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Nervous System
which usually is an early sign of uncal herniation.
Consciousness may be unimpaired because the reticular
activating system, which is responsible for wakefulness,
has not yet been affected. As uncal herniations progress,
there are changes in motor strength and coordination
of voluntary movements because of compression of the
descending motor pathways. It is not unusual for initial
changes in motor function to occur ipsilateral to the side
of the brain damage because of compression of the con-
tralateral cerebral peduncles. Changes in consciousness
and coma may follow due to compression of the mid-
brain against the opposite tentorial edge. Decerebrate
posturing (Fig. 37-4B) may develop, followed by dilated,
fixed pupils; flaccidity; and respiratory arrest.
Infratentorial Herniation.
Infratentorial compartment
lesions contributing to herniation are much less frequent
than those of the supratentorial region.
7
The infratentorial
compartment contains both the brain stem and cerebel-
lum. Herniation may occur superiorly (upward) through
the tentorial incisura or inferiorly (downward) through
the foramen magnum. Upward displacement of the brain
stem and cerebellum through the tentorium results maxi-
mum pressure on the midbrain. The most prominent
signs of upward herniation include: immediate onset
of deep coma; small equal, fixed pupils; and abnormal
respirations (slow rate with intermittent sighs or ataxia)
and other vital signs. Downward herniation involves dis-
placement of the midbrain through the tentorial notch
or the cerebellar tonsils through the foramen magnum
(Fig. 37-3C [4]). It often progresses rapidly and can cause
death because it is likely to involve the lower brain stem
centers that control vital cardiopulmonary functions.
Hydrocephalus
Hydrocephalus represents a progressive enlargement
of the ventricular system due to an abnormal increase
in CSF volume (see Chapter 34, Fig. 34-21). It can
result because of overproduction of CSF, impaired
A
Flexor or decorticate posturing
B
Extensor or decerebrate posturing
FIGURE 37-4.
Abnormal posturing.
(A)
Decorticate rigidity. In decorticate rigidity,
the upper arms are held at the sides, with
elbows, wrists, and fingers flexed.The legs
are extended and internally rotated.The feet
are plantar flexed.
(B)
Decerebrate rigidity.
In decerebrate rigidity, the jaws are clenched
and neck extended.The arms are adducted
and stiffly extended at the elbows with the
forearms pronated, wrists and fingers flexed.
(From Fuller J, Schaller-Ayers J. Health
Assessment: A Nursing Approach. 2nd ed.
Philadelphia, PA: J.B. Lippincott; 1994.)
1
2
3
4
3
2
1
A
B
C
Incisura
Cerebral
arteries
Oculomotor
nerve
FIGURE 37-3.
Supporting septa of the brain and patterns of herniation.
(A)
The falx cerebri [1],
tentorium cerebelli [2], and foramen magnum [3].
(B)
The location of the incisura or tentorial notch in
relation to the cerebral arteries and oculomotor nerve.
(C)
Herniation of the cingulate gyrus under the
falx cerebri [1], central or transtentorial herniation [2], uncal herniation of the temporal lobe into the
tentorial notch [3], and infratentorial herniation of the cerebellar tonsils [4].
