The overall frequency of severe developmental delay cannot be stated precisely. Rough estimates place the figure at 0.2 to 0.4 percent of the general population and at approximately 10 percent of the general developmentally delayed segment of society. It is important to emphasize that in a large proportion of individuals with severe developmental delay, a particular congenital abnormality or anomaly of development cannot at present be traced to any of the disorders reviewed in the preceding pages. More precisely stated, when groups of such severely impaired patients are studied clinically, a reasonably accurate etiologic determination of the underlying brain disease can be made in only slightly more than half. According to Penrose, chromosomal abnormalities account for 15 percent, single-gene disorders for 7 percent, and environmental agents for 20 percent. More recent studies of the subtelomeric parts of chromosomes reportedly find abnormalities in another 7 percent of severely retarded children (Knight et al). No cause is found for the remaining cases. Males outnumber females 3 to 1.
However, from the neuropathologic standpoint, the examination of the brains of the severely developmentally delayed by conventional histopathologic methods discloses lesions in approximately 90 percent, and in fully three-quarters of the entire group, an etiologic diagnosis can be determined or tentatively assigned. Many of the remaining 10 percent lack definite pathologic changes, but their brains are lighter in weight by 10 to 15 percent than age-matched normal brains. Interestingly, the proportion of vascular, hypoxic-ischemic, metabolic, and genetic lesions in this group of severely impaired individuals is much the same as is found in a group selected on the basis of “cerebral palsy.”
Here it is important to repeat a point made earlier and in Chap. 27: A few of the severely disabled and a large majority of the mildly affected do not have a recognizable cerebral pathology or exhibit any of the familiar and conventional signs of cerebral disease. Although the milder forms of developmental delay tend to be familial, this does not by itself separate them from the severe forms of developmental delay. There are several types of hereditary developmental delay, in which the cognitive impairment may be severe and in some of which there may be maldevelopment of the cerebral cortex. These are discussed further on.
Clinical Characteristics of the Severely Developmentally Delayed
These cases can be broadly divided into four groups. In one large group, designated dysmorphic developmental delay, a variety of physical deformities, including microcephaly, is commonly present. In a second group, with multiple-system retardation, the developmental delay is linked to nonskeletal abnormalities (e.g., hepatosplenomegaly, hematologic and skin disorders), which provide reliable clues to the underlying somatic disease. In a third group, neurologic retardation, somatic abnormalities are lacking but a configuration of neurologic signs leads to the diagnosis. In the fourth group, and the most difficult one to clarify, that comprising uncomplicated retardation, there are no or minimal somatic, visceral, or neurologic abnormalities. One is then forced to turn to special features of the developmental delay itself for identification of the underlying disease. Table 37-8 elaborates a reasonable classification of the types of developmental delay.
The profoundly developmentally delayed infant with a virtually untestable IQ is identified early in life because he does not sit, stand, or walk. If any one of these motor activities is acquired, it appears late and is imperfectly performed. Language never develops; at most a few spoken words or phrases are understood and fewer are uttered, or the patient only vocalizes in a meaningless way. Such a person may not even indicate bodily needs for food, drink, excretion, and so on. Usually the patient is continuously idle and interacts little with people and objects. Only the most primitive emotional reactions are exhibited, often without connection to an appropriate stimulus. Physical growth is usually retarded, nutrition may be poor, and susceptibility to infections is increased. Sphincteric control may never be attained and, if attained, is precarious.