Question 14

Describe the factors that influence intracranial pressure.

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College Answer

A structure approached works well for “describe the factors …” questions. Better answers
provided a definition of ICP, explained the Monro-Kellie doctrine and then detailed the factors
which affect the volume of each of the components - cerebro spinal fluid (CSF), cerebral blood
flow and brain parenchyma. Some candidates focused only on factors which cause intracranial
hypertension and were thus unable to score full marks. Many candidates stated that CSF
production was ICP dependant which is incorrect.

Discussion

CSF production is CPP-dependent, people.

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
Factors that affect ICP include:
- Factors affecting the CSF volume
  - CSF efflux: ability to drain the CSF
    - Into the spinal canal (eg. hydrocephalus)
    - Out of the CNS (eg. EVD, VP shunt, inadvertent dural tear)
    - Into the venous circulation (arachnoid granulations, blocked by meningitis and subarachnoid haemorrhage)
  - CSF production
    - Acetazolamide (decreases CSF production by chroid plexus)
    - Volume status of the patient, eg. 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)
- Factors affecting the cerebral blood volume
  - Cerebral arterial blood volume
    - Cerebral metabolic rate
      - Drugs, eg. sedatives
      - Temperature (eg. hypothermia and hyperthermia)
      - Pathological increased activity, eg. seizures
    - Cerebral arterial vasoactive agents:
      - CO₂, hypoxia,
    - Systemic increases in blood flow or blood pressure
      - Noxious stimuli, eg. pain, anxiety
  - Cerebral venous blood volume
    - Venous outflow obstruction:
      - C-spine collar, jugular or sinus venous thrombosis
      - Raised intrathoracic pressure, eg. Valsalva, cough, childbirth
      - Poor RV compliance, eg. right heart failure
      - Muscle contraction, eg. shivering
    - Venous reflux
      - Head-down position
    - Venous efflux
      - Upright posture

References

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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.

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.

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

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