Modern evaluation for aneurysmal subarachnoid hermorrhage, with some debate, may include definitive non-contrast CT performed within six hours of symptom onset. The traditional evaluation, and still recommended beyond six hours, involves a lumbar puncture, looking for red blood cells or xanthrochromia.
This latest tale of woe from Jeff Perry’s SAH data details the pragmatic effectiveness of the traditional pathway, focusing on the primary confounder: traumatic taps. They report on 1,739 patients undergoing lumbar puncture as part of this evaluation, and, unfortunately, the numbers are grim: 641 (36.8%) samples were abnormal in the final tube of CSF collected. However, it isn’t so bad – 476 of those had fewer than 100 RBCs x 10^6/L, with many having only a handful of cells. But, still, that leaves 165 patients with fairly substantial numbers of RBCs in their CSF.
Because, all told, only 15 received a final diagnosis of aneurysmal SAH.
Why is this so grim? Because 419 of these 626 patients with RBCs on their LP subsequently were subjected to angiography – with 404 of them negative.
And xanthrochromia? Some predictive value – 7 of 15 patients diagnosed with SAH displayed xanthrochromia, but, obviously there were 8 patients with SAH who did not, along with 16 instances of xanthrochromia in patients without SAH.
The final gist of the paper is to generate a 100% sensitive cut-off to exclude SAH – for which the authors choose 2000 x 10^6 and absent xanthrochromia. This results in a specificity of 91.2% and a positive LR or 11.4. This is a pretty good positive LR, but, unfortunately, given such a vanishingly rare disease, the PPV was only 21.4% in their cohort.
However, one major flaw in this study is it doesn’t usefully describe the population of true interest to Emergency Physicians – the test characteristics of those with a negative CT and a positive LP. There were 77 patients who did not undergo CT prior to LP, but, more importantly, 10 of the patients included in this cohort had visible SAH on CT recognized by the staff radiologist, but not the Emergency Physician. Therefore, if you practice in a setting without neuroradiology coverage, this is generalizable. Otherwise, we can exclude those 10 cases and boggle at the massive resource utilization in terms of LPs and angiography in order to pick up just 5 cases of occult aneurysmal SAH.
In patient-oriented terms – based on these data – the risk of SAH after a negative CT performed greater than 6 hours after onset is about 1 in 330. Using their cut-off of 2000 x10^6, the chance of a true positive LP is about 1 in 12. A vast improvement, to be sure, but probably still not a pathway very many patients are going to choose when presented with these odds.
“Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study”
http://www.ncbi.nlm.nih.gov/pubmed/25694274 (free fulltext)
Would welcome your thoughts on this article – Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study. BMJ 2011; 343. Perry JJ, et al.
many thanks, Chris
Given the poor prognosis of aneurysmal SAH in case of a subsequent larger bleed, I think I would still want that LP if the CT is negative (or read as negative).
I appreciate the data from the Perry 6-hour CT article. Most of the controversy with that article relates to the follow-up and "gold standard" for diagnosis, which is pragmatic survival-based, not LP + angiography based. It is certainly possible there were SAH missed in that study in the cohort that did not undergo LP. As long as they survived to the end of follow-up without complications, they would be, essentially, false negatives.
The trick is, as neurosciemed notes, is balancing the benefits and harms of imperfect tests in a very serious disease process with an extraordinarily low incidence. We see a little bit of this with aortic dissection – rare, with atypical presentations, but obvious morbidity/mortality. There is a wide range of practice variation regarding CT imaging for dissection, again, balancing the costs and resource utilization against the rarity of the disease.
With some debate, I think it's reasonable to adopt the 6-hour CT approach. It's a little hard to describe the utility of a diagnostic test when the pretest likelihood of disease is low, considering, again, the wide practice variation in terms of which headaches are evaluated for SAH. Beyond 6 hours, as above, it gets fuzzier. The key takeaway of studies like this is they don't "tell doctors what to do", but rather give us information to share with patients in order for them to help us decide which risks to assume – the risk of missed SAH, or the risks/costs of false-positive LP leading to detection of an asymptomatic aneurysm and endovascular coiling. Hard to claim there is a single right answer, for sure.