Headache Diagnostic Pathway

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Nurses Edition Commentary

Lisa Chavez, RN and Kathy Garvin, RN
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David W. -

I think there's a misleading statement in here at 10:06 minutes. The guest expert states "the possibility is approximately 1:1000 or less" regarding the risk of a SAH with a negative 6hr CT. I think he is conflating the sensitivity with the negative predictive value.

I think he is getting this 1:1000 number from the sensitivity of the CT, which he thinks is probably 99.9%, just shy of Perry's estimate of 100%. However, this means the chance of getting a negative CT in a patient whom we KNOW to have a SAH is 1:1000, which, of course, is not a helpful number. To explain to the patient, you really care about the negative predictive value. If we assume the 6hr CT has a 99.9% sensitivity, and, for arguments sake, 99.9% specificity (again, just shy of Perry's 100%) then the negative predictive value - which is the patient's chance of having a SAH with a negative CT head - is in fact about 1:125,000. The risk our expert is quoting to the patient is inflated by a factor of 100! (This estimate uses a prevalence of 7.7%).

David W. -

Correction: real risk is 1 in 12500. Which is lower than the risk quoted by the expert by a factor of 12.

Yulia S., Dr -

A question for Cam Berg - do you wait 12 hr to perform LP in your ADP?
What's your thoughts on spectrophotometry for exclusion of xantochromia?
Thank you.

Yulia

David H., M.D. (@BritFltDoc) -

Dr Berg,
Thank again for your segment. I think though the discussion about the next test, after a neg CT scan for acute thunderclap headache with an onset greater than 6 hours, needs to expand beyond that of SAH. While this is the subject of the ADP, for the true sudden in onset, maximal in onset, unusual headache, SAH is not the only DDx. When neck pain is added in to the symptom complex, arterial vessel dissection is also a worthy differential diagnosis. So I think the consideration of CT angiography is worthwhile, as you assess for both aneurysm and dissection with one test. As you mentioned, an LP is far from a perfect test, with problematic false positives, leading to the CTA anyway. And while serious complications are rare, post LP headache, requiring return ED visits and procedures are not uncommon. I think there are many factors to consider when deciding the next test after neg CT: is there another DDx you are considering, patient body habitus and likelihood of non traumatic tap, use of anti-platelet agents, and patient preference to an invasive procedure versus extra radiation and risk of incidental aneurysm. Thanks, David

Roee S. -

Question for Dr. Berg- I am a second year EM resident in Jerusalem, Israel, and am interested in implementing your ADPs as a teaching tool for medical students rotating in the ED and new interns. I was hoping that you could share the ADPs that you have implemented with me.
Thank you!

Rob O -

Hi Roee, I am sending you all of the ADPs right now. Enjoy!
-Rob O

Paul B. -

I am curious about your thoughts on the use of the Ottawa SAH rule and investigation rate. In the 2013 derivation paper by Perry et al. they show that implementation of the original rule (from the 2010 BMJ article) would have led to an investigation rate of 74%, however the modified rule (the one you are advocating using) had an investigation rate of 85%. The investigation rate in the study was 84% (without implementation of the rule). Additionally, in 2015 Bellolio et al. attempted to externally validate the modified rule via a retrospective chart review published in the American Journal of Emergency Medicine. I think there are a number of problems with the study, however, in the patients the rule was applied to it appeared to dramatically increase the rate of work up (and none were missed by standard work up).

Taken in aggregate it seems to me this rule does not decrease investigation rate from standard practice.
In the original 2010 BMJ derivation article the authors write "Such rules should be highly sensitive and reliable while also reducing the number of patients requiring investigation to rule out subarachnoid hemorrhage." It seems the rule has achieved the former, but failed at the latter. Given this, it seems to me that using this as an entry way to the diagnostic pathway will increase (or at least not decrease) CT utilization from standard care. It seems to me that in both the 2010 BMJ article and the 2013 JAMA article all patients with SAH received a CT scan with out using the rule, so I am uncertain the benefit of adding this over standard care.

I appreciate your thoughts!

Paul B. -

Additionally, you mention that after implementing this pathway you have reduced LP rates by 30%. I am curious what your before/after CT rate is this is most applicable to the Ottawa SAH rule.

Jeffrey F. -

Why not perform the CTA first for patients that present outside of 6 hours and then perform the lumbar puncture only after an aneurysm is found to get a better idea if the aneurysm is incidental or not? If maybe 15 % of LP's are going to be false positive, then these patients will get a CTA anyways and will be exposed to the harm of both LP and CTA. Unless the incidence of clinically insignificant aneurysms would result in us doing more LPS could CTA first be a reasonable approach for HA outside 6 hours?

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