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 Attention-Deficit/Hyperactivity Disorder (ADD, ADHD) Attention-deficit/hyperactivity disorder (ADHD) is a syndrome of inattention, hyperactivity, and impulsivity. The 3 types of ADHD are predominantly inattentive, predominantly hyperactive/impulsive... read more , autism spectrum disorders Autism Spectrum Disorders Autism spectrum disorders are neurodevelopmental disorders characterized by impaired social interaction and communication, repetitive and stereotyped patterns of behavior, and uneven intellectual... read more , and learning disorders Overview of Learning Disorders Learning disorders are conditions that cause a discrepancy between potential and actual levels of academic performance as predicted by the person’s intellectual abilities. Learning disorders... read more (eg, dyslexia Dyslexia Dyslexia is a general term for primary reading disorder. Diagnosis is based on intellectual, educational, speech and language, medical, and psychologic evaluations. Treatment is primarily educational... read more ).
Intellectual disability must involve early-childhood onset of deficits in both of the following:
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 weaknesses, 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.
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 the cause of an intellectual disability, a specific cause of intellectual disability 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 Overview of Learning Disorders Learning disorders are conditions that cause a discrepancy between potential and actual levels of academic performance as predicted by the person’s intellectual abilities. Learning disorders... read more , or deafness Hearing Impairment in Children Common causes of hearing loss are genetic defects in neonates and ear infections and cerumen in children. Many cases are detected by screening, but hearing loss should be suspected if children... read more rather than intellectual disability.
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* Some Chromosomal and Genetic Causes of Intellectual Disability* Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient 70 to 75) combined with limitations of adaptive functioning... read more ).
Congenital infections Overview of Neonatal Infections Neonatal infection can be acquired In utero transplacentally or through ruptured membranes In the birth canal during delivery (intrapartum) From external sources after birth (postpartum) Common... read more that can cause intellectual disability include rubella Congenital Rubella Congenital rubella is a viral infection acquired from the mother during pregnancy. Signs are multiple congenital anomalies that can result in fetal death. Diagnosis is by serology and viral... read more and those due to cytomegalovirus Congenital and Perinatal Cytomegalovirus Infection (CMV) Cytomegalovirus infection may be acquired prenatally or perinatally and is the most common congenital viral infection. Signs at birth, if present, are intrauterine growth restriction, prematurity... read more , Toxoplasma gondii Congenital Toxoplasmosis Congenital toxoplasmosis is caused by transplacental acquisition of Toxoplasma gondii. Manifestations, if present, are prematurity, intrauterine growth restriction, jaundice, hepatosplenomegaly... read more , Treponema pallidum, herpes simplex virus Neonatal Herpes Simplex Virus (HSV) Infection Neonatal herpes simplex virus infection is usually transmitted during delivery. A typical sign is vesicular eruption, which may be accompanied by or progress to disseminated disease. Diagnosis... read more , or HIV Human Immunodeficiency Virus (HIV) Infection in Infants and Children Human immunodeficiency virus (HIV) infection is caused by the retrovirus HIV-1 (and less commonly by the related retrovirus HIV-2). Infection leads to progressive immunologic deterioration and... read more . Prenatal Zika virus Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more infection causes congenital microcephaly Microcephaly Microcephaly is a head circumference 2 standard deviations below the mean for age. (See also Introduction to Congenital Craniofacial and Musculoskeletal Disorders and Overview of Congenital... read more and associated severe intellectual disability.
Prenatal drug and toxin exposure Drugs in Pregnancy Drugs are used in over half of all pregnancies, and prevalence of use is increasing. The most commonly used drugs include antiemetics, antacids, antihistamines, analgesics, antimicrobials, diuretics... read more can cause intellectual disability. Fetal alcohol syndrome Fetal Alcohol Syndrome Alcohol exposure in utero increases the risk of spontaneous abortion, decreases birth weight, and can cause fetal alcohol syndrome, a constellation of variable physical and cognitive abnormalities... read more is the most common of these conditions. Anticonvulsants such as phenytoin or valproate, chemotherapy drugs, radiation exposure, lead, and methylmercury are also causes.
Severe undernutrition during pregnancy may affect fetal brain development, resulting in intellectual disability.
