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 IQ of < 70, which is 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 mental retardation (MR). Severe MR occurs in families from all socioeconomic groups and educational levels. Less severe MR (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 even play roles in milder cognitive disabilities.

Etiology

Intelligence is both genetically and environmentally determined. Children born to parents with MR are at increased risk for a range of developmental disabilities, but clear genetic transmission of MR is unusual. Although advances in genetics have increased the likelihood of identifying the cause of a person's intellectual disability, in 60 to 80% of cases a specific cause still cannot be identified. 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 MR.

Prenatal

A number of chromosomal anomalies and genetic metabolic and neurologic disorders can cause MR (see Table 4: Learning and Developmental Disorders: Chromosomal and Genetic Causes of Mental Retardation*).

Table 4
Chromosomal and Genetic Causes of Mental Retardation* Chromosomal Abnormalities Genetic Metabolic Disorders Genetic Neurologic Disorders Cri du chat syndrome Down syndrome Fragile X syndrome Klinefelter's syndrome Mosaicisms Trisomy 13 (Patau's syndrome) Trisomy 18 (Edwards' syndrome) Turner's syndrome Autosomal recessive disorders: Aminoacidurias and acidemias Peroxisomal disorders: Galactosemia Maple syrup urine disease Phenylketonuria Lysosomal defects: Gaucher's disease Hurler's syndrome (mucopolysaccharidosis) Niemann-Pick disease Tay-Sachs disease X-linked recessive disorders: Lesch-Nyhan syndrome (hyperuricemia) Hunter's syndrome (a variant of mucopolysaccharidosis) Lowe's oculocerebrorenal syndrome Autosomal dominant disorders: Myotonic dystrophy Neurofibromatosis Tuberous sclerosis Autosomal recessive disorders: Primary microcephaly *This is a partial list of disorders.

Congenital infections that can cause MR include rubella virus, cytomegalovirus, Toxoplasma gondii, Treponema pallidum, and HIV.

Prenatal drug and toxin exposure (see Normal Pregnancy, Labor, and Delivery: Drugs in Pregnancy) can cause MR. 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 malnutrition during pregnancy may affect fetal brain development, resulting in MR.

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 MR. 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 MR, depending on gestational age, perinatal events, and quality of care.

Postnatal

Malnutrition 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 MR worldwide. Viral and bacterial encephalitides (including AIDS-associated neuroencephalopathy) and meningitides, poisoning (eg, lead, mercury), severe malnutrition, and accidents that cause severe head injuries or asphyxia may result in MR.

Symptoms and Signs

The primary manifestations are delayed intellectual development, immature behavior, and limited self-care skills. Some children with mild MR may not develop recognizable symptoms until preschool age. However, early identification is common in those with moderate to severe MR and in those in whom MR is accompanied by physical abnormalities or signs of a condition (eg, cerebral palsy) that may be associated with a particular cause of MR (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 MR 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 MR. Behavior problems are often situational, and precipitating factors can usually be found. Factors that predispose to unacceptable behavior include lack of training in socially responsible behavior, inconsistent discipline, reinforcement of faulty behavior, impaired ability to communicate, and discomfort from coexisting physical problems and mental health disorders such as depression or anxiety. 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 administered by a physician or others. Such measures should be used only for screening and not as substitutes for standardized intelligence tests, which should be administered 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, or suspected seizure disorders.

Establishing MR 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 MR (global developmental delay) has been proposed by the Child Neurology Society. Cranial imaging (eg, MRI) can demonstrate 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.

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 performed depending on the suspected cause (see Table 5: Learning and Developmental Disorders: Tests for Some Causes of Mental Retardation). Visual and auditory assessments should be performed at an early age, and screening for lead poisoning is often appropriate.

Table 5
Tests for Some Causes of Mental Retardation Suspected Cause Indicated Tests Single major anomaly or multiple minor anomalies, family history of retardation Chromosomal analysis Cranial CT and/or MRI* Failure to thrive, idiopathic hypotonia, genetic metabolic disorders HIV screen 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 12/60 Bone age, skeletal x-rays Seizures EEG Cranial CT and/or MRI* Blood Ca, phosphorus, magnesium, amino acids, glucose, and lead levels 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 Cranial CT and/or MRI* TORCH screen Urine culture for virus Chromosomal analysis *After neurologic consultation. SMA = sequential multiple analyzer; TORCH = toxoplasmosis, rubella, cytomegalovirus, herpes.

Prognosis and 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 the child must be established.

Family support and counseling are crucial. As soon as MR 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 MR; 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. Neurologists or developmental pediatricians, orthopedists, physical therapists, and occupational therapists assist in managing comorbidities in children with motor deficits. Speech pathologists and audiologists help with language delays or with a suspected hearing loss. Nutritionists can help with treatment of malnutrition and social workers can help reduce environmental deprivation. 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 MR. The use of psychoactive drugs without behavioral therapy and environmental changes is rarely helpful.

Every effort should be made to have the child live at home or in a community-based residence. Although the presence of a child with MR 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 daycare centers, homemakers, and respite services. The living environment must encourage independence and reinforce learning of skills needed to accomplish this goal. Whenever possible, a child with MR should attend an appropriately adapted daycare center or school with peers who are not mentally retarded. The Individuals with Disabilities Education Act (IDEA), a 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 MR 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.

Many people with mild to moderate MR can support themselves, can live independently, and can 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 MR are likely to require life-long support. The more severe the retardation and the greater the immobility, the higher is the mortality risk.

Prevention

Genetic counseling (see also Prenatal Genetic Counseling and Evaluation) may help high-risk couples understand possible risks. If a child has MR, 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 a couple to consider pregnancy termination and subsequent family planning. Amniocentesis or chorionic villus sampling may detect inherited metabolic and chromosomal disorders, carrier states, and CNS malformations (eg, neural tube defects, anencephaly). Ultrasonography may also identify CNS defects. Maternal serum α-fetoprotein is a helpful screen for neural tube defects, Down syndrome, and other abnormalities. 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 inborn metabolic disorders.

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 MR; the increase in survival of very-low-birth-weight infants has kept the prevalence constant.

Last full review/revision November 2005