(Figure 4-1) (Davidson et al., 1988). Hereditary and other prenatal causes accounted for between 31 and 52 percent of all hearing limitation; late pregnancy and perinatal causes for between 13 and 19 percent; and postnatal causes for between 5 and 41 percent. The underlying disorder (cause/pathology) was unknown in between 25 and 41 percent of the subjects.

In a population-based study of infants and toddlers, Williamson and colleagues reported that 1 child per 1,000 under age three had a severe vision impairment that could not be corrected (Figure 4-1) (Williamson et al., 1987). Twenty-five percent of these cases were attributable to prenatal disorders, 20 percent to late pregnancy/perinatal conditions, and 25 percent to postnatal infection or injury. The cause was unknown in 15 percent of the cases.

Multiple Impairments and Functional Limitations A person who has a serious impairment that causes functional limitation in one area may have additional impairments causing other functional limitations. For example, a child with mental retardation may also have mobility limitations caused by cerebral palsy. About two-thirds of all children with developmental disabilities have more than one clinical disorder. Of children with mental retardation, 10 percent have cerebral palsy (and mobility limitation), 3 percent have serious hearing impairment, 1 percent have visual loss, and 40 percent have emotional or behavioral disorders (Accardo and Capute, 1979). Children with more serious mental retardation are more likely to have additional functional limitations.

Disability

At least two survey indicators are useful in determining childhood disability: activity limitation (measured in the National Health Interview Survey) and use of special education services (measured by public education agencies). Based on the 1983-1985 National Health Interview Surveys, 40.2 children per 1,000 aged 5 through 17 were limited in their major activities (LaPlante, 1988). The prevalence of children who needed help to carry out activities of daily living (ADL) was 3.3 per 1,000. Of the 15 per 1,000 children aged 5 through 17 who were found to have mental retardation, 90 percent (13 per 1,000) were limited in their major activity (school), and 6 percent (0.9 per 1,000) needed help in ADLs. Of the 2.4 per 1,000 children who had cerebral palsy, 74 percent (1.8 per 1,000) had activity limitations, and 13 percent (0.3 per 1,000) needed help in ADLs. Children with mental retardation and cerebral palsy accounted for 26 and 10 percent, respectively, of all children needing help in ADLs.

The prevalence of children aged 3 through 21 who received special educational services in 1987-1988 was 66 per 1,000 (U.S. Department of



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