Question 15

College Answer

A definition and a normal value were expected. A description of the Monro-Kellie doctrine was expected. Better answers divided into the various components of the cranium with the answer focussing on cerebral blood volume and CSF volume as the brain tissue as no capacity to change its volume.

Discussion

• Intracranial pressure (ICP) is the pressure within the intracranial space relative to atmospheric pressure
  • It is determined by the three components of the Monro-Kellie relationship, which states that an increase in the volume of one intracranial compartment will lead to a rise in ICP unless it is matched by an equal reduction in the volume of another compartment
  • These compartments are:
    • Brain tissue: 1400ml on average
    • Cerebral blood volume: 150ml
    • Cerebrospinal fluid: 150ml
• Normal intracranial pressure regulation
  • Intracranial pressure is normally ~ 10 mmHg in the supine person, and probably 0-2 mmHg in the upright person
  • ICP is well regulated within the normal physiological range by these main mechanisms:
    • Displacement of venous blood out of the CNS
      • Venous blood is a relatively large and relatively mobile component of the intracranial cavity, and can be moved out of the brain at short notice.
    • Displacement of CSF out of the brain and into the spinal cord
      • The compliance of the spinal meninges is usually better than the compliance of the cerebral meninges, and CSF can reflux into the spinal spaces if intracerebral pressure is raised
    • Distension of the meninges
      • dura-covered openings in the skull and spinal column are flexible, i.e. the dura can bulge out of these openings to accommodate a (small) volume.
    • Venting of the CSF into the venous circulation by increased reabsorption through arachnoid granulations
  • Intracranial compliance becomes poor if even small increases in volume occur, as these mechanisms are quickly overwhelmed (i.e. the intracranial compliance curve is hyperbolic)

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