A concussion is basically a mild traumatic brain injury.  To be considered a concussion, there must be some alteration of mental status such as confusion, difficulty concentrating, amnesia, or even a loss of consciousness.  Studies on animals suggest that after a head injury, the brain requires more oxygen, but blood flow to the brain actually decreases.  This mismatch in the supply and demand for oxygen for the brain may persist for many days.  As a result, subsequent injuries during this time may not be as well tolerated as they otherwise would, resulting in severe, possibly fatal outcomes.

The most common symptom of a concussion is a headache.  However, a headache after head trauma does not equate to a concussion if there is no alteration of mental status.  Furthermore, it is important to remember that headaches that do follow concussions may not manifest for several hours.  Sometimes the headaches are so severe, and accompanied by nausea or hypersensitivity to noise or light, that they have been described as being post-traumatic migraines.

Those who are not truly confused, but just feel “foggy”, after concussions process information slower, and therefore have slower reaction times.  Returning to play at that point may not only result in less effective performance, but also risk further trauma by not being able to avoid potential injurious situations as well as otherwise.

Amnesia following concussion can involve memory loss of events for a time period after the injury (referred to as “antegrade” or “post-traumatic”) or for a time period prior to the injury (referred to as “retrograde” or "pre-traumatic"), or both.  Although the time period covered by the amnesia will usually shrink as the individual recovers, there typically will be some permanent loss of memory. It can be very difficult to evaluate amnesia in the face of ongoing confusion.

Fortunately, loss of consciousness following concussion is typically brief, and occurs in less than 10% of these injuries.  For prolonged loss of consciousness, imaging of the central nervous systems is indicated, especially if the loss of consciousness begins some time after the injury, rather than immediately following the traumatic event.  This makes it imperative to observe the injured person for a couple of hours post injury, to ensure that the level of consciousness is not declining, which is often a marker of significant arterial bleeding inside the skull cavity.

One major concern about concussions is that premature return to play and subsequent re-injury before fully recovering from the first injury can lead to severe symptoms and even death – even though the second injury may not be very severe.  This “second impact syndrome” occurs more often in younger (e.g. high school) athletes than in somewhat older competitive athletes (e.g. college and professional).  Furthermore, younger athletes take longer to return to baseline levels of performance.  One study reported high school athletes returned to normal verbal memory skills 7 days following concussions compared to 3 days for college athletes playing similar sports and having similarly severe head injuries.

Classification systems have been developed to help grade the severity of concussion, and by so doing help guide time to return to contact sports.  There are numerous grading schemes currently in use, indicating a significant lack of consensus among physicians evaluating and treating athletes with concussions.