Incidence is the number of newly diagnosed cases occurring in a defined period, usually expressed with reference to a base of 100,000 persons. An incidence study is one in which only newly diagnosed TBI cases in a specified period of time in a population of known size have been enumerated and are included in the study group. Some 30 population-based TBI incidence studies conducted in the United States have been published since 1980 (Table 3.4). Early studies were limited to counties (Kraus et al., 1984), cities (Cooper et al., 1983; Whitman et al., 1984), and states, such as Oklahoma, Massachusetts, Louisiana, and Alaska. National or subnational estimates of the incidence of TBI have recently been published from the Centers for Disease Control and Prevention (CDC) TBI surveillance system (Langlois et al., 2003) or from existing national administrative datasets (Langlois et al., 2006). Methods used for incidence studies have varied. For example, some earlier studies (e.g., Rimel, 1981; Kraus et al., 1984) relied on hospital or coroner records for case findings based on discharge codes, reviewed original institutional records, and abstracted pertinent data. Later studies used hospital discharge records and electronic files; in a few instances, a trauma registry was the source of data on TBI (Warren et al., 1995).
On the basis of the data available from those studies, the incidence of hospitalizations for TBI in the United States is about 140/100,000 persons per year. If the highest reported rate (367/100,000) and the lowest reported rate (69/100,000) are excluded, the average rate of hospitalization for TBI (plus cases of immediately fatal TBI) in the United States is about 130/100,000 persons per year. Those estimates do not include ED-based studies, with rates of 444/100,000 (Jager et al., 2000) or 392/100,000 (Guerrero et al., 2000). The rates given in Table 3.4 represent three case-finding methods: hospitalized cases and those identified from medical-examiner records, hospital discharge records only, and trauma-registry files. The differences in case-finding approaches and other methodologic differences result in different rates.
Some 36 TBI incidence studies conducted outside the United States have been published since the middle 1970s, most coming from Europe and Australia (Table 3.5). As in the US studies, a wide variety of methods were used in TBI case definition and ascertainment methods. Even when ICD TBI codes were used in existing hospital electronic discharge files, the codes selected were not uniform. About half the incidence studies did not evaluate TBI severity. Therefore, it is difficult to synthesize findings from the non-US studies.
Few incidence studies have collected data beyond a single year or two. MacKenzie and associates (1990) reported an increase in TBI incidence in Maryland from 1979 to 1986. There did not appear to be any remarkable changes in TBI identification procedures in the state’s database, and only patients admitted to the state’s 56 acute-care nonfederal hospitals were counted. Using the US National Hospital Discharge Survey, a yearly survey sampled in such a way as to be representative of the US general population, Thurman and Guerrero (1999) reported a 51% decline in incidence from 1980 through 1995. The change over that period was from 199/100,000 to 98/100,000. They noted that the TBI-associated death rate also declined, possibly because of the preventive measures associated with motor-vehicle crash outcomes. The authors noted also that the greatest change in hospitalization rates was in those with mild TBI; that suggested a change in hospital admission practices.