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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.
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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.
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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.
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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 embryonic ...