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Medically, mental retardation (MR) or “intellectual disability” (ID, the currently preferred term) is a highly variable, heterogeneous manifestation of central nervous system dysfunction. According to the Diagnostic and Statistical Manual, 4th edition (DSM-IV) (1994), the diagnostic criteria are (1) onset before age 18; (2) an IQ of approximately 70 or below; and (3) concurrent deficits or impairments in two or more of the following areas: communication, self-care, home living, social and interpersonal skills, use of community resources, self-direction, functional academics, health and safety, and work and leisure.

MR or ID (for the purposes of this chapter we will use the traditional abbreviation, MR, recognizing that it likely will become outmoded in the future) is grouped into four degrees of severity by measure of tested IQ. Mild MR is defined as “educable”; patients possess an IQ level from 50 to approximately 70. Moderate MR is considered a “trainable” severity level and is seen in individuals with IQs of 35 to 55. In severe MR, the IQ level is 20 to 40, and profound MR is most frequently defined by an IQ level below 20 to 25. About 85% of individuals with MR function within the mild range, whereas about 10% function within the moderate range, and only 5% are severely to profoundly disabled. Recently, the American Association on Mental Retardation proposed that a different system, one that utilizes the intensity of the support needed by the individual, would better express the functional limitations of the individual and thus hold more practical use. Intensity of intervention is quantified as intermittent, limited, extensive, or pervasive. However, grouping by degrees of severity is still useful from the clinical point of view.

Mental retardation may become evident during infancy or early childhood as developmental delay (DD), which is a common clinical problem in pediatrics and is estimated to occur in approximately 2% to 10% of the population (see Chapter 91). However, recent data from the United States Department of Education indicate that the prevalence of MR among school-aged children (ages 6–17) is 1.14%. The different rates of prevalence of MR depend on definitions used, methods of ascertainment, and population studied. The individual’s cultural and socioeconomic environments should also be taken into consideration when testing procedures are applied. It is important to note that the prevalence of mild retardation varies inversely with socioeconomic status, whereas moderate to severe disability does not. The approach to the evaluation of developmental delay is further discussed in Chapters 91 and 547.


Etiologic factors may be biological or socioenvironmental; in some cases, there may be combination of the two. The biological factors can be prenatal, perinatal, or postnatal. The prenatal factors can be further subdivided into preconceptional, embryonic, and fetal factors. Preconceptional factors include single gene abnormalities such as neurocutaneous disorders; malformation syndromes; inborn errors of metabolism; and chromosome aberrations, including trisomy syndromes and polygenic familial syndromes. In the ...

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