C h a p t e r 3 7
Disorders of Brain Function
929
Decorticate (flexion) posturing is characterized by
the arms being held tightly to the sides, with flexion of
the arms, wrists, and fingers; and extension and internal
rotation of the legs with plantar flexion of the feet (see
Fig. 37-4A). Decorticate posturing results from lesions
of the cerebral hemisphere or internal capsule.
Decerebrate (extensor) posturing results from
increased muscle excitability (see Fig. 37-4B). It is char-
acterized by rigidity of the arms with the wrists and fin-
gers flexed and turned away from the body and with
stiffly extended legs and plantar flexion of the feet. This
response occurs with rostral-to-caudal deterioration,
when lesions of the diencephalon extend to involve the
midbrain and upper brain stem.
Respiratory Responses.
Early respiratory changes
include yawning and sighing, with progression to Cheyne-
Stokes breathing, in which there is waxing and waning
of respirations with variable periods of apnea. When the
progression of injury continues to the midbrain, respi-
rations change to neurogenic hyperventilation, in which
the frequency of respirations may exceed 40 breaths per
minute because of uninhibited stimulation of inspiratory
and expiratory centers. With medullary involvement, res-
pirations become ataxic (i.e., totally uncoordinated and
irregular). Apnea may occur because of a lack of respon-
siveness to carbon dioxide stimulation. Complete venti-
latory assistance is often required at this point.
A
B
FIGURE 37-11.
The doll’s-head eye response demonstrates
the always-present vestibular static reflexes without forebrain
interference or suppression. Severe damage to the forebrain
or to the brain stem rostral to the pons often results in loss of
rostral control of these static vestibular reflexes. If the person’s
head is moved from side to side or up and down, the eyes will
move in conjugate gaze to the opposite side
(A)
, much like
those of a doll with counterweighted eyes. If the doll’s-head
phenomenon is observed, brain stem function at the level of
the pons is considered intact (in a comatose person). In the
unconscious person without intact brain stem function and
vestibular static reflexes, the eyes stay in midposition (fixed) or
turn in the same direction
(B)
as the head is turned.
SUMMARY CONCEPTS
■■
Many of the agents that cause brain damage
do so through common and often interrelated
pathways, including hypoxia or ischemia,
accumulation of excitatory amino-acid
neurotransmitters, and cerebral edema.
■■
Deprivation of oxygen (i.e., hypoxia) or blood
flow (i.e., ischemia) can have deleterious effects
on the brain structures. Ischemia can be focal,
as in stroke, or global as occurs during cardiac
arrest when blood flow is inadequate to meet the
metabolic needs of the entire brain.
■■
Excitotoxicity is a final common pathway for
neuronal cell injury and death. It is associated
with excessive activity of excitatory amino-acid
neurotransmitters, particularly glutamate.
■■
Cerebral edema represents an increase in brain
volume secondary to abnormal fluid accumulation.
Vasogenic edema occurs when integrity of the
blood–brain is disrupted allowing intravasular
fluid to move into the extracellular fluid surround
brain cells; whereas, cytotoxic edema involves
swelling of brain cells due to the movement of the
extracellular fluid into the brain cells.
■■
The intracranial pressure (ICP) is the pressure
exerted by the essentially incompressible tissue
and fluid volumes of the three compartments
contained within the rigid confines of the skull—
brain tissue, blood, and cerebral spinal fluid
(CSF). Excessive ICP can obstruct cerebral blood
flow, destroy brain cells, displace brain tissue
as in herniation, and otherwise damage delicate
brain structures.
■■
Hydrocephalus represents enlargement of
the CSF compartment owing to an abnormal
CSF volume. It can result from impaired
reabsorption from the arachnoid villi into the
venous system (communicating hydrocephalus)
or from obstruction of the ventricular system
(noncommunicating hydrocephalus), which
prevents the CSF from reaching the arachnoid
villi.
■■
The term traumatic brain injury refers to injuries
to the skull, brain, or both.The brain injuries
can be primary, because of direct impact, or
secondary, resulting from complicating processes
that were initiated at the time of injury.They
can be focal, as occurs with contusions and
hematoma formation; or diffuse, as in concussion
or diffuse axonal injury.
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