Sport-Related Concussion

Recognizing, treating and preventing this common injury

This occurs due to a rotational or acceleration/deceleration forces transmitted to the head, either as a result of a direct blow to the head, neck or face or a blow to another area of the body that is transmitted to the head.3,4,5

Brain injuries are typically classified as either focal or diffuse, and either primary or secondary.  Concussion is considered to be a diffuse brain injury, as the entire brain is generally affected, as opposed to only the specific, direct area of injury causing symptoms. This type of injury is also classified as a secondary brain injury, meaning that the symptoms of concussion are a result of the pathophysiological events that occur after the moment of impact, as opposed to injury resulting from the trauma immediately and directly.3,6,7

The rotational and acceleration/deceleration forces in a concussion are believed to cause stretching and disruption of neuronal membranes.2,3  This allows for a massive efflux of potassium from the neuronal intracellular space to the extracellular space, triggering the release of glutamate. An excitatory neurotransmitter, glutamate subsequently prompts continued potassium release.

The depolarization of a massive number of neurons by this mechanism leads to the suppression of normal, physiologic neuronal activity. In an attempt to achieve homeostasis, sodium-potassium pump activity is increased, resulting in utilization of substantially more than normal amounts of cellular fuel in the forms of adenosine triphosphate (ATP) and glucose. As a result, lactate accumulates within the brain and cerebral blood flow is decreased, leading to an “energy crisis” in the brain.3

The massive efflux of potassium previously described also leads to an accumulation of calcium within neurons, which can interfere with oxidative metabolism and activate biochemical pathways that lead to catabolic processes, free radical accumulation, and cell death.3,8  These events lead to a hypometabolic state within the brain, which can persist for four weeks.3

The Centers for Disease Control and Prevention estimates that in sports alone 1.6-3.8 million concussions occur annually in the United States, accounting for 8.9% of sport-related injuries in high school athletes.1-4

Most of these reported injuries occur during participation in organized sports, specifically during competition rather than in practice.4 Concussion is most likely to occur during player-to-player contact; unsurprisingly, male athletes have the highest concussion risk while playing football, while female athletes are at greatest risk while participating in soccer.3,4 Specific positions within these and other sports that lend themselves to increased opportunities for player-to-player contact are also associated with an increased risk of concussion.4

A previous history of a concussion is the single greatest risk factor for sustaining a subsequent concussion, creating a 2-5.8 times increased risk, and a 6 times increased risk if the previous concussion was accompanied by loss of consciousness.4,9-11 This risk is greatest in the first 7-10 days following a concussion.4

While the reason for this profound increased risk is not fully understood, it is postulated that it may be due to the athlete’s style of play, the pre-existing, personal susceptibility of the athlete, including the athlete’s age, level of participation, and exposure time, and the brain’s increased susceptibility to concussion while recovering from previous concussion.4

In sports with comparable rules and exposure, females are more likely to sustain a concussion than males. Females also report greater numbers, severity and duration of symptoms of concussion compared to males.4,9 The reasons for these observations are not clear, but are thought to be due to differences in head-neck segment mass between males and females, as well as to differences in estrogen levels and cerebral blood flow that may influence the pathophysiology of symptom development and recovery.4

Younger athletes appear to be at an increased risk for concussion, potentially due to differing physiology of developing brains, but studies in athletes under the age of 15 are minimal.4,9

Specific genetic mutations have been thought to be associated with an increased risk of concussion, but studies of these mutations to date have failed to show a significant increased risk.4,9 A personal history of mood disorders, learning disabilities, attention disorders and migraine headaches have all been shown to be associated with an increased risk for concussion, as well as an increased risk for prolonged recovery time following an injury.4,9