially rejected by other children or if they are disturbed by the realization that others see them as different and deficient. Well-managed, inclusive school programs can help maximize social integration, thereby minimizing such emotional responses.
Diagnosis of Intellectual Disability
Prenatal testing
Intelligence and developmental assessment
Imaging of the central nervous system
Genetic testing
Prenatal testing can be done to determine whether the fetus has abnormalities, including genetic disorders, that predispose to intellectual disability.
From birth on, growth and development, including cognitive ability, are routinely assessed at well-child visits. For suspected cases of intellectual disability, development and intelligence are assessed in greater detail, typically by early intervention or school staff.
Establishing intellectual disability is followed by efforts to determine a cause, often including central nervous system (CNS) imaging and genetic and metabolic testing. Accurate determination of the cause may provide a developmental prognosis, suggest plans for educational and training programs, help in genetic counseling, and relieve parental guilt.
Prenatal testing
Genetic counseling may help high-risk couples understand possible risks. If a child has intellectual disability, evaluation of the etiology can provide the family with appropriate risk information for future pregnancies.
Prenatal testing may be done in high-risk couples who choose to have children. Prenatal testing enables couples to consider pregnancy termination and subsequent family planning. Testing includes
Amniocentesis or chorionic villus sampling
Quad screen
Ultrasonography
Maternal serum alpha-fetoprotein
Noninvasive prenatal screening
Amniocentesis or chorionic villus sampling may detect inherited metabolic and chromosomal disorders, carrier states, and CNS malformations (eg, neural tube defects, anencephaly) and may be considered for all pregnant women > 35 years of age because their risk of having an infant with Down syndrome is increased and for pregnant women with a family history of inherited metabolic disorders.
The quad screen (ie, measurement of maternal levels of beta-hCG, unconjugated estriol, alpha-fetoprotein, and inhibin A) is recommended for most pregnant women to evaluate risk of Down syndrome, trisomy 18, spina bifida, or abdominal wall defects.
Ultrasonography may also identify CNS defects.
Maternal serum alpha-fetoprotein is a helpful screen for neural tube defects, Down syndrome, and other abnormalities.
Noninvasive prenatal screening (NIPS) methods can be used to identify numerical chromosome abnormalities and have been used to identify some larger microdeletion syndromes such as 22q11 deletion.
Intelligence and developmental assessment
Standardized intelligence tests can measure subaverage intellectual ability but are subject to error, and results should be questioned when they do not match clinical findings; illness, motor or sensory impairments, language barriers, and/or cultural differences may hamper a child’s test performance. Such tests also have a middle-class bias but are generally reasonable in appraising intellectual ability in children, particularly in older ones.
Developmental screening tests such as the Ages and Stages Questionnaire (ASQ) or the Parents’ Evaluation of Developmental Status (PEDS) provide gross assessments of development for young children and can be given by a physician or others. Such measures should be used only for screening and not as substitutes for standardized intelligence tests, which should be given by qualified psychologists. A neurodevelopmental assessment should be initiated as soon as developmental delays are suspected.
A developmental pediatrician or pediatric neurologist should investigate all cases of
Moderate to severe developmental delays
Progressive disability
Neuromuscular deterioration
Suspected seizure disorders
Diagnosis of cause
History (including perinatal, developmental, neurologic, and familial) may identify causes. (See also the 2011 evidence report on genetic and metabolic testing on children with global developmental delay from the American Academy of Neurology and the Practice Committee of the Child Neurology Society.)
Cranial imaging (eg, MRI) can show central nervous system malformations (as seen in neurodermatoses such as neurofibromatosis or tuberous sclerosis), treatable hydrocephalus, or more severe brain malformations such as schizencephaly.
Genetic tests may help identify disorders:
Standard karyotyping shows Down syndrome (trisomy 21) and other disorders of chromosome number.
Chromosomal microarray analysis identifies copy number variants such as might be found in 5p- syndrome (5p minus syndrome or cri du chat syndrome) or DiGeorge syndrome (chromosome 22q deletion).
Direct DNA studies identify Fragile X syndrome.
Chromosomal microarray analysis is the preferred investigative tool; it can be used to identify specifically suspected syndromes and also when no specific syndrome is suspected. It affords opportunities for identifying otherwise unrecognized chromosome disruptions but requires parental testing to interpret positive findings. Whole genome sequencing of the coding regions (whole exome sequencing) is a newer, more detailed method that may uncover additional causes of intellectual disability.
Clinical manifestations (eg, failure to thrive, lethargy, vomiting, seizures, hypotonia, hepatosplenomegaly, coarse facial features, abnormal urinary odor, macroglossia) may suggest a genetic metabolic disorder. Isolated delays in sitting or walking (gross motor skills) and in pincer grasp, drawing, or writing (fine motor skills) may indicate a neuromuscular disorder.
Specific laboratory tests are done depending on the suspected cause ( see Table: Tests for Some Causes of Intellectual Disability). Visual and auditory assessments should be done at an early age, and screening for lead poisoning is often appropriate.
Prognosis for Intellectual Disability
Many people with mild to moderate intellectual disability can support themselves, live independently, and be successful at jobs that require basic intellectual skills.
