to 50 percent at 3 to 6 months postinjury, and 50 percent to 60 percent at one year (Oddy and Humphrey, 1980; Rimel et al., 1982).
Profiles of individuals who are at highest risk of sustaining TBI are consistent in the research literature (reviewed by Annegers et al., 1980; Frankowski et al., 1985; and Whitman et al., 1984). Adults aged 15 to 24 years are at highest risk of sustaining a traumatic brain injury, but the elderly, aged 65 and over, and very young children are also at high risk. Compared with females, males are twice as likely to sustain TBI; thus, more than 70 percent of all TBIs occur among males. Demographic studies indicate that the incidence of TBI is highest for nonwhite, urban populations (ranging from 250 to 400 per 100,000). White populations living in rural and suburban areas, on the other hand, have the lowest rates (200 per 100,000).
Motor vehicle crashes constitute the leading cause of TBI, accounting for one-third to one-half of all new cases. The second leading cause of TBI is falls, accounting for an additional 20 percent to 30 percent of total incidence. Intentional injuries also represent a major cause, although the contribution of assaults to the overall incidence of head injuries varies among populations according to socioeconomic composition. Studies of inner city Chicago and Bronx County, New York, for instance, indicate that motor vehicle crashes and violence contribute equally to the incidence of head injury (Cooper et al., 1983; Whitman et al., 1984).
Given the force involved in motor vehicle crashes, resulting TBIs generally lead to a higher percentage of diffuse brain damage. Falls and blows to the head, on the other hand, are associated with a higher frequency of hematomas and focal paralysis.
The incidence of SCI is considerably lower than TBI; however, SCI substantially affects both the individual and society. Each year an estimated 10,000 to 20,000 people in the United States sustain an SCI (2.8 to 5 cases per 100,000 people). These incidence figures translate into a prevalence of approximately 200,000 people in any given year (Kraus, 1985). As with TBI, however, estimates of SCI incidence and prevalence vary considerably across studies because of differences in methods of case ascertainment and in characteristics of study populations. On average, lifetime costs for medical treatment and rehabilitation range from an estimated $210,379 to $751,854 (1989 dollars) per individual, depending on the extent of the injury. The average present value of forgone earnings due to premature death and disability ranges from $151,250 to $308,000 per person. Total lifetime costs of all new cases of SCI in 1989 will amount to an estimated $6 billion (1989 dollars) (DeVivo, 1989).
In contrast to TBI, the major impairments resulting from spinal cord