Porth's Essentials of Pathophysiology, 4e

922

Nervous System

U N I T 1 0

1

Cerebral arteries

Incisura

1

Oculomotor nerve

2

3

2

4

3

A

B

C

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].

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

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.)

A

Flexor or decorticate posturing

B

Extensor or decerebrate posturing