Life expectancy may be shortened, depending on the etiology of the disability, but health care is improving long-term health outcomes for people with all types of developmental disabilities. People with severe intellectual disability are more likely to require life-long support. The more severe the cognitive disability and the greater the immobility, the higher the mortality risk.
Treatment of Intellectual Disability
Early intervention program
Multidisciplinary team support
Treatment and support needs depend on social competence and cognitive function. Referral to an early intervention program during infancy may prevent or decrease the severity of disability resulting from a perinatal insult. Realistic methods of caring for affected children must be established.
Family support and counseling are crucial. As soon as intellectual disability is confirmed or strongly suspected, the parents should be informed and given ample time to discuss causes, effects, prognosis, education and training of the child, and the importance of balancing known prognostic risks against negative self-fulfilling prophecies in which diminished expectations result in poor functional outcomes later in life. Sensitive ongoing counseling is essential for family adaptation. If the family’s physician cannot provide coordination and counseling, the child and family should be referred to a center with a multidisciplinary team that evaluates and serves children with intellectual disability; however, the family’s physician should provide continuing medical care and advice.
A comprehensive, individualized program is developed with the help of appropriate specialists, including educators.
A multidisciplinary team includes
Neurologists or developmental-behavioral pediatricians
Orthopedists
Physical therapists and occupational therapists (who assist in managing comorbidities in children with motor deficits)
Speech pathologists and audiologists (who help with language delays or with suspected hearing loss)
Nutritionists (who help with treatment of undernutrition)
Social workers (who help reduce environmental deprivation and identify key resources)
Psychologists (who oversee planning of behavioral interventions)
Affected children with concomitant mental health disorders such as depression may be given appropriate psychoactive drugs in dosages similar to those used in children without intellectual disability. Use of psychoactive drugs without behavioral therapy and environmental changes is rarely helpful.
Every effort should be made to have children live at home or in a community-based residence. Living at home with the family is usually better for the child than alternative placements unless marked behavior difficulties require a higher level of supervision than the family can provide. The family may benefit from psychologic support and help with daily care provided by day care centers, homemakers, and respite services. The living environment must encourage independence and reinforce learning of skills needed to accomplish this goal.
Whenever possible, children with intellectual disability should attend an appropriately adapted day care center or school with peers without cognitive disability. The Individuals with Disabilities Education Act (IDEA), the primary US special education law, stipulates that all children with disabilities should receive appropriate educational opportunities and programs in the least restrictive and most inclusive environments. The Americans with Disability Act and Section 504 of the Rehabilitation Act also provide for accommodations in schools and other public settings.
As people with intellectual disability reach adulthood, an array of supportive living and work settings is available. Large residential institutions are being replaced by small group or supported individual residences matched to the individual's functional abilities and needs.
Prevention of Intellectual Disability
Vaccines have all but eliminated congenital rubella and pneumococcal and H. influenzae meningitis as causes of intellectual disability.
Fetal alcohol syndrome is a highly common and totally preventable cause of intellectual disability. Because it is unknown when during pregnancy alcohol is most likely to harm the fetus and whether there is a lower limit of alcohol use that is completely safe, pregnant women should be advised to avoid all alcohol intake.
Folate supplementation (400 to 800 mcg orally once a day) in women beginning 3 months before conception and continuing through the 1st trimester reduces the risk of neural tube defects (see prevention of congenital neurologic anomalies).
Continuing improvements in, and increased availability of, obstetric and neonatal care and the use of exchange transfusion and Rho(D) immune globulin to prevent hemolytic disease of the newborn have reduced the incidence of intellectual disability; the increase in survival of very low-birth-weight infants has kept the prevalence constant.
Key Points
Intellectual disability involves slow intellectual development with subaverage intellectual functioning, immature behavior, and limited self-care skills that in combination are severe enough to require some level of support.
A number of prenatal, perinatal, and postnatal factors can cause intellectual disability, but a specific cause often cannot be identified.
Deficits in language and personal–social skills may be due to emotional problems, environmental deprivation, learning disorders, or deafness rather than intellectual disability.
Screen using tests such as the Ages and Stages Questionnaire (ASQ) or the Parents’ Evaluation of Developmental Status (PEDS) and refer suspected cases for standardized intelligence testing and neurodevelopmental assessment.
Search for specific causes with cranial imaging, genetic tests (eg, chromosomal microarray analysis, exome sequencing), and other tests as clinically indicated.
Provide a comprehensive, individualized program (including family support and counseling) using a multidisciplinary team.
More Information
The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
American Academy of Neurology and the Practice Committee of the Child Neurology Society: Evidence report on genetic and metabolic testing on children with global developmental delay (2011)
Individuals with Disabilities Education Act (IDEA): A US law that makes available free appropriate public education to eligible children with disabilities and ensures special education and related services to those children
Americans with Disability Act: A US law that prohibits discrimination based on disability
Section 504 of the Rehabilitation Act: A US law that guarantees certain rights to people who have disabilities
American Association on Intellectual and Developmental Disabilities (AAIDD): An organization providing research, support, and advocacy resources for people with intellectual and developmental disabilities
March of Dimes: An organization aiming to improve the health of mothers and babies through research, advocacy, and education
