In this observational series: 204.
This study describes the findings of 2,248 patients with “headache, suggestive of subarachnoid hemorrhage” who underwent computed tomography, followed by lumbar puncture. CT is quite sensitive for aneurysmal hemorrhage in the few hours following onset, but fades with time. Spectrophotometric analysis of CSF is the gold standard for diagnosis in the United Kingdom, and was used by these facilities to describe positive, negative, and indeterminate results on LP.
Of these 2,248 patients, LP were negative in 1,507, uninterpretable in 350, indeterminate in 299, and positive in 92. All patients with positive LP had follow-up CTA or MRA, and there were 9 vascular abnormalities. Then, a subset of the uninterpretable and indeterminate patients underwent CTA or MRA, as well, resulting in an additional two vascular abnormalities.
Thus, 204.
There are a few surprising bits regarding these data. The ~4% rate of “SAH” detected in this study is lower than most other observational series. Some of these, however, were neurosurgical referral centers, with much higher rates. Then, of course, the incidence will further depend on various definitions of positivity and the rate at which patients with lower pre-test likelihood are evaluated for SAH. Additionally, the rate of vascular abnormality in those with “SAH” was also quite low. In such a small sample, the confidence intervals are quite wide, so it’s hard to estimate the generalizability of these findings.
Regardless of the precise numbers, their overall conclusion is reasonable: CT + LP is extremely low yield for true pathology. There is a definite need for decision instruments with improved specificity to prevent such extensive resource utilization.
“An Observational Study of 2,248 Patients Presenting With Headache, Suggestive of Subarachnoid Hemorrhage, Who Received Lumbar Punctures Following Normal Computed Tomography of the Head”
http://www.ncbi.nlm.nih.gov/pubmed/26480290
Thanks for the excellent summary. I had a few concerns about the article that made me a bit uneasy.
1) 92 true positive taps in CT negative patients, about 1 in 25. I know the literature suggests that non aneurysmal bleeding has better prognosis, however, we dont know what the other 80+ patients actually were diagnosed with. A few of the differential diagnoses for non aneurysm bleeds they list, dural sinus thrombosis, AV malformation, cerebral dissection I imagine are still worthy of diagnosis (all presumably by CT-A after the positive LP)
2) 350 "uninterpretable samples" – my read of the paper, of the 350, they talk about 84 at 2 centers, in which 22 were actually investigated (5 at one site, no aneurysm, and 17 at the other with 2 – of the total of 11) were found. Oh, and 1 person walked out with meningitis. Obviously the question is regarding those 327 patients who werent investigated. They say in the discussion they would have expected 17 "incidental" aneurysms from that group – depending on how many missed aneurysms in this group, the numbers start to look less appealing, from 1 in 200 to (adding 17) 1 in 80.
Would be interested in your thoughts. Agree that with 6 hour CT, LP likely headed out, however, am worried for technology creep (lets just CT all headaches before 6 hours), and also forgetting all the other diagnostic benefits of LP.
Thanks
Bill