Cerebral edema is an increase in brain volume caused by an absolute increase in cerebral tissue water content.Diffuse cerebral edema may develop soon after head injury. Vasogenic edema arises from transvascular leakage caused by mechanical failure of the tight endothelial junctions of the BBB. Vasogenic edema is frequently associated with focal contusions or hematomas. It eventually resolves as edema fluid is reabsorbed into the vascular space or the ventricular system.
Cytotoxic edema is an intracellular process that results from membrane pump failure. It is very common after head injury and is frequently associated with posttraumatic ischemia and tissue hypoxia. Normal membrane pump activity depends on adequate CBF to ensure adequate substrate and oxygen delivery to brain tissue. If the CBF is reduced to 40% or less of baseline, cytotoxic edema begins to develop. If CBF drops to 25% of baseline, membrane pumps fail and cells begin to die. Congestive brain swelling can contribute to cytotoxic edema if it becomes severe enough to increase ICP and reduce CPP so that cerebral circulation cannot be maintained.
Alteration in Consciousness
Consciousness is a state of awareness of the self and of the environment and requires intact functioning of the cerebral cortices and the reticular activating system (RAS) of the brain stem. An altered level of consciousness is the hallmark of brain insult from any cause and results from an interruption of the RAS or a global event that affects both cortices.
A patient who has sustained TBI commonly has an altered level of consciousness. Head-injured patients may be hypoxic from injury to respiratory centers or from concomitant pulmonary injury. Hypotension from other associated injuries can compromise CBF and affect consciousness. Global suppression may be present as a result of an intoxicating substance consumed before the injury. With increasing ICP from brain swelling or an expanding mass lesion, brain stem compression and subsequent RAS compression can occur.
Patients with altered levels of consciousness require careful monitoring and observation. Reversible conditions that can alter mental status, such as hypoxia, hypotension, hypoglycemia, should be corrected as they are identified.