Mental Retardation
There are 2 to 4 million mentally retarded persons in the United States (1% to 2% or more of population, depending on upper cutoff; M:F = 1.5:1; coded on Axis II in DSM), of whom 80% to 85% are only mildly retarded (1). The diagnosis of mental retardation requires decreased intellectual functioning (measured by standard IQ tests—two standard deviations below the mean IQ of 100) and impaired general functioning. Its presentation is modified by age, experience, and the environmental and cultural setting. When mild, it is most commonly first identified in gradeschool children. Most mildly retarded adults, when no longer in school, are indistinguishable from the lowest socioeconomic segment of the general population and may no longer receive the diagnosis of mental retardation [i.e., they develop skills, adapt, and “grow out of it” (2)], but, although self-sufficient, they tend to be poor and to experience stress and emotional problems (3). Adults with intellectual impairment that developed before age 18 have (usually nonprogressive) retardation, and those developing it after age 18 (i.e., regressing from a higher level) have dementia.
CLASSIFICATION
MILD MENTAL RETARDATION (DSM, p. 43, 317): IQ 50 to 70; considered “educable.” Usually recognized when they enter school (and are tested) and require special education. Constitutes 85% of the retarded (but this is the group that decreases markedly with adulthood). Many become self-supporting, with help, although they have limited judgment, social sensitivity, and insight.
MODERATE MENTAL RETARDATION (DSM, p. 43, 318.0): IQ 35 to 50; 10% of all retarded. Usually recognized during preschool years. They are “trainable,” can learn simple work skills, can read at a 2nd grade level and speak simply, and can be partly self-supporting in sheltered settings. They tend to be clumsy and uncoordinated.
SEVERE MENTAL RETARDATION (DSM, p. 43, 318.1): IQ 20 to 35; 3% to 4% of all retarded. These are the dependent retarded: They are capable of simple speech but require institutional or other intensely supportive care. Malformations and severe physical handicaps are frequent.
PROFOUND MENTAL RETARDATION (DSM, p. 44, 318.2): IQ below 20; 1% of all retarded. They are totally dependent on others for survival and usually have significant neurologic damage; cannot walk or talk.
A presumably retarded patient who is untestable is considered to have MENTAL RETARDATION, SEVERITY UNSPECIFIED (DSM, p. 44, 319).
CAUSES
Distinct causes (usually biologic) are identified in fewer than 50% of patients; these are most of the moderately-to-profoundly retarded patients. Other causes include environmental factors (e.g., pre- and perinatal problems, infant illness, psychosocial neglect, malnutrition), with an uncertain polygenic contribution in some cases. Moderate-to-profound retardation is distributed uniformly across social classes, whereas mild retardation (usually from sociocultural etiology) is weighted toward the lower classes. Retardation is a familial disorder (genetics or environment or both); the risk of retardation in a child with normal parents and siblings is less than 2%, whereas the risk if both parents and one sibling are afflicted may be as great as 40% to 70% (4).
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