Problems in Epidemiological Profiles of Concussion
Although the number of studies examining the epidemiology of concussion has increased rapidly over the past 10 years, there are barriers and issues related to the collection of these data. One of the most notable is the definition of concussion. Ten to 15 years ago, the definition of concussion involved a loss of consciousness47; therefore, these studies grossly underestimated the incidence of concussion across all levels of sport and activity. Because concussion has no gold-standard measure for diagnosis, many clinicians and researchers use varying definitions. Many model a multimodal approach that encompasses the clinical evaluation, symptom assessment, cognitive assessment, and balance assessment.3,4 Furthermore, many concussions are identified through self-report or disclosure, and studies have indicated that more than 50% of concussive injuries may go unreported and unidentified.1,2 Despite the recent advances in head impact biomechanics, including measurement of head impact acceleration magnitudes, location, and direction (Figure 6-1), there is currently no threshold for concussive injury, also making it difficult to identify all concussions.
Various settings in which concussions may be managed, such as the emergency department, clinic, or school, also make interpretation of findings difficult. However, an epidemiologic profile and knowledge of patients in various settings is essential to better understanding the injury.
Adding further issue to the full understanding of the epidemiology of concussion are the definitions concerning recovery. Varying definitions are used in studies to define recovery, ranging from asymptomatic to full return to sport.45,48,49 These definitions can make it difficult to determine the course of recovery because times for these variables may be affected by self-reporting and clinician bias and decision making. In addition, there are issues of studying recurrent and multiple concussions. Studies show that once an athlete sustains a concussion, he or she is more likely to sustain another.50 Recent studies attempt to address recurrent injuries, at least within the context of a single season. However, over longer study periods, this becomes more difficult. Understanding recurrent concussion is important from epidemiological and clinical standpoints, given the growing body of evidence linking multiple concussions to issues later in life. Sports medicine professionals should be aware of these potential limitations when reading the literature and attempting to apply them to clinical practice.
Concussion Etiology and Assessment
The complex pathophysiology of concussions makes it difficult to clearly diagnose and define. Signs and symptoms and the duration of disability vary greatly among athletes. This section describes the known mechanism, neurophysiology, and risk factors, as well as the current assessment protocols, including symptom scales, computerized assessment, and balance measures.
Mechanism and Pathophysiology
A concussive injury results from acceleration-deceleration forces transmitted through the brain. Coup (same side) and/or contrecoup (opposite side) injury may also result from these forces. The coup-contrecoup pressure phenomenon states that contrecoup injury may produce the majority of deficits in a closed head injury.51 However, some studies concerning concussion have theorized that rotational forces result in more severe brain injury and produce increased dysfunction as rotational forces (Figure 6-2) cause shearing of the tissue within the brain.52,53 Although the majority of what is known regarding the neurophysiology of concussion comes from animal studies, new functional imaging techniques are giving insight and paralleling these findings in humans.54-56 Recent preliminary in vivo (in living person) head impact research has found no relationships between concussive injury and impact magnitude or location.57-59
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The forces causing concussion produce what is known as a complex neurometabolic cascade of events leading to the variety of signs and symptoms following a concussion. Acutely following injury, there is an abrupt ionic shift, resulting in changes in cellular physiology. During this complex cascade, the sodium potassium pump goes into overdrive, leading to increased energy needs (need for adenosine triphosphate). As a result, there is a significant increase in glucose metabolism, which occurs while there is also decreased cerebral blood flow. This disparity between glucose supply and demand triggers an ongoing energy crisis, which is thought to be the cause of postconcussive vulnerability.60 During this period of vulnerability is when individuals often present with the initial symptoms of concussion, such as headache and difficulty concentrating. Next, the brain then goes into a phase of depressed metabolism. Increases in calcium may further impair cell function and fuel the ongoing energy crisis. The increased and unchecked calcium accumulation may also lead to cell death. The diffuse nature of the injury and the level at which it occurs leads to poor identification, which is a problem in understanding the epidemiology of concussion across all levels of sport.60
Due to the complex individualized nature of concussive injuries, there are few prospective risk factor studies in the literature. A primary risk factor among athletes is participation in a collision sport, such as football, boys’ ice hockey, and boys’ lacrosse.26,61-63 One of the most cited studies concerning risk highlights previous concussion as a risk factor for subsequent injury. This study suggested the risk of sustaining a concussion is 5.8 times greater for individuals with a history of previous concussion (95% CI, 4.8, 6.9).50 An additional study identified concussion history, participation in contact sports, and being in the bottom quintile for body mass index to be some of the strongest predictors of concussion rates.28
More recent studies also suggest that females may be more at risk for concussion than male athletes, specifically among female soccer and basketball athletes.26,64 The growing body of literature suggests that in comparable female sports, the risk may be 2 times greater.25,62 However, when all males and females are grouped together, few gender differences have been observed. Schulz et al28 reported that the high school concussion injury rate per 100,000 athlete practices was 4.01 (95% CI, 1.25, 6.77) for females and 5.81 (95% CI, 3.30, 8.31) for males overall. The only gender difference observed in this study was between male (2.67/100,000 AEs) and female (7.94/100,000 AEs) soccer athletes.
Gender differences concerning reporting of concussions are not well understood or studied. Covassin et al64 reported that female athletes sustain a higher percentage of concussions during games than males. Many theories exist around why these differences are observed. Concussion disclosure behaviors may contribute to the increased relative incidence among females reported in the literature. Health behavior literature reports that females are more likely to seek medical care and report symptoms of many medical conditions than males.65,66 These studies are largely focused on more severe health problems and chronic pain, which may not directly translate to athletic injury. A study related to TBI in individuals of all ages found no difference in gender on seeking care for concussion. However, many of these individuals were older, and some had more severe brain injuries.67 There continues to be speculation concerning other potential risk factors for concussion, including neck strength, physical characteristics, location, and cumulative magnitude of impacts; however, limited data exist concerning these factors. Other factors, such as fair play, equipment, and play through pain cultural standards may be risk factors. These are discussed in the rules and equipment sections in Chapter 11.