Concussion, from the Latin concutere ("to shake violently") or the Latin concussus ("action of striking together"), is the most common type of traumatic brain injury. The terms mild brain injury, mild traumatic brain injury (MTBI), mild head injury (MHI), and minor head trauma and concussion may be used interchangeably, although the latter is often treated as a narrower category. The term 'concussion' has been used for centuries and is still commonly used in sports medicine, while 'MTBI' is a technical term used more commonly nowadays in general medical contexts. Frequently defined as a head injury with a transient loss of brain function, concussion can cause a variety of physical, cognitive, and emotional symptoms.

Treatment of concussion involves monitoring and rest. Symptoms usually go away entirely within three weeks, though they may persist, or complications may occur. Repeated concussions can cause cumulative brain damage such as dementia pugilistica or severe complications such as second-impact syndrome.

Due to factors such as widely varying definitions and possible underreporting of concussion, the rate at which it occurs annually is not known; however it may be more than 6 per 1,000 people. Common causes include sports injuries, bicycle accidents, car accidents, and falls; the latter two are the most frequent causes among adults. Concussion may be caused by a blow to the head, or by acceleration forces without a direct impact. The forces involved disrupt cellular processes in the brain for days or weeks.

It is not known whether the concussed brain is structurally damaged the way it is in other types of brain injury (albeit to a lesser extent) or whether concussion mainly entails a loss of function with physiological but not structural changes. Cellular damage has reportedly been found in concussed brains, but it may have been due to artifacts from the studies. A debate about whether structural damage exists in concussion has raged for centuries and is ongoing.

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