Under which circumstances must one be so interested in intracranial pressure, so as to introduce things into the patient's skull? This question, in a variety of permutations, is a College favourite. For instance, it has recently appeared in Question 27 of the first paper of 2014, less recently in Question 16 of the first paper of 2009, and Question 27.2 from the first paper of 2008. The advantages and disadvantages of various ICP monitoring techniques are discussed elsewhere; this is the chapter which debates the very need for something like this.
In brief summary:
(Recommendations of The Brain Trauma Foundation)
The Brain Trauma Foundation recommends (without any Level 1 evidence) that any severe brain injury patient (i.e. GCS 3-8) should have an ICP monitor of some sort. That's pretty obvious. ICP monitoring both guides treatment, reveals evolving pathology and predicts outcome.
The more salient question is, who requires ICP monitoring among those patients who have a normal CT, but still have a reduced level of consciousness?
The BTF gives us Level III guidelines for this. This is not a very strong set of recommendations. They are based on a retrospective observational study by Narayan et al. The overall risk of intracranial hypertension was only about 13% among patients with normal CT scans, but the authors found that the patients who met two of the abovementioned criteria (old age, posturing, hypotension) this risk increased to 60%.
I should think that any sort of abnormal posturing (or any focal neurological deficit) combined with a decreased level of consciousness following a traumatic brain injury should be a concern. The normal CT, in these circumstances, may reflect the deficts of CT in visualising posterior fosssa damage, like some sort of hideous brainsteam contusion, vertebral artery dissection, and what have you. Or perhaps the patient is not waking up because of a diffuse axonal injury. None of these pathologies benefit from ICP monitoring because raised ICP is not usually a feature.
Non-traumatic indications for intracranial pressure monitoring
Cochrane reviewers still have not found any studies of this topic worth combining into a meta-analysis.
However, a slightly dated opinion piece on this topic makes a broad suggestion:
"...we conclude that all patients with non-traumatic cerebral insult complicated by raised intracranial pressure or at risk of developing it, should be considered for measurement of intracranial pressure"