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Intellectual disability (ID, previously called mental retardation) is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient < 70 to 75) combined with limitations of > 2 of the following: communication, self-direction, social skills, self-care, use of community resources, and maintenance of personal safety. Management consists of education, family counseling, and social support.
Basing severity on intelligence quotient (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 ID. Severe ID occurs in families from all socioeconomic groups and educational levels. Less severe ID (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
Intelligence is both genetically and environmentally determined. Children born to parents with ID are at increased risk of a range of developmental disabilities, but clear genetic transmission of ID is unusual. Although advances in genetics, such as chromosomal microarray analysis, have increased the likelihood of identifying the cause of an ID, a specific cause cannot be identified in 60 to 80% of cases. A cause is most likely to be identified in severe cases. Deficits in language and personal-social skills may be due to emotional problems, environmental deprivation, learning disorders, or deafness rather than ID.
Prenatal
A number of chromosomal anomalies and genetic metabolic and neurologic disorders can cause ID (see Table 4: Learning and Developmental Disorders: Some Chromosomal and Genetic Causes of Intellectual Disability* ).
Congenital infections that can cause ID include rubella and those due to cytomegalovirus, Toxoplasma gondii,
Treponema pallidum, or HIV.
Prenatal drug and toxin exposure (see Drugs in Pregnancy) can cause ID. Fetal alcohol syndrome (see also Metabolic, Electrolyte, and Toxic Disorders in Neonates: Alcohol) 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 ID.
Perinatal
Complications related to prematurity, CNS bleeding, periventricular leukomalacia, breech or high forceps delivery, multiple births, placenta previa, preeclampsia, and perinatal asphyxia may increase the risk of ID. The risk is increased in small-for-gestational-age infants; intellectual impairment and decreased weight share the same cause. Very low- and extremely low-birth-weight infants have variably increased chances of having ID, depending on gestational age, perinatal events, and quality of care.
Postnatal
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 ID worldwide. Viral and bacterial encephalitides (including AIDS-associated neuroencephalopathy) and meningitides, poisoning (eg, lead, mercury), and accidents that cause severe head injuries or asphyxia may result in ID.
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Table 4
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| Some Chromosomal and Genetic Causes of Intellectual Disability* |
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Cause
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Example
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Chromosomal abnormalities
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Cri du chat syndrome
Down syndrome
Fragile X syndrome
Klinefelter syndrome
Mosaicisms
Trisomy 13 (Patau syndrome)
Trisomy 18 (Edwards syndrome)
Turner syndrome
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Genetic metabolic disorders
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Autosomal recessive disorders
Aminoacidurias and acidemias
Maple syrup urine disease
Hurler syndrome (mucopolysaccharidosis)
Peroxisomal disorders
X-linked recessive disorders
Hunter syndrome (a variant of mucopolysaccharidosis)
Lesch-Nyhan syndrome (hyperuricemia)
Oculocerebrorenal (Lowe) syndrome
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Genetic neurologic disorders
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Autosomal dominant disorders
Autosomal recessive disorders
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*This is a partial list of disorders.
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Symptoms and Signs
The primary manifestations are
Some children with mild ID may not develop recognizable symptoms until preschool age. However, early identification is common among children with moderate to severe ID and among children in whom ID is accompanied by physical abnormalities or signs of a condition (eg, cerebral palsy) that may be associated with a particular cause of ID (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. Although developmental patterns may vary, it is much more common for children with ID to experience slow progress than developmental arrest.
Some children may have 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.
Behavioral disorders are the reason for most psychiatric referrals and out-of-home placements for people with ID. Behavioral problems are often situational, and precipitating factors can usually be identified. Factors that predispose to unacceptable behavior include
In institutional settings, overcrowding, understaffing, and lack of activities contribute.
