What is a concussion?

what is concussion

There has been a tremendous amount of media coverage on concussions in sports, but few of these sources ever completely answer the basic question: What is a concussion? The American Academy of Neurology defines concussion as a trauma-induced alteration in mental status that may or may not involve loss of consciousness.1 A more recent definition offered by an International Conference on Concussion in Sport2is more precise, but much more complex (see Table). The American Medical Society for Sports Medicine Concussion defines concussion as a traumatically induced transient disturbance of brain function and involves a complex pathophysiological process.3

In basic terms, a concussion occurs when a forceful injury to the brain temporarily impairs brain functioning.

Signs and symptoms of concussion

It is somewhat easier to understand concussion by the symptoms it causes rather than its definition. The core symptoms of concussion are confusion and memory loss. The symptoms may occur right after the head injury or several minutes later. Confusion may be a blank stare, the inability to pay attention, slowness in responding to questions, disorientation, and slurred speech. In some cases, a person with a concussion will temporarily lose coordination. The amnesia that occurs in a concussion almost always involves memories of the traumatic event. In other words, the sufferer cannot remember what happened to him or her. The memory loss may be more extensive than that, however. The person may not remember important details of the game that occurred before or even after the injury.

Aside from these core symptoms, a concussed player may also experience headache, dizziness, feeling disoriented and/or overly emotional. The concussion may also interfere with sleep, causing insomnia. Concussion can also cause problems with concentration and school work making it difficult to return to school or work after a concussion.

Concussion versus traumatic brain injury

Physicians and scientists tend to disagree about whether or not concussion is different from mild traumatic brain injury. Some people consider traumatic brain injury to be a separate and more permanent phenomenon than concussion. However, most professionals consider concussion to be a form of mild traumatic brain injury. Why is this important? Because symptoms, tests, and treatment recommendations apply to both concussion and mild traumatic brain injury, regardless of which term is used. It also highlights the severity of concussion by calling it what it really is: a traumatic brain injury.

Brain changes in concussion

The signs and symptoms that occur in concussion fully resolve in 9 out of 10 people within 1 to 2 weeks.4 Thus, for the most part, concussion is viewed as an acute process; there is a problem in the brain that soon goes away. However, there may be subtle, permanent changes that also take place in the brain.5

In the short term, concussion causes something called a post-concussive metabolic cascade. This cascade includes three main abnormalities:

  • Disruption of normal chemical function of brain tissue
  • Impaired energy production in brain cells
  • Altered blood flow to the brain

All of these changes help explain why people undergo behavioral changes and memory loss during a concussion. It also partially explains why people are much more vulnerable to developing a second concussion if they experience head trauma soon after an initial concussion. In other words, if a second injury occurs during this metabolic cascade, the consequences can be more severe and more long-lasting.5

Definitions of Concussion
Fourth International Conference on Concussion in Sport American Medical Society for Sports Medicine
Concussion is a brain injury and is defined as a complex pathophysiological process affecting the brain, induced by biomechanical forces. Several common features that incorporate clinical, pathologic and biomechanical injury constructs that may be utilised in defining the nature of a concussive head injury include:

1. Concussion may be caused either by a direct blow to the head, face, neck or elsewhere on the body with an ‘‘impulsive’ force transmitted to the head.

2. Concussion typically results in the rapid onset of short-lived impairment of neurological function that resolves spontaneously. However, in some cases, symptoms and signs may evolve over a number of minutes to hours.

3. Concussion may result in neuropathological changes, but the acute clinical symptomslargely reflect a functional disturbance rather than a structural injury and, as such, no abnormality is seen on standard structural neuroimaging studies.

4. Concussion results in a graded set of clinical symptoms that may or may not involve loss of consciousness. Resolution of the clinical and cognitive symptoms typically follows a sequential course. However, it is important to note that in some cases symptoms may be prolonged.

Concussion is defined as a traumatically induced transient disturbance of brain function and involves a complex pathophysiological process. Concussion is a subset of mild traumatic brain injury (MTBI) which is generally self-limited and at the less-severe end of the brain injury spectrum.

On the other hand, research suggests that the structure of brain cells may actually change at the microscopic level after concussion. Concussion can injure an important part of a brain cell called the axon. The axon is what transmits electrical signals along a neuron (brain cell). When axons are damaged, brain cells cannot “fire” appropriately, which means they cannot communicate with one another. This damage may have long-lasting effects on the person who experiences the concussion.


A concussion is a temporary impairment in brain functioning that is caused by trauma to the head/brain. It causes confusion, headache, and memory loss. Concussion may also cause the sufferer to be disoriented, overly emotional, and have difficulty sleeping. While symptoms usually resolve within 1 to 2 weeks in most people, some changes that take place can be permanent. A single concussion increases a person’s vulnerability to later concussions.


  1. Practice parameter: the management of concussion in sports (summary statement). Report of the Quality Standards Subcommittee. Neurology. Mar 1997;48(3):581-585.
  2. McCrory P, Meeuwisse WH, Aubry M, et al. Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012. Br J Sports Med. Apr 2013;47(5):250-258.
  3. Harmon KG, Drezner JA, Gammons M, et al. American Medical Society for Sports Medicine position statement: concussion in sport. Br J Sports Med. Jan 2013;47(1):15-26.
  4. McCrory P, Johnston K, Meeuwisse W, et al. Summary and agreement statement of the 2nd International Conference on Concussion in Sport, Prague 2004. Br J Sports Med. Apr 2005;39(4):196-204.
  5. Shrey DW, Griesbach GS, Giza CC. The pathophysiology of concussions in youth. Phys Med Rehabil Clin N Am. Nov 2011;22(4):577-602, vii.

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