Concussion is a trauma-induced change in mental status, with confusion and amnesia, and with or without a brief loss of consciousness.

A concussion occurs when the head hits or is hit by an object, or when the brain is jarred against the skull, with sufficient force to cause temporary loss of function in the higher centers of the brain. The injured person may remain conscious or lose consciousness briefly, and is disoriented for some minutes after the blow. According to the Centers for Disease Control and Prevention, approximately 300,000 people have mild to moderate sports-related brain injuries each year, most of them young men between 16 and 25.

While concussion usually resolves on its own without lasting effect, it can set the stage for a much more serious condition. “Second impact syndrome” occurs when a person with a concussion, even a very mild one, suffers a second blow before fully recovering from the first. The brain swelling and increased intracranial pressure that can result is potentially fatal. More than 20 such cases have been reported since the syndrome was first described in 1984.

A state following injury in which there is temporary functional impairment without physical evidence of damage to the impaired tissues. The term usually refers to cerebral concussion produced by any type of trauma.

From a clinical point of view cerebral concussion is produced by a head injury which causes temporary unconsciousness but with complete recovery within 24 h. This temporary alteration is believed to result from one of several mechanisms. In all of these a sudden acceleration or deceleration appears to be aprerequisite. The sudden movement is thought to cause an unequal shifting of tissues of different specific gravities within the skull, between skull and brain, or between different brain tissues.

This word originally meant severe shaking, or the shock of an impact, but has come to mean the effect of such violence on the brain. The immediate effect of such an impact — usually when the moving head meets an immovable object, most commonly the ground — is unconsciousness. After a mild injury this lasts only a minute or so and the person is then dazed or confused for a few more minutes before recovering normal consciousness; occasionally recovery may take hours. After more severe impact injury, the patient may remain in coma for many days and remain confused for many more days thereafter. In either event there will be no memory for the moment of impact, often for a period immediately before this, and always for the period of unconsciousness and confusion: this is known as post-traumatic amnesia.

It is now recognized that the effect of the jelly-like brain being distorted by these forces is to stretch or even tear delicate nerve fibres, resulting in some permanent damage. After mild injury this is very limited, but after more severe impact there is more severe and more widespread damage to fibres. There can therefore be both mild and severe concussion.

After only mild concussion there are often symptoms for several days, sometimes weeks — headache, fatigue, dizziness, and poor concentration. In a few patients these post-concussional symptoms give rise to anxiety and other psychological symptoms that can aggravate and prolong the organically-impaired function that the patient suffers. In contact sports there is the risk of repeated concussions, and the small amount of damage sustained each time can be cumulative. Moreover, soon after one concussion the brain may be more susceptible to a second blow, and this is why most sports have rules about waiting 2-3 weeks before playing again, for example after concussion on the football field or in the boxing ring. The repeated concussions over a period of years that boxers can experience may result in progressive brain damage, evident in altered mental function and control of the limbs — the so-called ‘punch-drunk’ syndrome. This is now rare, as there are stringent regulations to limit exposure to such a hazard.