– Primary Injury
There are two main kinds of primary injury: penetrating or open head injury and closed head injury, which is much more common.
1. Penetrating or open head injury
A penetrating or open head injury is caused when an object fractures the skull and penetrates the brain – resulting in direct physical damage to the soft tissue.
Examples include colliding with the sharp edge of a brick wall or a bullet piercing the skull and entering the brain. In this kind of injury the damage is usually confined to the immediate area of the injury (localised damage), resulting in quite specific problems similar to those caused by a stroke. Unfortunately, the head is also often shaken which may result in widespread damage.
2. Closed head injury
A closed head injury involves rapid acceleration and deceleration or rotation of the head. The damage occurs when the head quickly changes speed of motion and is stopped abruptly, for example, by hitting the dashboard of a car.
The sudden and violent movement causes the brain to shift and rotate within the skull. This stretches and shears the delicate connecting nerve fibres, resulting in wide-spread damage called a diffuse axonal injury.
With the violent movement, blood vessels tear and the surface of the brain – mainly the frontal and temporal areas – is lacerated as it rotates across the bony ridges inside the front of the skull, so causing more damage.
Further damage and bruising can be caused to the front and back of the brain as it rebounds forward and backward against the rough inside surface of the skull.
Additionally, the skull may be fractured in the area where the head hit the fixed surface – causing further localised damage. If the fracture causes a piece of bone to exert pressure on the underlying brain, this is called a depressed fracture.
Mild brain injury is likely to involve the full range of damage discussed above as the brain is shaken around inside the skull. This can cause significant problems. The diffuse axonal damage can be devastating and result in permanent disability.