The distribution of severity of brain injury as assessed by the GCS (or other parameters) is shown in Table 3.8. Of the more than 60 population-based incidence studies published worldwide since 1980 only 22 address the degree of TBI severity in the study populations; 10 are from US and 12 from non-US countries. Most studies used the GCS to evaluate brain injury severity but some also used the AIS. The majority of hospital-admitted brain injuries are classified as "mild" (generally, a GCS score of 13 to15 or AIS of 1 or 2). However, the mild category is viewed differently by different researchers some of whom use mild to describe any GCS score above 7 while others refer to GCS scores above 8, above 10, above 13, or 15 only (Kraus and Chu, 2004). Studies published in the 1980s, with the exception of the report from Oklahoma, showed a ratio of mild to moderate to severe of about 8:1:1.
With one or two exceptions almost all studies in the United States show less than 20% of patients admitted to a hospital are in the severe TBI range, and mild TBI is diagnosed 60% or more of the time. However, researchers outside the US report severity distribution proportions at even more consistent levels. A study by Hillier et al. (1997) evaluated TBI severity using three different measures: the GCS, LOC, and PTA; results were very similar, which provides support to the acceptance of severity classification when each of those measures is used. Severity distribution findings from non-US studies (Table 3.8) are similar to those from the United States with a ratio of mild to moderate to severe of 7:1:1. The high percentage of severe TBI admissions for Northeast Italy (Baldo et al., 2003) and the Romagna region of Italy (Servadei et al., 2002a) may reflect the referral practice of the acute medical care treatment institutions involved.
Several risk factors have been examined in connection with the incidence of TBI: age, sex, ethnicity, and socioeconomic status. Data on age and sex in TBI can be found in 60 of the 66 papers reviewed (Tables 3.9 and 3.10). Although the papers do not necessarily group ages similarly, findings are remarkably consistent; the age group with the highest incidence of TBI is 15–24 years. In some reports, age groups at highest risk depend on TBI severity. For example, the very young (0–4 years old) and the very old (at least 85 years old) present to an ED with a brain injury most frequently, whereas those 15–24 years old and over the age of 65 years are hospitalized with TBI most frequently. The age-specific rates tend to reflect differences in exposure, particularly to motor-vehicle crashes and falls. Males are at greater risk for TBI than females at all ages in all incidence studies. Every report that gives data on sex-specific incidence shows that males have much higher TBI rates than females, and the ratio of male incidence to female incidence often exceeds 2. In one report (Nell and Brown, 1991), the incidence ratio of males to females exceeded 4 in both blacks and whites in Johannesburg, South Africa. The researchers posit that men in Johannesburg are involved in much higher levels of aggressive activities than women in the same city. The sex-specific mortality ratio is about 3.5:1, strongly indicative of more severe injuries among males (Adekoya et al., 2002).
The US TBI death rate in 1989–1998 averaged 27/100,000 in American Indians and Alaskan Natives, 25/100,000 in blacks, and 20/100,000 in whites (Adekoya et al., 2002). The nonfatal-TBI hospitalization rate in 1997 (based on a 14-state surveillance system) was