C h a p t e r 3 7
Disorders of Brain Function
921
3
Compliance and the Volume-
Pressure Curve.
Compliance,
which refers to the ease with which
a substance can be compressed
or deformed, is a measure of the
brain’s ability to maintain its ICP
during changes in intracranial vol-
ume. Compliance (C) represents the
ratio of change (
Δ
) in volume (V) to
change in pressure (P): C =
Δ
V/
Δ
P.
The dynamic effects of changes in
intracranial volume and compliance
on ICP can be illustrated on a graph
with the volume represented on
the horizontal axis and ICP on the
vertical axis. The shape of the curve
demonstrates the effect on ICP of
addingvolumetotheintracranialcavity.
From points A to B, the compensatory
mechanisms are adequate, compliance
is high, and the ICP remains relatively
constant as volume is added to the
intracranial cavity. At point B, the
ICP is relatively normal, but the
compensatory mechanisms have
reached their limits, compliance is
decreased, and ICP begins to rise with
each change in volume. From points C
to D, the compensatory mechanisms
have been exceeded and ICP rises
significantly with each increase in
volume as compliance is lost.
90
80
70
60
50
40
30
20
10
1 2 3 4 5 6 7 8 9 10
mm Hg
Units of volume
A
B
C D
ICP
(From Hickey JV. Neurological and Neurosurgical Nursing. 5th ed.
Philadelphia, PA: Lippincott Williams &Wilkins; 2003:286.)
and eventually medullary (respiratory) function. The
pattern of deterioration is fairly predictable unless vas-
cular or obstructive complications occur and exaggerate
the condition. Pupillary reactions to light is upper brain
stem function (oculomotor nerve [CNIII]) and are espe-
cially useful when assessing the unconscious patient.
There are two major types of supratentorial hernia-
tion: cingulate and transtentorial.
1,4,7
A
cingulate hernia-
tion
(see Fig. 37-3C [1]), which poses the less serious
threat in terms of clinical outcomes, involves displace-
ment of the cingulate gyrus and hemisphere beneath the
sharp edges of the falx cerebri to the opposite side of
the brain.
7
This displacement can compress local blood
supply and cerebral tissue, causing edema and ischemia,
which further increase the degree of ICP elevation. There
may also be compression of branches of the anterior
cerebral artery with unilateral or bilateral leg weakness.
Transtentorial herniations
result in two distinct syn-
dromes: a central syndrome and an uncal syndrome,
which is the most common herniation syndrome.
7
Clinically, they display distinct patterns early in their
course, but both merge in a similar pattern once they
begin to involve structures at the level of the midbrain
and below.
Central transtentorial herniations
(see Fig.
37-3C [2]) involve the downward displacement of the
cerebral hemispheres, basal ganglia, diencephalon, and
midbrain through the tentorial incisura. Bilateral small,
reactive pupils and drowsiness are heralding signs. The
uncal herniation syndrome
occurs when an expanding
lesion pushes the medial aspect of the temporal lobe,
which contains the uncus and hippocampal gyrus,
through the incisura of the tentorium (see Fig 37-3C
[3]). As a result, the diencephalon and midbrain are
compressed and displaced laterally to the opposite side
of the tentorium. The oculomotor nerve (CN III) and the
posterior cerebral artery are frequently compressed. The
oculomotor nerve controls pupillary constriction; entrap-
ment of this nerve results in ipsilateral pupillary dilation,
(
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