Subarachnoid, MRI, and the Case of the Vanishing LP

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Nurses Edition Commentary

Lisa Chavez, RN and Kathy Garvin, RN

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LP for SAH has a 4.8% positive rate. I don't believe in stabbing 100 people to benefit 4.
At best, I would have radiology do a fluoroscopic guided LP.
Due to this low positive rate on an invasive procedure, I find an unguided ED LP to be harmful to the patient.

Anand S., M.D. -

I think we need to be careful with how we extrapolate information. If it was LP 100 to find 4 SAH (essentially LP 25 patients to find 1 positive) I think most Emergency providers would go for the LP. While time consuming and painful, LPs don't have a high rate (or even a moderate or low rate) of significant complications. SAH, on the other hand, is a life-threat. Best estimates for the yield of LP after a negative NCHCT w/in 6 hours of onset, though, are far lower, though. You probably need to LP many more patients in that group to find a positive.


The above article cites this article:
This article shows that the positive LPs, without aneurysms/vascular abnormalities to be low risk findings, with 0/181 (0%) patients rebleeding in 2-10 years.

It also states LP has a 9.9% "complication" rate of headache, neck, back, leg pains.

The first article also notes of the LPs performed, only 0.45% of patients had vascular abnormality SAH.
0.45% on an LP is quite a low rate to find a truly dangerous SAH.
That is 1 positive LP, in 222 LPs performed.

The podcast is trying to say SAH are dangerous. But if only vascular abnormality SAH is truly dangerous, I'm still being sold on CT/CTA/MRI.

What is the treatment for the not so dangerous, non-vascular abnormality SAHs? pain control, bp control, obs?


In the article you guys cited,
Perry, JJ et al. Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study. BMJ. 2015 Feb 18;350:h568.BMJ 2015;350:h568
Positive LP rate is 15/1739 for aneurysmal SAH. 1 positive LP in 116.
Is this invasive procedure worth it at 1 in 116-222?

Bad headache, admit, CTA, MRI?
You would not send a bad headache home anyways, would you? Press Ganey.
Plus, a differential of a bad headache is an ischemic stroke, which would use this CTA/MRI workup anyways.

Ben S. -

I agree with the challenges described for CT angio, but sometimes I'm also worried about cervical artery dissection. In those cases, it seems like you can kill two birds with one stone, and so I'll often order the angio all the way up through the brain. Any thoughts on this, from Evie or otherwise? Reasonable practice?

Anand S., M.D. -

From Dr. Marcolini:
You are absolutely correct in putting cervical artery dissection in your differential, and the CTA is the correct test to order when looking for it, but as for killing two birds, you may come up short. CTA sensitivity is not 100% for aneurysm, it can find aneurysms that didn't bleed, resulting in angst for the patient, or worse, complications from subsequent diagnostic procedures. Here is a new article by someone I consider the neuro expert in our field that gives a great summary on the differential diagnosis and best approach to assessment of the patient with a thunderclap headache. In addition to cervical artery dissection, other diagnoses, such as RCVS and venous sinus thrombosis should be considered, which all have diagnostic hallmarks, but to diagnose SAH after a negative CT, LP is still the standard. The bottom line is that whatever we use, we should engage the patient in shared decision making, and confirm as best we can that we understand the limitations of each study, and that the patient understands the risks/benefits of our diagnostic strategy.

Ann Emerg Med. 2017 Jun 7. pii: S0196-0644(17)30537-1. doi: 10.1016/j.annemergmed.2017.04.044. [Epub ahead of print]

Karine B. -

When I see a patient with a rapid-onset headache, but which remains quite severe or possibly worsening, but the CT noncontrast is negative at, for example 8 hrs, can this really be a sentinel bleed? I understand that the protocol is to LP to keep looking for SAH (and of course other dangerous diagnoses), but I've never understood how a tiny bleed that resolves could cause a headache that doesn't resolve. Is there any literature on this? Shouldn't these sentinel bleed patients have improving symptoms? Thanks for pondering with me.

Evie M., M.D. -

Karine, thanks for your question
The term sentinel bleed is really a retrospective term describing a bleed that happened typically within 1-3 weeks prior to the patient coming in with a headache and SAH. The patient has usually been treated for the pain of the headache and is sent home after the first episode. The ruptured aneurysm tamponades and stops bleeding (as they all do), but if/when it re-ruptures, the patient returns and is then diagnosed with SAH. It is only then that the 'sentinel bleed' is invoked, with the presumption that the headache that was treated a week prior was the 'sentinel bleed'. The significance of the sentinel bleed is that a re-ruptured aneurysm results in a big increase in mortality to 70% (from the 20% at the initial event), which is why we even think about the term.
As for the question of tiny bleeds causing unremitting headaches, it's challenging to correlate the amount of blood to the amount of pain. I have seen aneurysms <3mm create disastrous symptoms, and while it makes sense that more blood = more pain, I don't know of any data to support this. Yes, the pain of a sentinel bleed usually does resolve for the most part, with or without analgesia, but we can't rely on the lack of resolution to rule out SAH. Everyone's brain is exquisitely different with respect to sensitivity to blood and pain, which I imagine has something to do with inflammation (doesn't everything?).

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