Question 18

College Answer

It was expected answers would include an explanation of the Monro-Kellie Doctrine. Many candidates gave insufficient details of compensatory mechanisms especially regarding decreased total cerebral blood volume (primarily venous) in response to increased intracranial pressure. Most candidates had all the information but had difficulty synthesising the information to write a  cohesive answer. Factors affecting ICP could be divided into factors affecting CBV, factors  affecting CSF and factors affecting brain tissue. Under factors affecting CBV the effect of blood  gases, autoregulation, temperature, metabolism, drugs and venous obstruction could have been detailed. 

Discussion

"Most candidates had all the information but had difficulty synthesising the information to write a 
cohesive answer", the examiners complained, implying that they were specifically looking for a particular structure or format which they did not mention in the question stem. This is a great example of an exam question designed to generate poor performance even in candidates who are armed with all the relevant knowledge. To prevent the occurrence of any further such comments from the examiners, a structures approach is taken here, focusing on arranging the information rather than the information itself.

• 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)
• Measurement of intracranial pressure
  • Fluid-filled transducer connected to extraventricular drain
  • Fiberoptic pressure transducer
  • Subdural bolt

Gomes, Joao A., and Anish Bhardwaj. "Normal intracranial pressure physiology." Cerebrospinal fluid in clinical practice (2008): 19-25.

Timofeev, Ivan. "The intracranial compartment and intracranial pressure." Essentials of Neuroanesthesia and Neurointensive Care. WB Saunders, 2008. 26-31.

Gergelé, Laurent, and Romain Manet. "Postural Regulation of Intracranial Pressure: A Critical Review of the Literature." Acta neurochirurgica. Supplement 131 (2021): 339-342.

Boulton, M., et al. "Raised intracranial pressure increases CSF drainage through arachnoid villi and extracranial lymphatics." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 275.3 (1998): R889-R896.

Mann, J. Douglas, et al. "Regulation of intracranial pressure in rat, dog, and man." Annals of Neurology: Official Journal of the American Neurological Association and the Child Neurology Society 3.2 (1978): 156-165.

Czosnyka, Marek, and John D. Pickard. "Monitoring and interpretation of intracranial pressure." Journal of Neurology, Neurosurgery & Psychiatry 75.6 (2004): 813-821.

Davson, H., G. Hollingsworth, and M. B. Segal. "The mechanism of drainage of the cerebrospinal fluid." Brain 93.4 (1970): 665-678.

Löfgren, Jan, Claes von Essen, and Nicolaus N. Zwetnow. "The pressure‐volume curve of the cerebrospinal fluid space in dogs." Acta Neurologica Scandinavica 49.4 (1973): 557-574.

Avezaat, C. J., J. H. Van Eijndhoven, and D. J. Wyper. "Cerebrospinal fluid pulse pressure and intracranial volume-pressure relationships." Journal of Neurology, Neurosurgery & Psychiatry 42.8 (1979): 687-700.

Langfitt, Thomas W., James D. Weinstein, and Neal F. Kassell. "Transmission of increased intracranial pressure: I. Within the craniospinal axis." Journal of neurosurgery 21.11 (1964): 989-997.

Langfitt, Thomas W., et al. "Transmission of increased intracranial pressure: II. Within the supratentorial space." Journal of neurosurgery 21.11 (1964): 998-1005.

Petersen, Lonnie Grove, et al. "Postural influence on intracranial and cerebral perfusion pressure in ambulatory neurosurgical patients." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 310.1 (2016): R100-R104.

Petersen, Lonnie Grove, et al. "Postural influence on intracranial and cerebral perfusion pressure in ambulatory neurosurgical patients." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 310.1 (2016): R100-R104.

Haughton, V., and K-A. Mardal. "Spinal fluid biomechanics and imaging: an update for neuroradiologists." American Journal of Neuroradiology 35.10 (2014): 1864-1869.