Question 10

College Answer

In the good answers to this question, and there were a number, the candidates included the volumes of the cranium and a correct description of the Monroe Kellie doctrine. A good answer should have included the compensations and consequences of increases in intra-cranial volumes; a discussion of all three components (brain tissue, blood, and CSF) and how they affect intracranial pressure; and then information on intra-ventricular and parenchymal devices in measuring ICP, briefly including their pros and cons. A common issue was writing quite a lot more than was needed on the relationship of cerebral blood flow to cerebral blood volume, and/or on the physiological consequences of raised ICP, which seemed to leave little time for discussion elsewhere. A few candidates did not provide any response for ICP measurement (worth 20% of the marks). Few candidates provided the intra-cranial elastance equation. A significant proportion of candidates missed out a part of the question, either the factors that affect CBV or ICP measurement.

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
  • Intracranial compliance becomes poor if even small increases in volume occur, as regulatory mechanisms are quickly overwhelmed (i.e. the intracranial compliance curve is hyperbolic)
• Factors which affect the ICP include:
  • Factors affecting the CSF volume
    • CSF efflux (eg. hydrocephalus, EVD, VP shunt)
    • CSF production (acetazolamide, diuretics, dehydration)  
  • Factors affecting the brain tissue
    • Age (decreased mass)
    • Space occupying lesions (eg. tumour, abscess)
    • Cerebral oedema 
  • Factors affecting the Monro-Kellie doctrine
    • Continuity of the cranial vault (i.e. decompressive craniectomy, open skull fracture)
  • Factors affecting the cerebral blood volume
    • Cerebral metabolic rate (eg. hyper or hypthermia, seizures, sedation, 
    • Cerebral arterial vasoactive agents (CO₂, hypoxia) 
    • Systemic increases in blood flow or blood pressure (eg. pain)
    • Venous outflow obstruction (eg. shivering, coughing, Valsalva, C-spine collar, jugular or sinus venous thrombosis, head-down position)
• Measurement of the ICP, for 20% of the total mark, would have to be in the form of literally a couple of sentences, and there would be little room for "briefly including their pros and cons", but let us try it anyway:
  • External ventricular drain: pressure is transmitted to a Wheatsone bridge transducer via fluid-filled non-compressible tubing; also allows therapeutic drainage and sampling, but is large and insertion can be traumatic
  • Fiberoptic pressure transducer: piesoelectric strain gauge pressure sensor connected to the monitor via fiberoptic cable. Less traumatic and easier to insert but does not allow drainage or sampling, and can drift from factory calibration over time

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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.

Khan, S. H., et al. "Comparison of percutaneous ventriculostomies and intraparenchymal monitor: a retrospective evaluation of 156 patients."Intracranial Pressure and Neuromonitoring in Brain Injury. Springer Vienna, 1998. 50-52.