5-3. PATHOPHYSIOLOGY

Pathophysiology is the physiology of disordered function. When there is trauma to the central nervous system in the form of a head injury, a variety of pathophysiological responses can occur.

a. Head Injury. The words "head injury" usually refer to an injury to the portion of the skull (cranium) that encloses the brain, the overlying scalp, or the contents of the cranial cavity (brain, cranial nerves, meninges, and associated blood vessels). This definition focuses attention on that portion of the head that is at or above the level of the eyebrows anteriorly, the zygomatic arches laterally, and an imaginary line between the tips of the mastoid processes posteriorly. These are approximate external landmarks for the skull base, which is the floor of the cranial cavity. Nevertheless, physical signs of injury of the brain or of its soft tissue or bony coverings may also be detected in adjacent structures of the head (eyes, ears, and nose) or even in portions of the body that are remote from it.

b. Brain Injuries. Most brain injuries occur due to movement of the brain inside the skull. The level of damage to the brain depends on the speed the head was traveling and the head's position just prior to contact.

c. Responses to Brain Injury. The base of the skull is rough; therefore, movement over this area will cause various degrees of injury to the brain or blood vessels. Possible responses to brain injury include the following:

(1) Initial response to a bruised brain is swelling. The swelling is caused by:

(a) Increased blood volume due to vasodilation and increased cerebral blood flow to the injured areas.

(b) Buildup of extra blood volume putting pressure on the brain and decreasing blood flow to the injured part.

NOTE: Since the edema builds over a period of 24 to 48 hours, early care and efforts to decrease the vasodilation is important.

(2) Carbon dioxide may build up, having a critical effect on cerebral vessels. This buildup causes more vasodilation.

(3) Hyperventilation may occur, causing a decrease in the carbon dioxide, vasoconstriction, and better perfusion (passage of a fluid through the vessels of an organ) for the brain.

NOTE: Hyperventilation -- a condition marked by fast, deep breathing, which tends to remove increased amounts of carbon dioxide from the body and lower the partial pressure of the gas, causing buzzing in the ears, and tingling of the lips and fingers.

(4) Unconsciousness may occur due to injury to the cerebral cortex or the brain stem.

(5) If there is increased intracranial pressure (ICP) and decreasing cerebral blood flow, no matter what the cause, the level of consciousness is depressed.

(6) The intracranial cavity is filled to capacity with contents that cannot be compressed -- cerebral spinal fluid, intravascular blood, brain tissue water (interstitial fluid). If the volume of one of the constituents of the intracranial cavity increases, a reciprocal decrease in volume of one or both of the others must occur. Otherwise, the result is an increase in intracranial pressure.