Within the severely delayed there are gradations. Because of the objectionable implications of the previously used terms idiot, imbecile, and moron, the developmentally delayed can be grouped instead into four categories: (1) those with profound deficiency, incapable of self-care (IQ below 25); (2) those with severe deficiency, incapable of living an independent existence and essentially untrainable (IQ 25 to 39); (3) those with moderate deficiency, trainable to some extent (IQ 40 to 54); and (4) those with mild deficiency, who are impaired but trainable and to some extent educable. The above newer terms, while in common use, satisfy neither neurologists nor psychologists because, in their generality, they do not capture deficits in performance in other spheres. Moreover, they express only one aspect of impaired mental function—the cognitive—and ignore the inadequate development of personality, social adaptation, and behavior. A more comprehensive view is provided by assessing the individual’s adaptive abilities along the lines of conceptual, social, and practical skills that allow planning for maximizing independence and productivity.
When the brains of severely affected people are examined by conventional methods, gross lesions are found in approximately 90 percent of cases. Just as noteworthy is the fact that among the remaining 10 percent of the severely delayed, the brains are grossly and microscopically normal. Despite the recent discovery of many mutations that may give rise to a delay in cognitive development only a modest proportion of cases of those with milder deficiency can presently be traced to one of the congenital abnormalities of development that are described in Chap. 37 and the vast majority of the less-severely delayed also lack a recognizable tissue pathology and have not exhibited any of the conventional signs of cerebral disease.
In our view, a more acceptable view of the mildly affected group is that they represent the proportion of the population that is on the low end of the Gaussian curve of intelligence, that is, they constitute the group that falls between 2 and 3 standard deviations (SD) below the mean (Fig. 27-4) and are the opposite in this respect to genius. Lewis was one of the first to call attention to this large group of mildly delayed individuals and he referred to them by the ambiguous term subcultural. The term familial retardation was in the past applied to this group, because in some of the families, members of the same and previous generations have reduced cognitive ability.
An important advance in the understanding of developmental delay emerged from careful genetic studies that have identified specific loci at which deletions or duplications result in intellectual disability. Some of these express particular syndromes such as cognitive delay with autism or epilepsy; however, the same genetic changes can be found among individuals who have only autism or schizophrenia. While a unifying explanation for these disparate disorders has yet to be defined, they probably have subcellular and synaptic changes in common as summarized by Mefford, Batshaw, Hoffman in their review that is recommended to the reader.
Both the milder and more-severe forms of developmental delay are associated with physical abnormalities and diseases of the brain, as well as nondysmorphic and genetic forms of delayed development are discussed in Chap. 37.
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