Porth's Essentials of Pathophysiology, 4e

921

Disorders of Brain Function

C h a p t e r 3 7

ICP

mm Hg

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.

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Units of volume

(From Hickey JV. Neurological and Neurosurgical Nursing. 5th ed. Philadelphia, PA: Lippincott Williams &Wilkins; 2003:286.)

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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,