Epidemiology of traumatic brain injury

Chapter 10
Epidemiology of traumatic brain injury

Ramon Diaz-Arrastia and Kimbra Kenney

Center for Neuroscience and Regenerative Medicine, Uniformed Services University of the Health Sciences, Bethesda, MD, USA

Traumatic brain injury (TBI) is among the oldest and most common medical afflictions affecting humankind. Injuries to the cranium are commonly found in skeletal remains of prehistoric humans. While some of these injuries occurred in the perimortem period, the majority are healed skull fractures, a consequence of nonlethal head injuries from which the victims survived for many months and years. Between 10 and 50% of skulls of prehistoric humans show evidence of cranial trauma [1, 2], with the higher number corresponding to times of climactic stress and political instability. Since most cranial trauma does not result in skull fractures, this number represents an underestimate of the true prevalence of TBI in our ancestors. It is likely that most of these injuries were a consequence of warfare, as high levels of interpersonal violence were a universal feature of early human civilizations. However, it is also likely that many of these TBIs were accidental and occurred during hunting or otherwise interacting with a harsh environment. TBI remains a common and frequently disabling feature of modern life in industrialized as well-industrializing societies. This chapter discusses the existing data on incidence of TBI, most of which comes from industrialized nations, but evolving evidence from developing countries is also included. Recent trends related to a decline in traffic-related injuries and an increase in fall-related injuries in the elderly are highlighted. Additionally, more recent information on lifetime prevalence of TBI is reviewed, as it is becoming apparent that consequences of brain trauma manifest over many years [3]. As elsewhere in this book, this information is presented with a focus on information relevant to practicing clinicians encountering patients with TBI in different medical settings.

Incidence of TBI

In the USA, the Centers for Disease Control and Prevention have carried out surveillance studies of TBI incidence using standardized case definitions and methods of data collection for the past two decades [4, 5]. The most readily ascertained data are collected from the National Ambulatory Medical Care Survey (NAMCS), the National Hospital Discharge Survey (NHDS), and the National Vital Statistics System and focus on emergency department (ED) visits, hospitalizations, and deaths. The most recent data indicate that each year 1.37 million Americans are treated and released from an ED, 275 000 are hospitalized and discharged alive, and 52 000 die as a consequence of TBI [5]. It is further estimated that TBI is a contributing factor in a third (30.5%) of all injury-related deaths in the USA. It is more difficult to estimate how many individuals are seen in outpatient departments and office-based clinical settings, but recent estimates using National Hospital Ambulatory Medical Care survey and NAMCS data indicate that an additional 84 000 patients with TBI are seen annually in hospital outpatient departments and 1 080 000 are seen by office-based physicians and in community health clinics. Thus, the total number of TBIs for which individuals seek medical attention in the USA annually approaches 3.5 million [6].

Data from other developed countries are comparable to US data. Age-adjusted incidence rates in the USA (per 100 000 population) are 468 for ED visits, 93.6 for hospitalizations, and 17.4 for deaths. A systematic review of European data [7] identified rates (per 100 000) of 235 for ED visits and 15 for TBI-related mortality. Similar rates are reported from Australia [8] and Sweden [9]. Differences in rates are likely due in part to differences between countries in case ascertainment procedures and definitions. However, part of the difference is also likely due to differences in socioeconomic status, transportation and safety regulation, and the delivery of emergency medical services. Within the USA, there are wide differences among states in reported rates of hospital discharges for TBI, with a high of 99 per 100 000 in Maryland to a low of 52 per 100 000 in Rhode Island [10]. This variation is mostly accounted for by discordant rates of TBI among different American ethnic groups [11].

Epidemiologic data from low- and middle-income countries are incomplete. Many nations, particularly developing countries, do not have systematic hospital registration and reporting systems. Trauma registries and surveillance programs are in their infancy in most of the developing world. However, reviews of available data [12] indicate that TBIs due to motor vehicle accidents are significantly higher in Latin America and sub-Saharan Africa, primarily due to underdeveloped road and transportation systems.

The vast majority of these injuries are classified as mild, although it is recognized that the traditional definitions of mild, moderate, and severe TBI leave much to be desired and are not well suited for epidemiologic studies. It is also clear that the term mild does not mean inconsequential and that some mild TBIs result in significant and disabling long-term deficits. The magnitude of deficits resulting from mild TBI remains to be determined in carefully conducted, population-based studies.

Trends in incidence

One of the more gratifying trends of modern life has been a striking reduction in mortality from TBI resulting from motor vehicle accidents over the past four decades, at least in developed countries [13]. The experience of Australia is typical. Between 1970 and 1995 there was a reduction in fatalities from traffic accidents of 47%, despite a 40% increase in population and a 140% increase in the number of motor vehicles. Fatalities per registered vehicle fell over fourfold during that period, from 8.05 to 1.84% [14], and the fatality rate per billion kilometers traveled fell by 90% from 1960 to 2010 [15]. This primarily reflects increased use of safety devices (seat belts, airbags, motorcycle and bicycle helmets, child safety seats) and other improvements in road and automotive engineering, as well as improved enforcement of traffic laws, particularly those against driving while intoxicated [16]. It is also likely that improvements in medical care, both pre- and in-hospital, played a role. A retrospective review of neurosurgical databases in the USA found that mortality from severe TBI declined from 39 to 27% from 1984 to 1996 [17].

However, as the population ages throughout the world, particularly in developed countries, fatalities from TBI in individuals aged greater than 75 years have largely counterbalanced the recent decrease in deaths from road traffic accidents [18]. In the elderly, most TBIs result primarily from falls, and while these injuries are usually classified as mild, they often result in significant disability and even death.

Figure 10.1 represents CDC data [5] for TBI-related ED visits, hospitalizations, and death, by cause and age. In early life (before age 10), falls are the predominant cause of TBI requiring medical attention, and most of these are mild injuries that do not require hospitalization and rarely result in death. In adolescence and young adulthood, motor vehicle accidents predominate, and these are frequently severe injuries that often require hospitalization and are associated with high death rates. Finally, in late life (>65 years), falls are again the predominant mechanism, but in the elderly, hospitalization and death rates are high.

c10-fig-0001 — **Figure 10.1** TBI-related ED visits (a), hospitalization (b), and deaths (c) by cause and age. Adapted from Ref. 5.

Prevalence of TBI

While high-quality and generally consistent data are available to estimate TBI incidence rates, at least in industrialized countries, much less is known about lifetime history or lifetime occurrence of TBI [11]. This is particularly important given the recent recognition of long-term and often delayed effects of neurotrauma [19, 20], as well as the appreciation that multiple brain injuries have an additive effect.

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