Complications related to prematurity Premature Infants An infant born before 37 weeks gestation is considered premature. Prematurity is defined by the gestational age at which infants are born. Previously, any infant weighing read more , central nervous system bleeding Intracranial Hemorrhage The forces of labor and delivery occasionally cause physical injury to the infant. The incidence of neonatal injury resulting from difficult or traumatic deliveries is decreasing due to increasing... read more , periventricular leukomalacia, breech Breech presentation Fetal dystocia is abnormal fetal size or position resulting in difficult delivery. Diagnosis is by examination, ultrasonography, or response to augmentation of labor. Treatment is with physical... read more or high forceps delivery, a multifetal pregnancy Multifetal Pregnancy Multifetal pregnancy is presence of > 1 fetus in the uterus. Multifetal (multiple) pregnancy occurs in up to 1 of 30 deliveries. Risk factors for multiple pregnancy include Ovarian stimulation... read more , placenta previa Placenta Previa Placenta previa is implantation of the placenta over or near the internal os of the cervix. Typically, painless vaginal bleeding with bright red blood occurs after 20 weeks gestation. Diagnosis... read more , preeclampsia Preeclampsia and Eclampsia Preeclampsia is new-onset or worsening of existing hypertension with proteinuria after 20 weeks gestation. Eclampsia is unexplained generalized seizures in patients with preeclampsia. Diagnosis... read more , and perinatal asphyxia may increase the risk of intellectual disability. The risk is increased in small-for-gestational-age infants Small-for-Gestational-Age (SGA) Infant Infants whose weight is the 10th percentile for gestational age are classified as small for gestational age. Complications include perinatal asphyxia, meconium aspiration, polycythemia, and... read more ; 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.
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 Pneumococcal Infections Streptococcus pneumoniae (pneumococci) are gram-positive, alpha-hemolytic, aerobic, encapsulated diplococci. In the US, pneumococcal infection is a major cause of otitis media, pneumonia, sepsis... read more , Haemophilus influenzae infection Haemophilus Infections The gram-negative bacteria Haemophilus species cause numerous mild and serious infections, including bacteremia, meningitis, pneumonia, sinusitis, otitis media, cellulitis, and epiglottitis... read more ), poisoning (eg, lead Lead Poisoning Lead poisoning often causes minimal symptoms at first but can cause acute encephalopathy or irreversible organ damage, commonly resulting in cognitive deficits in children. Diagnosis is by whole... read more , mercury), and accidents that cause severe head injuries Traumatic Brain Injury (TBI) Traumatic brain injury (TBI) is physical injury to brain tissue that temporarily or permanently impairs brain function. Diagnosis is suspected clinically and confirmed by imaging (primarily... read more or asphyxia may result in intellectual disability.
The primary manifestations of intellectual disability are
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 Cerebral Palsy (CP) Syndromes Cerebral palsy refers to nonprogressive syndromes characterized by impaired voluntary movement or posture and resulting from prenatal developmental malformations or perinatal or postnatal central... read more ) that may be associated with a particular cause of intellectual disability (eg, perinatal asphyxia). Delayed development is usually apparent by preschool age. 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
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 are common, particularly attention-deficit/hyperactivity disorder Attention-Deficit/Hyperactivity Disorder (ADD, ADHD) Attention-deficit/hyperactivity disorder (ADHD) is a syndrome of inattention, hyperactivity, and impulsivity. The 3 types of ADHD are predominantly inattentive, predominantly hyperactive/impulsive... read more , mood disorders (depression Depressive Disorders in Children and Adolescents Depressive disorders are characterized by sadness or irritability that is severe or persistent enough to interfere with functioning or cause considerable distress. Diagnosis is by history and... read more , bipolar disorder Bipolar Disorder in Children and Adolescents Bipolar disorder is characterized by alternating periods of mania, depression, and normal mood, each lasting for weeks to months at a time. Diagnosis is based on clinical criteria. Treatment... read more ), autism spectrum disorders Autism Spectrum Disorders Autism spectrum disorders are neurodevelopmental disorders characterized by impaired social interaction and communication, repetitive and stereotyped patterns of behavior, and uneven intellectual... read more , anxiety disorder Overview of Anxiety Disorders in Children and Adolescents Anxiety disorders are characterized by fear, worry, or dread that greatly impairs the ability to function normally and that is disproportionate to the circumstances at hand. Anxiety may result... read more , and others.
