Perhaps the most basic response is “Brain injury is injury to the brain”, the easy response to what is in reality a very complex question and concept. Injury to the brain takes many forms. Many patients experience non-traumatic brain injury (e.g., brain tumors, infections, seizures, stroke, hypoxia, toxic exposure ((alcohol, drugs)) and others). The most common perception is that only an individual who is “hit in the head” has “brain damage”. Often in sports, being hit in the head is treated as of no major consequence, the “victim” just needs to “shake it off” and get back in the game. Someone hit in the head does not necessarily have brain injury, the spectrum of injury ranging from “no injury” to death. Often the consequence of brain injury has been over-looked but many athletes have become disabled by repeated brain injury, the effect being a cumulative effect, multiple concussions. Mohammad Ali’s primary diagnosis was not “Parkinson’s Disease”. Compared to most boxers, he took relatively few “direct” hits. Sometimes a relatively mild blow is lethal. Natasha Richardson’s tragic death skiing is such a reminder. Mild injury in a fall skiing, she died of a cerebral hemorrhage the next day, a direct result of the blow to the head.

A major misconception is that if you have a brain injury, you “look” brain-injured. You “drag a leg and drool”. The reality is most patients with acquired brain injury are not obvious. The range of brain injury is from very mild to very severe. There are multiple systems for evaluating and rating brain injury relative to severity, but it is not always accurate.

Perhaps the most basic function of the brain is that of “regulation”. When a brain injury occurs, the most predictable consequence of the injury is that of loss of “regulation”. One of the most common problems patients experience is a rather marked disruption of mood and emotion, including "neurogenic irritability", low stress and frustration tolerance, reactive anger, becoming less inhibited, and so on. For some, dysregulation of sleep is a problem, unable to fall asleep and stay asleep. Others experience other physiological dysregulation (e.g., menstrual cycles, dysregulation of attention/concentration). Injury may also disrupt both motor and sensory function (e.g, loss of muscle function, vision, smell, hearing).

Another area disrupted to varying degrees is disruption of the brain’s ability to think, reason, and problem-solve. Memory, particularly working and short-term memory, is disrupted, a significant problem in day-to-day function (e.g., school, employment, socialization). The brain’s ability to learn is often a problem. A major contributing factor is that of disruption to the brain’s ability to organize information, allowing us to reason and problem-solve. In some cases, the disruption is relatively mild and short-lived; at the other extreme, the consequences are severe and long-term. Treatment interventions are often of benefit, short and long-term.