There are several hundred possible causes of mental retardation. They include genetic conditions (e.g., Down syndrome, fragile X syndrome); prenatal problems (e.g., fetal alcohol syndrome, rubella, malnutrition); problems apparent at birth (e.g., low birth weight and prematurity); and problems that occur after birth (e.g., injuries and childhood diseases like measles that can lead to meningitis and encephalitis). The most commonly identified causes of mental retardation are Down syndrome, fragile X syndrome, and fetal alcohol syndrome. In many cases the cause is never known.
Most mentally retarded children are capable of learning new things, both in and out of a formal school setting, but they may learn at a slower pace than other children. Schools are responsible for providing an appropriate education for retarded children. Many teachers and parents feel that the practice of mainstreaming, or inclusion, which places such children in standard classrooms for at least part of the day, helps them to feel more a part of society and helps others to better understand their special needs and capabilities.
Many cases of mental retardation are now prevented by improved health care. Vaccines against rubella and measles prevent an estimated 3,000 cases of mental retardation in the United States yearly. Vaccination against Haemophilus influenzae b (Hib), a cause of childhood meningitis, is expected to prevent 3,000 more. Prevention of Rh disease (see Rh factor), screening and treatment for phenylketonuria, and emphasis on prenatal care and the dangers of poor nutrition or alcohol consumption during pregnancy have also resulted in a decrease in cases of mental retardation in the United States. Mental retardation rates in poor nations, however, are rising.
The treatment of mentally retarded people has always reflected the changes in society. They have been officially referred to as idiots and as the feebleminded. The introduction of the IQ test was followed by a classification system that used such terms as moron (IQ of 51-70), imbecile (26-50), and idiot (0-25); later these terms were softened and classifications redefined somewhat to mild (IQ of 55-70), moderate (40-54), severe (25-39), and profound (0-24) retardation. The term mentally retarded itself, although still commonly used, has been replaced in some settings by the term developmentally disabled.
Mentally retarded people have been subjected to unnecessary institutionalization and, as a result of the eugenics movement, involuntary sterilization. The deinstitutionalization movement of the 1970s reflected a concern for the civil rights of mentally retarded. Very few of the mentally retarded are now institutionalized; most now live independently, with their families, or in group homes. The emphasis on education and self-sufficiency seen in the late 20th cent. mirrors a similar movement in the 1840s.
See M. Adams, Mental Retardation and Its Social Dimensions (1971); A. Clarke et al., ed., Mental Retardation: The Changing Outlook (1985); E. Zigler, Understanding Mental Retardation (1986); American Association on Mental Retardation, Mental Retardation: Definition, Classification, and Systems of Support (1992).
Subaverage intellectual ability that is present from birth or infancy and is manifested by abnormal development, learning difficulties, and problems in social adjustment. A standardized intelligence test is a common method of identification. Individuals with IQ scores of 53–70 are usually classified as having mild intellectual disability and are able to learn academic and prevocational skills with some special education. Those with scores of 36–52 are classified as having moderate intellectual disability and are able to learn functional academic skills and undertake semiskilled work under supervised conditions. Those in the severe (21–35) and profound (below 21) ranges require progressively more supervision or full-time custodial care. Intellectual disability can be caused by genetic disorders (such as Down syndrome), infectious diseases (such as meningitis), metabolic disorders, poisoning from lead, radiation, or other toxic agents, injuries to the head, and malnutrition.
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Overactive disorder associated with mental retardation and stereotyped movements is a pervasive developmental disorder (PDD) in Chapter V(F) of the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10); its diagnostic code is F84.4.
Diagnosis depends on the combination of developmentally inappropriate severe overactivity, motor stereotypies, and moderate to severe mental retardation; all three must be present for the diagnosis. If the diagnostic criteria for F84.0 (childhood autism), F84.1 (atypical autism) or F84.2 (Rett's syndrome) are met, that condition should be diagnosed instead.