Some children have comorbid motor or sensory impairments, such as cerebral palsy Cerebral Palsy (CP) Syndromes Cerebral palsy refers to nonprogressive syndromes characterized by impaired voluntary movement or posture and resulting from prenatal developmental malformations or perinatal or postnatal central... read more 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.
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 Childhood Development Development is often divided into specific domains, such as gross motor, fine motor, language, cognition, and social/emotional growth. These designations are useful, but substantial overlap... read more , 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.
Genetic counseling Prenatal Genetic Counseling Prenatal genetic counseling is provided for all prospective parents, ideally before conception, to assess risk factors for congenital disorders. Certain precautions to help prevent birth defects... read more 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 Genetic Evaluation Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more 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 Amniocentesis Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more or chorionic villus sampling Chorionic Villus Sampling Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more 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 Prenatal Ultrasonography Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more 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 Screening Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes... read more (NIPS) methods can be used to identify numerical chromosome abnormalities and have been used to identify some larger microdeletion syndromes such as 22q11 deletion.
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, 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
History (including perinatal, developmental, neurologic, and familial) may identify causes. (See also the 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 Diagnosis Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and... read more shows Down syndrome (trisomy 21) and other disorders of chromosome number.
Chromosomal microarray analysis Diagnosis Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes... read more 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).
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 Genetic Diagnostic Technologies Genetic diagnostic technology is rapidly improving. A small amount of DNA can be amplified using the polymerase chain reaction (PCR) process, which can produce millions of copies of a gene or... read more 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 Approach to the Patient With a Suspected Inherited Disorder of Metabolism Most inherited disorders of metabolism (inborn errors of metabolism) are rare, and therefore their diagnosis requires a high index of suspicion. Timely diagnosis leads to early treatment and... read more . Isolated delays in sitting or walking (gross motor skills) and in pincer grasp, drawing, or writing (fine motor skills) may indicate a neuromuscular disorder Introduction to Inherited Muscular Disorders Muscular dystrophies are inherited, progressive muscle disorders resulting from defects in one or more genes needed for normal muscle structure and function; dystrophic changes (eg, muscle fiber... read more .
Specific laboratory tests are done depending on the suspected cause (see Table: Tests for Some Causes of Intellectual Disability Tests for Some Causes of Intellectual Disability Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient 70 to 75) combined with limitations of adaptive functioning... read more ). Visual and auditory assessments should be done at an early age, and screening for lead poisoning is often appropriate.
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 likely to require life-long support. The more severe the cognitive disability and the greater the immobility, the higher the mortality risk.
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
Physical therapists Physical Therapy (PT) Physical therapy aims to improve joint and muscle function (eg, range of motion, strength) and thus improve the patient’s ability to stand, balance, walk, and climb stairs. For example, physical... read more and occupational therapists Occupational Therapy (OT) Occupational therapy (OT) focuses on self-care activities and improvement of fine motor coordination of muscles and joints, particularly in the upper extremities. Unlike physical therapy, which... read more (who assist in managing comorbidities in children with motor deficits)
Speech pathologists Speech Therapy Speech therapists can identify the most effective methods of communication for patients who have aphasia, dysarthria, or verbal apraxia or who have had a laryngectomy: Expressive aphasia: A... read more 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 community-based residences. 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.
Vaccines Childhood Vaccination Schedule Vaccination follows a schedule recommended by the Centers for Disease Control and Prevention (CDC), the American Academy of Pediatrics, the American Academy of Family Physicians, and the American... read more have all but eliminated congenital rubella and pneumococcal and H. influenzae meningitis as causes of intellectual disability.
Fetal alcohol syndrome Fetal Alcohol Syndrome Alcohol exposure in utero increases the risk of spontaneous abortion, decreases birth weight, and can cause fetal alcohol syndrome, a constellation of variable physical and cognitive abnormalities... read more 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 Prevention Spina bifida is defective closure of the vertebral column. Although the cause is not known, low folate levels during pregnancy increase risk. Some children are asymptomatic, and others have... read more ).
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.
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 disorders 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.
American Academy of Neurology and the Practice Committee of the Child Neurology Society's evidence report on genetic and metabolic testing on children with global developmental delay
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