Porth's Essentials of Pathophysiology, 4e

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Nervous System

U N I T 1 0

Intracranial Pressure

U N D E R S T A N D I N G

The intracranial pressure (ICP) is the pressure within the intracranial cavity. It is determined by (1) the pressure-volume relationships among the brain tissue, cerebrospinal fluid (CSF), and blood in the intracranial cavity; (2) the Monro-Kellie hypothesis, which relates to reciprocal changes among the intracranial volumes; and (3) the compliance of the brain and its ability to buffer changes in intracranial volume.

Nonexpandable skull

Intracranial Volumes and Pressure. The ICP represents the pressure exerted by the essentially incompressible tissue and fluid vol- umes of the three compartments contained within the rigid confines of the skull—the brain tissue and interstitial fluid (80%), the blood (10%), and the CSF (10%). 1

Brain tissue

Cerebrospinal fluid

Blood

Monro-Kellie Hypothesis. Nor­ mally, a reciprocal relationship exists among the three intracranial vol- umes such that the ICP is maintained within normal limits. Because these volumes are practically incompress- ible, a change in one component must be balanced by an almost equal and opposite effect in one or both of the remaining components. This is known as the Monro-Kellie hypothesis. Of the three intracranial volumes, the fluid in the CSF compartment is the most easily displaced. The CSF (A) can be displaced from the ventricles and cerebral subarachnoid space to the spinal subarachnoid space, and it can also undergo increased absorp- tion or decreased production. Because most of the blood in the cranial cav- ity is contained in the low-pressure venous system, venous compression (B) serves as a means of displacing blood volume. 2

A

B