Diagnosis
For suspected cases, development and intelligence are assessed, typically by early intervention or school staff. 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 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
Establishing ID is followed by efforts to determine a cause. 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. History (including perinatal, developmental, neurologic, and familial) may identify causes. An algorithm for the diagnostic evaluation of the child with ID (global developmental delay) has been proposed by the Child Neurology Society. Cranial imaging (eg, MRI) can show CNS 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 such as Down syndrome (trisomy 21) via standard karyotyping, 5p-deletion (cri du chat syndrome) or DiGeorge syndrome (chromosome 22q deletion) via fluorescent in situ hybridization (FISH), or fragile X syndrome via direct DNA studies. Recently, chromosomal microarray analysis has become more widely available, affording opportunities for identifying otherwise unrecognized chromosome disruptions. This technique may shortly become the preferred investigative tool when no specific syndrome is suspected. Exome sequencing is a newer method and may uncover additional causes of ID.
Genetic metabolic disorders may be suggested by their clinical manifestations (eg, failure to thrive, lethargy, vomiting, seizures, hypotonia, hepatosplenomegaly, coarse facial features, abnormal urinary odor, macroglossia). 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 5: Learning and Developmental Disorders: 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.
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Table 5
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| Tests for Some Causes of Intellectual Disability |
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Suspected Cause
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Indicated Tests
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Single major anomaly or multiple minor anomalies
Family history of cognitive disability
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Chromosome analysis
Chromosomal microarray analysis
Cranial CT and/or MRI*
Possibly exome sequencing
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Failure to thrive
Idiopathic hypotonia
Genetic metabolic disorders
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HIV screening in high-risk infants
Nutritional and psychosocial history
Urine and/or blood amino acid and enzyme studies for storage diseases or peroxisomal disorders
Muscle enzymes
SMA 12 (includes albumin, alkaline phosphatase, AST, total bilirubin, BUN, Ca, cholesterol, creatinine, glucose, P, total protein, and uric acid)
Bone age, skeletal x-rays
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Seizures
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EEG
Cranial CT, MRI*, or both
Blood Ca, P, Mg, amino acids, glucose, and lead levels
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Cranial abnormalities (eg, premature closure of the sutures, microcephaly, macrocephaly, craniostenosis, hydrocephalus)
Cerebral atrophy
Cerebral malformations
CNS hemorrhage
Tumor
Intracranial calcifications due to toxoplasmosis, cytomegalovirus infection, or tuberous sclerosis
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Cranial CT, MRI*, or both
TORCH screening
Urine culture for virus
Chromosome analysis
Chromosomal microarray analysis
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*After neurologic consultation.
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SMA = sequential multiple analyzer; TORCH = toxoplasmosis, rubella, cytomegalovirus, herpes.
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Prognosis
Many people with mild to moderate ID 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 ID are likely to require life-long support. The more severe the cognitive disability and the greater the immobility, the higher the mortality risk.
Treatment
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 ID 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 ID; 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
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 ID. 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. Although the presence of a child with ID in the home can be disruptive, it can also be extremely rewarding. 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 ID 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 programming in the least restrictive and most inclusive environments. As people with ID reach adulthood, an array of supportive living and work settings is available. Large residential institutions are being replaced by small group or individual residences matched to the affected person's functional abilities and needs.
Prevention
Genetic counseling (see also Prenatal Genetic Counseling and Evaluation: Prenatal Genetic Counseling) may help high-risk couples understand possible risks. If a child has ID, evaluation of the etiology can provide the family with appropriate risk information for future pregnancies.
High-risk couples who choose to have children often undergo prenatal testing, which enables couples to consider pregnancy termination and subsequent family planning. Testing includes
Amniocentesis or chorionic villus sampling may detect inherited metabolic and chromosomal disorders, carrier states, and CNS malformations (eg, neural tube defects, anencephaly). Amniocentesis is indicated for all pregnant women > 35 yr (because their risk of having an infant with Down syndrome is increased) and for women with family histories of inherited metabolic disorders. Ultrasonography may also identify CNS defects. Maternal serum α-fetoprotein is a helpful screen for neural tube defects, Down syndrome, and other abnormalities.
Rubella vaccine has all but eliminated congenital rubella as a cause. A vaccine for cytomegalovirus infection is being sought. Continuing improvements in and increased availability of obstetric and neonatal care and the use of exchange transfusion and Rh0(D) immune globulin to prevent hemolytic disease of the newborn have reduced the incidence of ID; the increase in survival of very low-birth-weight infants has kept the prevalence constant.
Key Points
Last full review/revision March 2013 by Stephen Brian Sulkes, MD
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