The skull and vertebral canal form a rigid covering for the brain, spinal cord, cerebrospinal fluid (CSF) and blood. All of these intracranial compartments are non-compressible, thus the intracranial volume is essentially constant (the "Monro-Kellie doctrine"). Volume expansion of any compartment can only occur at the expense of compression of other compartments. The only buffering capacity is secondary to compression of the venous sinuses and the caudal displacement of CSF to the lumbosacral axis. Once this is exhausted, any tendency to increase volume in any of the compartments (as in an expanding mass) will result in an increased intracranial pressure (ICP).
The compliance of the intracranial system is often expressed as the "compliance curve" by plotting ICP against the expanding volume, although it should more accurately be described as the "elastance curve" (Fig. 5.1). A steep curve implies increased elastance or decreased compliance, as occurring with a rapidly expanding mass, as in a subdural or intracranial
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