TBI occurs as the result of some external force being applied to the brain in an accident or trauma.
The mechanism of the injury itself is a key factor in what makes rehabilitation different for people with TBI from other neurological brain injuries.
In TBI there is an external force from an accident or trauma that may result in the brain being penetrated by objects, cut by internal bony skull ridges, torn, stretched, bruised or become swollen from the twisting and shearing forces as the brain moves around inside the skull, or is squashed when the skull is compressed or split open. Oxygen may not be able to get through to brain cells and there may be bleeding. Some brain cells will die while others recover partially or completely. The pattern of intact cells will be quite scattered.
Depending on the type and velocity of the forces affecting the brain, different areas throughout the whole brain are affected. This is different from stroke where a clot prevents oxygen travelling to the cells supported by that blood vessel, or where a blood vessel might burst and affect the immediate area and possibly the area distal to the bleed if the blood supply is cut off. The damage is limited to the area of the brain supplied by that vessel, e.g. middle cerebral artery, and other areas remain intact.
Common mechanisms of TBI
- Crush injuries with compression of the skull caught between two hard surfaces.
- Acceleration and deceleration injuries: injuries in which the head is in motion and then abruptly comes to a halt. For example, if a car hits a tree, the car suddenly stops, and the driver's head hits the steering wheel. The brain within the skull still has forward momentum and can hit the inner surface of the skull (which has bony edges) with some force, causing internal bruising, lacerations and bleeding.
- Diffuse axonal injury: the twisting motions or sudden changes in brain momentum that accompany some accidents may cause the bundles of nerve fibres (brain white matter) to be stretched or shear. The term closed head injury is often used to describe this type of TBI because the damage to the brain occurs as the result of these internal mechanisms but usually with the skull remaining intact.
- Penetrating injuries: this describes cases where some external object, e.g. knife or arrow, pierces the skull and there is direct contact with underlying brain tissue.