Epidemiology

Excerpt

Traumatic brain injury (TBI) is an important public health concern that is one of the leading causes of death and disability annually around the world., Several factors have fueled increasing interest in TBI over the last several years, including rising awareness around the acute and chronic effects of sport-related concussion (SRC) and the reported incidence of head injuries sustained by U.S. military service members deployed to Iraq and Afghanistan. Sporting and military cohorts are now known to be at elevated risk of mild TBI (mTBI), characterized by more subtle neurocognitive and neurobehavioral symptoms that are often challenging to assess and characterized in a clinical setting.

A strong epidemiological framework for TBI is vital to improving our understanding of the injury’s occurrence, populations at risk, and effective strategies for injury prevention. The purpose of the current chapter is to review the most recent epidemiological literature on TBI in both civilian and military populations.

The epidemiological literature on TBI is limited by a number of factors, not the least of which is that the definition of a TBI varies across studies. Though there are a number of operational definitions, TBI is commonly and most basically defined as an alteration in brain functioning or the emergence of evidence of brain pathology caused by an external force. One of the methodological limitations of many epidemiological studies over the last 20 to 30 years has been suboptimal or inconsistent classification and inclusion criteria. Many studies have included individuals who sustained an injury to the head that may not have involved any alteration in brain function or physiological damage to the brain. For example, Bazarian and colleagues used emergency department records for a study of mTBI that included individuals with diagnostic codes such as “skull fracture,” “intracranial injury of unspecified nature,” and “head injury, unspecified.” Even though many individuals with these diagnoses likely met the criteria for mTBI, there are others who did not sustain a TBI at all or may have sustained an injury that was more severe than an mTBI.

Population-based epidemiological studies are typically based on hospital admission or discharge records, emergency department records, or death certificates, which creates a selection bias. It has long been held that many patients with less severe injuries do not present for medical treatment and therefore are not included in many of these studies. In addition, changes in hospitalization patterns over the past several years, with fewer less severe patients being hospitalized, has also likely lead to an underestimation of the true incidence of milder forms of TBI (for a review, see the study by McCrea6).

TBI severity classification can also vary among different studies. Commonly, clinical indicators related to acute injury characteristics are used to indicate injury severity such as mild, moderate, or severe. The indicators most commonly used are alteration in consciousness, loss of consciousness (LOC), and post-traumatic amnesia (PTA) following the injury. The Glasgow Coma Scale (GCS) is the most commonly used clinical tool for assessing consciousness following an injury. A GCS score of 13 to 15 is considered a mild injury, 9 to 12 is considered a moderate injury, and 8 or less is considered a severe injury. Methods used to classify TBI also have varied utility. The GSC as a highly effective tool to differentiate the severity of TBI has been well founded and the GCS has been shown to be useful in predicting morbidity and mortality in more severe injuries,– but it has been less useful as a prognostic indicator for mTBI. Historically, not all epidemiological studies have employed a common or standardized classification system, which complicates interpretation of epidemiological data and comparisons across studies.

In terms of injury reporting, there is no single system in the United States or most other countries that tracks the occurrence of head injury over large populations. As a result, a true appreciation for TBI’s national or worldwide incidence is lacking. The Centers for Disease Control and Prevention (CDC) is the most comprehensive data source of epidemiological studies of TBI in the United States, as it monitors death records (National Vital Statistics System), hospitalizations (National Hospital Discharge Survey), and emergency department visits (National Hosptial Ambulatory Medical Care). The data for military TBI injuries comes from the Department of Defense (DoD) in collaboration with the Armed Forces Health Surveillance Center. Where available, other sources from well-designed epidemiological studies in the United States and around the world were also reviewed.

The first section of the this chapter will review the civilian literature, including sport-related concussions, and the second section will focus on the military epidemiological literature.