====== Intracranial compliance ======

Intracranial [[compliance]] refers to the ability of the [[brain]] and [[cerebrospinal fluid]] (CSF) within the [[skull]] to change [[volume]] in response to changes in pressure. It is affected by factors such as the amount of CSF within the skull, the [[elasticity]] of [[brain tissue,]] and the degree of [[brain swelling]]. Low intracranial compliance can occur in conditions such as [[brain tumor]]s, hydrocephalus (excess fluid within the skull), and [[traumatic brain injury]], and can lead to increased pressure within the skull, which can cause brain damage. High intracranial compliance, on the other hand, can lead to decreased pressure within the skull, which can also cause brain damage.

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The brain is enclosed in a rigid container, and any transfer of pulsatility from the arterial walls into the surrounding tissue is felt almost instantaneously everywhere throughout the cranium. This leads to the observation that intraparenchymal and CSF pressure [[waveform]]s tend to be similar and independent of location. This is sometimes over generalized to suggest that pressures are everywhere equal [[intracranial]]ly, but this obviously does not apply to the very important arterial and venous compartments. Secondly, this leads to the interesting and potentially important phenomenon of measurable [[flow pulsatility]] in the microvasculature 
((Seki J, Satomura Y, Ooi Y, Yanagida T, Seiyama A. Velocity profiles in the rat cerebral microvessels measured by optical coherence tomography. Clin Hemorheol Microcirc. 2006;34:233–239.))
and in the venous system. In the brain, the substitute for tissue compliance, which dissipates arterial pulsations in non-cranial tissues, is the overall intracranial [[compliance]]. This compliance, is comprised of four main components: actual [[brain tissue]] compliance (which is small), arterial compliance, venous compliance (veins have highly compliant walls) and compliance of the spinal thecal sac (which communicates with the brain via the [[cerebrospinal fluid]] spaces). Traditionally, intracranial compliance is assumed to decrease primarily with increased ICP, due to the exponential pressure-volume relationship
((Marmarou A, Shulman K, Rosende RM. A nonlinear analysis of the cerebrospinal fluid system and intracranial pressure dynamics. J Neurosurg. 1978;48:332–344. doi: 10.3171/jns.1978.48.3.0332.)).

Decreased compliance with elevated ICP leads to increased pressure [[pulsatility]]. However, an additional factor which must be considered is the transfer of pulsations out of the cranium through either venous or CSF outflow pathways; while usually not considered as a factor which affects intracranial compliance, this is another way in which pulsatility is modified in the brain. Thus, intracranial pulsatility can also be affected by restriction of these flow pathways (which can manifest itself as a change in either pressure or flow pulsatility), such as with venous hypertension or a blockage in the outflow CSF pathways at the [[craniocervical junction]] (e.g., in [[Chiari malformation]] or Dandy-Walker variant)
((Wagshul ME, Eide PK, Madsen JR. The pulsating brain: A review of experimental 
and clinical studies of intracranial pulsatility. Fluids Barriers CNS. 2011 Jan
18;8(1):5. doi: 10.1186/2045-8118-8-5. PubMed PMID: 21349153; PubMed Central
PMCID: PMC3042979.
)).
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see Application of non-invasive [[intracranial pressure waveform]] analysis in acute [[brain injury]]: [[Intracranial Compliance]] Scale
((Frigieri G, Robba C, Machado FS, Gomes JA, Brasil S. Application of non-invasive ICP waveform analysis in acute brain injury: Intracranial Compliance Scale. Intensive Care Med Exp. 2023 Jan 27;11(1):5. doi: 10.1186/s40635-023-00492-9. PMID: 36703025.)).