Question 6(p.2)
Describe the physiology of intracranial pressure and the physiological mechanisms that limit a rise in intracranial pressure.
College Answer
Candidates who did well in this question used graphs to describe the various
concepts, described normal physiology and covered the breadth of the topic. A good
answer made mention of normal values of ICP, it’s variation with respiration and
blood pressure and illustrated a trace of the ICP. An explanation of the Monroe Kelly
doctrine was expected, CSF production and absorption and it’s relationship to raised
ICP as well other compensatory mechanisms for a high ICP (eg displacement of
CSF into spinal canal, displacement of venous blood into the jugular veins, rise in
ICP leads to ischaemia if the brain. Critical ischaemia invokes the Cushing reflex.
Major omissions by candidates was the use of diagrams, description of normal
variation and only a superficial knowledge of compensatory mechanisms.
Syllabus: G1, 2d,g
References: Textbook of Medical Physiology, Guyton, Chp 61
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
- Displacement of venous blood out of the CNS
- 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):
References
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