Page 540 - Clinical Application of Mechanical Ventilation

506 Chapter 15

Acceleration and Deceleration Brain Injuries

A direct blow to the head causes acceleration brain injury. This type of injury
causes the skull to move away from the blow. Since the brain does not move away
from the blow at the same speed as the skull, the skull makes impact to the brain
and causes brain injury. In deceleration brain injury, the skull makes impact to a
stationary object (e.g., a fall on concrete). The skull stops suddenly when it makes
impact on the concrete. The brain continues to move toward the concrete and
make impact with the skull. In both acceleration and deceleration injuries, the
various cellular components of the brain or the axons are injured. In some cases,
rotational forces applied to the skull and brain can also cause shear injuries on the
axons (Crippen, 2011).
Explosive blasts are a common cause of TBI in active military personnel. Re-
searchers believe the pressure wave from explosives passing through the brain di-
rectly disrupts brain function (Mayoclinic, 2012). In addition to the pressure wave,
TBI may also result from acceleration (e.g., shrapnel) and deceleration (e.g., falls or
head collisions with stationary objects) brain injuries.

Brain Herniation. Within the skull, the two dural structures (falx cerebri and ten-
torium cerebella) divide the intracranial compartment into three sections. These
two dural structures have central openings with prominent edges at the borders
of these openings. When the ICP becomes significantly elevated, the brain can
become herniated and slide through these openings within the falx or tentorium.
Brain injury often occurs when the brain makes contact with the dural edge
(Crippen, 2011). There are five types of brain herniation and each type has its dis-
tinct characteristics. Transtentorial herniation is one example and it is discussed
below. Readers should refer to a neurology reference source for other types of
brain herniation.
Transtentorial herniation is a type of brain injury that causes the downward dis-
transtentorial herniation: A
type of brain injury that causes placement of the medial aspect of the temporal lobe (uncus) through the tentorial
the downward displacement of notch by a mass above. This condition exerts pressure on the underlying structures,
the medial aspect of the temporal
lobe (uncus) through the tentorial including the brainstem and the third cranial nerve. Compression of the third cranial
notch by a mass above. nerve suppresses the parasympathetic input to the eye, resulting in a dilated pupil. A
unilateral (one side only) dilated pupil is a classic sign of transtentorial herniation of
the brain. Eighty percent of the dilated pupil occurs ipsilateral (same side) to the side
of herniation. If a unilateral dilated pupil is not observed, unilateral posturing can
be a sign of transtentorial herniation. Unilateral dilated pupil or/and unilateral pos-
1
turing requires immediate evaluation for surgical intervention (trauma.org , 2011).
Clinical Evaluation and Assessment

TBI is graded on the basis of the level of consciousness (Table 15-8) or Glasgow
Glasgow coma scale (GCS): A scor-
ing system to determine the degree coma scale (GCS) score (Table 15-9). The level of consciousness ranges from cloudy
of traumatic brain injury; score of 13 consciousness (mild) to brain death (most severe). Mild TBI has a GCS score of 13
to 14 (mild), score of 9 to 12 (moder-
ate), score of 3 to 8 (severe). to 14. Moderate injury has a score of 9 to 12, and severe injury has a score of 3 to 8
(Teasdale et al., 1974).

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