Following significant head injury, whether clinically mild or severe, swelling of the brain can occur. Brain swelling may be focal, adjacent to an area of brain injury; or diffuse, involving one or both cerebral hemispheres. Brain swelling is due to an increase in intravascular cerebral blood volume secondary to vasodilatation (congestive brain swelling), or an absolute increase in the water content of the brain tissue, or a combination of the two. An increase in tissue water content, or cerebral edema, is often incorrectly considered synonymous with brain swelling. If continued long enough, brain swelling caused by an increase in the intravascular cerebral blood volume progresses to cerebral edema, presumably due to increased vascular permeability. The magnitude of the brain swelling does not necessarily correspond to the severity of the injury. Massive cerebral (congestive) swelling can occur within 20 minutes following head trauma.41
Swelling of one cerebral hemisphere is seen most commonly with an ipsilateral subdural hematoma. The secondary swelling may, in fact, cause a more serious mass effect than the hematoma. The rapid onset of the swelling suggests that the etiology is congestive.
With severe brain injury, diffuse brain swelling of a severe degree may occur immediately without the individual regaining consciousness. Brain swelling, however, might not occur immediately after an injury, but rather develop minutes to hours later. Delayed brain swelling of a significant degree is rare. It is usually diffuse and more often associated with the less severe forms of brain injury. Typically, the patient receives a concussion, regains consciousness, only to become stuporous and lapse into coma minutes to hours later. Until recently, it was felt that children were more susceptible than adults to developing diffuse swelling, even after minor trauma.42,43 Recent studies have challenged this contention. Lang et al. found that, while diffuse swelling might occur more readily in children it is more benign.44 Thus, in their study, 75% of children with diffuse swelling had a benign course, while two thirds of adults had a poor outcome The researchers believed that a number of the previous studies had problems in that they were not able to adequately study changes in the brain in children because the studies were performed without CT scans.
If brain swelling develops to a severe degree and continues over a sufficient time, there can be herniation of the brain or secondary brain stem hemorrhage. A rapidly expanding intracranial mass or severe brain swelling
can produce tonsillar, transtentorial, or subfalcial herniation of the brain, with resultant necrosis, secondary infarction, and Duret hemorrhages (Figure 6.15). Herniation may be either symmetrical, due to brain swelling, or asymmetrical, due to a mass in one side of the brain or subdural space, e.g., a subdural hematoma or intracerebral hemorrhage. In the case of diffuse brain swelling, there is usually symmetrical herniation of the cerebellar tonsils without brain stem hemorrhage. The brain stem and cerebellar tonsils are forced into the foramen magnum, with resultant dysfunction or even infarction of the brain stem. The individual becomes unconscious and develops respiratory difficulty that proceeds to arrest and death. Severe herniation of the cerebellar tonsils can result in infarction. In some individuals with prolonged survival, the authors have seen the upper spinal cord encased in necrotic cerebellar tissues shed into the cerebrospinal fluid. In dealing with an asymmetrical herniation caused by a subdural hematoma, in addition to ipsilateral cerebellar tonsil herniation, one often has a secondary brain stem hemorrhage (a Duret hemorrhage) involving the midbrain and pons.
Transtentorial or uncal herniation is due to a rapidly expanding suprat-entorial mass lesion. It may be either unilateral or bilateral, though unilateral herniation is more common because rapidly expanding lesions are usually unilateral. A rapidly expanding mass in a cerebral hemisphere means that ipsilateral uncal herniation can be expected. If severe enough, there will be displacement of the brain stem against the contralateral tentorial edge with injury to the brain stem and production of Kernohan's notch.
The third type of herniation is subfalcial or transfalcial herniation. This occurs when there is a rapidly expanding mass in one cerebral hemisphere or at least in the subdural space on one side. This causes herniation of the cerebral hemisphere across the midline below the edge of the falx. The her-niating tissue is most often the cingulate or supracingulate gyrus.
As previously noted, herniation with compression of the brain stem can result in Duret hemorrhages. These are secondary herniation hemorrhages of the midbrain and pons. They might range from small streaks to massive confluent hemorrhage. They are in the midline and are most commonly associated with asymmetrical herniation of the brain stem. Duret hemorrhages may develop in only 30 min.
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