Trauma is the most common cause of acquired brain injury particularly in young people, of course there are other ways in which the brain can acquire injury, stroke is particularly common from middle age onwards then there are rare conditions of the brain like infections and tumors. The brain is very sensitive to the amount of blood getting through to it, so anything which impedes the flow of blood to the brain or lowers oxygen to the brain, can damage it. Poisons in the blood, such as carbon monoxide from car fumes, can poison the brain. Finally there are treatments people can receive in good faith, which can damage the brain, a surgeon operating on the brain to remove a tumor that inadvertently damage it and radiation therapy, which is often used to treat malignant brain tumors, which can damage the it especially in young people.
There are two broad guidelines to asses the severity of a brain injury. The first is how unconscious the patient is at their very worst, and this is measured with something called the Glasgow Coma Score, this is usually measured out of 15, 15 is fully conscious and three is as deeply unconsistent as you can be and still be alive. The Glasgow Coma Score has three levels in it, one is to do with response to eye movement and eye opening, another is to do with response to speech, and the third is to do with response to movement. These three levels can all be given a number, and that gives you the total figure. Broadly speaking, any head injury in which the Glasgow Coma Score goes to eight or below is classified as a very severe brain injury.
The second things that used to asses the severity if a brain injury is what is called the post traumatic amnesia (PTA), this is the period of time from the accident to when continuous memory returns. The PTA does not shrink, so if a month after a head injury a person has a PTA of an hour, its still going to be an hour a year later. The PTA has a very close connection to time taken to get back to work and it's also one of the factors in the risk of getting late traumatic epilepsy.
The temporal lobes, on either side of the brain, are concerned predominately with memory, but they also have input into mood and emotions. The most devastating effect of serious damage to the temporal lobes is loss of memory, particularly the loss of short-term memory.
The occipital lobes at the back of the brain are where vision is perceived. They're not often damaged in acquitted brain injury, but when they are damaged, there can be very severe impairments of how the brain sees the world.
The cerebellum, at the back part of the head, controls coordination of movement and coordination of the muscles of the larynx (the voice box). If they're damaged the injured person will be clumsy with their arms and their legs and their speech may also be slurred and clumsy, they may also have difficulty with swallowing too.
One of the most obvious problems that can develop from a brain injury is weakness of an arm or a leg or lack of coordination of an arm or a leg. Speech and language functions can be damaged either in terms of getting out words, understanding words or just in the mechanics of producing speech, so speech may become blurred or slurred. The special senses (hearing, vision, smell and taste) are not often damaged with the exception of smell, when smell is lost the finer points of taste are also lost.
When it comes to recovering from a brain injury there is no fixed period of time. The time is longer in children in the first 10 – 15 years of life. Essentially most recovery occurs in the first 18 months. A better way of looking at it is that when someone has reached the plateau and has stayed at that plateau for six months, that's probably how they're going to stay.