Is the 6-Hour CT for SAH Debate Over?

There has been a fair bit of back and forth about the validity of early CT in the setting of “thunderclap headache” to obviate a lumbar puncture in the search for aneurysmal subarachnoid hemorrhage.  David Newman and Kevin Klauer debated this subject a few years ago – and that was in all-comers, not simply those in the few hours following onset.

This most recent meta-analysis and systematic review gathers together all the published literature regarding early CT and the incidence of SAH on follow-up.  Including 8,907 patients from five publications, based on a few assumptions from retrospective studies, there were up to 13 missed cases of aneurysmal SAH occurring despite a negative CT within 6 hours of onset.  Worst-case sensitivity based on these data, then, was 0.987 (95% CI 0.971-0.994).

The prevalence of SAH in patients presenting with true thunderclap headache is estimated at ~10%.  The post-test odds, then, after a negative CT, are on the order of 0.1% – in line with David Newman’s posit of requiring 1000 LPs to catch one missed SAH.  The problem, then, lies in taking the next step in Bayesian reasoning – how likely is the positive LP to be true SAH?  If prevalence has dropped to 1 in 1000 after a negative CT, and the specificity of LP for SAH is only 65%, even a positive result barely budges the likelihood of disease.

How do you consent a patient for an invasive procedure in a setting in which a positive result has only the tiniest fraction of a chance of being real – and the treatments based on findings of follow-up examinations may be more likely to harm the patient than the magnitude of benefit associated with detection of a true positive?

“Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis”

13 thoughts on “Is the 6-Hour CT for SAH Debate Over?”

  1. Great post and I agree wih you Ryan. The other flip side of the question is what do you do with those >6 hours out? Tap them all? With super long wait times in the winter. Scans are often >6 hours after onset

  2. I think at that point it becomes a conversation with the patient. Do they want to undergo an invasive procedure with the possibility that it doesn't end up helping us or would they rather take the 0.1% risk.

  3. While it's a bit taboo, I think we should consider extending the 6 hour window as the "golden hours" for which a CT is performed. In the Sayer study from Academic Emergency Medicine from Oct 2015, out of 1898 patients, 9 had a +LP and an aneurysm. 6 of them had a headache for a week or longer. One had a previously coiled embolism, and only one of the 9 had a negative CT performed within 3hrs of onset.

    So there was a whole one patient (of 1898 patients) with symptoms under a week that was CT negative,LP+, confirmed on MRI. Yeesh.

  4. I think it depends on your suspicion as well. One of my partners did an LP within a couple of hours of headache onset, and a negative CT. CSF was obviously positive. CTA showed large aneurysm. The story was classic though.

  5. It's almost always possible to come up with an anecdote of a real patient that was helped by swimming against the tide … but harder to make an accounting of all the other equivocal patients, false-positive taps, and incidental aneurysms picked up.

    No prudent strategy will ever be zero-miss – the goal is high-value care with minimal population harm.

  6. I consider the initial non-contrast CT-brain as the "D-dimer" of SAH investigation. It has an excellent, but probably not-quite-100% NPV. If my pre-test probability is low, and CT at < 6 hours is normal, I think it's reasonable to stop there. If pre-test probability / gestalt clinical suspicion is high, they should have a second-line investigation. That can either be an LP (_if_ the local lab does quantitative spectrophotometry for xanthochromia, but _not_ if all they do is visual inspection) or an immediate CTA.

  7. A one-way sensitivity analysis was performed by varying the cost of LP assuming a cost of $498.02 for CTA, as it might differ from institution to institution. The result showed that LP remains as the more cost-effective strategy until its cost exceeds $364.45

  8. One thing that I struggle with is the definition of the thunderclap headache; my working definition is a headache that instantly reached peak intensity (patients have described this like an "explosion" or like being struck in the head) versus more liberal definitions (eg peaking < 1h). Are patients with slow to peak headaches buffing up these values.

    An advantage of CTA is that it can provide definitive diagnosis for other causes of thunderclap headache (eg reversible vasospasm), and a disadvantage that it may disclose an asymptomatic aneurysm.

    I think that the avoidance of LP makes sense in most circumstances, but I do think that with an increasingly certain thunderclap headache, consideration could be given to further non invasive testing or LP.

  9. Described in: Perry JJ, Spacek A, Forbes M, et al. Is the combination of negative computed tomography result and negative lumbar puncture result sufficient to rule out subarachnoid hemorrhage? Ann Emerg Med. 2008; 51:707–13.

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