Question 27

College answer

Aim: To explore the options available for neuromonitoring in TBI.
Key sources include: Paper 2010.1 Q8 compare and contrast EVD and fibre optic. 2009.2 Q25.1 - clinical assessment. CanMEDS Medical Expert.
Discussion: Parts a, b and c are repeat explorations of this topic. Many candidates did not gain marks by not attempting parts of the question. Marks were gained by answering questions discussing first principles. Whilst some techniques described in the question are not available in all centres, the principles underpinning their use are simple and well described in textbooks. Candidates are reminded that there is no  negative marking, and it is recommended to attempt all parts of the question. First principle examples would include details on availability, cost, invasiveness, reproducibility, type, and volume of information gathered and complications.

Discussion

The examiners referenced Question 8 from the first paper of 2010, which asked for a comparison between an EVD and a fiberoptic pressure transducer, and Question 25.1 from the second paper of 2009, which actually asked for the sort of clinical features that would suggest raised intracranial pressure and imply that a head CT would be needed prior to a lumbar puncture. The added twist this time was "role", which would have made some of the candidates think carefully and choose their words. For example, considering c) and d) are not well-accepted or widespread, one could legitimately argue that they have no well-established role. One way of addressing this SAQ could have been to list the possible methods of each assessment and then discuss their advantages and disadvantages (eg. comparing EVD and fibreoptic catheters) but this would have missed an opportunity to showcase some higher-order analysis. What follows is an attempt to answer this question in a way that takes advantage of the stem to discuss the advantages and disadvantages of the whole concept, rather than specific applications of it. 

a) Clinical assessment:

• Role: part of the ongoing tertiary survey of ICU patients with TBI 
• Advantages:
  • Gold standard of assessment
  • Easily repeatable
  • Cheap (i.e. minimal equipment cost)
  • Thorough (can identify subtle deficits, eg. cranial nerve palsies)
  • Reliable (especially when using validates scales, eg. the GCS)
• Disadvantages:
  • Interpreter-dependent
  • A full examination may take longer than other methods of assessment
  • Confounded by sedation
  • Often limited to the GCS, which may miss subtle changes

b) ICP monitoring:

• Role: titration of neuroprotective therapies to prevent secondary brain injury
• Advantages:
  • Well accepted preventative methods, eg. supported by BTF guidelines
  • An established range of therapies (eg. osmotherapy) are guided by ICP monitoring
  •  Well-practiced, and generally safe
• Disadvantages:
  • Invasive, particularly EVDs
  • Gradually less and less accurate (fibreoptic monitors)
  • May not allow recalibration (fibreoptic monitors)
  • Susceptible to infection (eg. ventriculitis)

c) Cerebral blood flow monitoring:

• Role: titration of neuroprotective therapies to prevent secondary brain injury
• Advantages:
  • Non-invasive (in the case of transcranial doppler) or minimally invasive (in case of jugular oximetry)
  • Supported by solid physiological principles, i.e plausible 
  • Specifically target a critically important endpoint (whereas ICP and CPP are surrogates for CBF, which is what we are really interested in)
  • Can be used to assess the safety of hyperventilation
  • Allows assessment of cerebral blood flow autoregulation
  • Is prognostically important (as an extreme example, the absence of CBF can identify brain death)
• Disadvantages:
  • Limited support from guidelines
  • Expensive to implement (requires specific devices or consumables)
  • Requires some training for use, and can be operator-dependent (transcranial Doppler), including being dependent on the availability of the operrator
  • Therapies designed to prevent secondary brain injury are mostly described in the setting of ICP monitoring; to substitute CBF monitoring would mean to extrapolate from ICP-based treatment recommendations 
  • CBF monitoring quantifies the flow, but does not assess whether this quantity is adequate

d) Cerebral function and metabolism:

• Role: assessment of the adequacy of cerebral blood flow and extent of cerebral dysfunction
• Advantages:
  • Can identify otherwise undetectable pathology (eg. EEG can detect non-convulsive status epilepticus)
  • Can be used to target neuroprotective therapies (eg. where EEG is monitored to confirm the depth of thiopentone coma)
  • Regional assessment of cerebral pathology becomes possible (eg. cerebral microdialysis can target specific at-risk areas)
  • Functional impairment can be more easily detected or prognosticated (eg. with somatosensory evoked potentials)
  • Not confounded by muscle relaxants
• Disadvantages:
  • Limited to centres with experience, and mostly used for research purposes
  • Expensive equipment
  • Requires expertise and availability of specialist services
  • Little support from guidelines and professional bodies

An extension of this SAQ into e) neuroimaging is also a plausible future avenue.

Kirkness, Catherine J. "Cerebral blood flow monitoring in clinical practice." AACN Advanced Critical Care 16.4 (2005): 476-487.

Casault, Colin, et al. "Multimodal brain monitoring following traumatic brain injury: A primer for intensive care practitioners." Journal of the Intensive Care Society 23.2 (2022): 191-202.

Feyen, B. F., et al. "Neuromonitoring in traumatic brain injury." Minerva anestesiologica 78.8 (2012): 949-958.

Lazaridis, Christos, and Brandon Foreman. "Management Strategies Based on Multi-Modality Neuromonitoring in Severe Traumatic Brain Injury." Neurotherapeutics (2023): 1-15.

Ropper, Alexander E., and John H. Chi. "Treatment of traumatic brain injury without direct intracranial pressure monitoring." Neurosurgery 72.4 (2013): N19-N20.