In a compound fracture the scalp has been lacerated and directly communicates with the brain via the bony fragments of the skull. This type of fracture is associated with the worst prognosis and is often accompanied by severe brain damage. A concussion is often associated with a brief period of amnesia. If the loss of consciousness persists for a prolonged period of time, then it is assumed that there has been structural injury of brain tissue, and the patient is considered to be in coma. Therefore a concussion, in contrast to a coma, results from an interruption of the physiological functioning of the brain, and not structural brain injury.
Cerebral edema refers to brain swelling due to an accumulation of water.
This is often caused by an excessive movement of fluid into the brain tissue from the blood vessels supplying that tissue. Critical brain swelling can develop hours or days after a serious head injury, and can be detected with an intracranial pressure monitor. A patient may be in a persistent coma because of a widespread stretching or shearing of brain cell processes known as axons. Such generalized axonal damage is known as diffuse axonal injury. This refers to bleeding within the brain as it scrapes across the inner surface of the skull.