Acute headache is a common emergency department presentation that accounts for 1-2% of all encounters (Goldstein 2006) The differential includes many life-threatening diagnoses, one of which is aneurysmal subarachnoid hemorrhage (SAH). With an estimated overall prevalence of 2% and a 1-month mortality of 40-45%, SAH is a high-risk disease that no emergency physician wants to miss (Linn 1996). 30% of those patients that survive will have severe disabilities (Hop 1997). Rebleeding following initial presentation is the largest cause of death. There is a 1.5% risk of rebleeding per day and a 15-20% risk in the first two weeks (Kassell 1983, Samandouras 2010). Early diagnosis and aneurysm repair using endovascular or surgical treatment before rebleeding occurs improves outcomes. However, about 5% of SAH are misdiagnosed on the first Emergency Department (ED) assessment because up to 50% of SAH patients present with no neurological deficits, providers fail to correctly interpret test results, or providers fail to appreciate the wide spectrum of symptoms that a SAH can present with (Vermeulen 2007, Weir 1994).

The current guidelines put forth by the American Heart / Stroke Association in 2012 state that the acute diagnostic workup for suspected SAH should include a noncontrast head CT (NCHCT), which, if nondiagnostic, should be followed by a lumbar puncture (LP). (Class I; Level B evidence) (Connolly 2012). The sensitivity of a NCHCT for SAH has been shown to be as high as 100% if performed within 6 hours of HA onset (Perry 2011), but falls to 82% after 12 hours (Sidman 1996), and is close to 50% by the end of the first week (Van der Wee 1995). MRI offers a higher sensitivity for late diagnosis, but the sensitivity is not 100%. An LP gives providers the ability to perform CSF analysis for red blood cells, xanthochromia by visual inspection, and bilirubin via spectrophotometry. Because the threshold is low for investigation of SAH due its high-risk nature, a high number of LPs are performed for each positive diagnosis.

Clinical Question

In ED patients presenting with acute headache, what is the rate of diagnosis of SAH by LP after a negative head CT?


Adult patients over 17 years old presenting to one of six urban EDs in the United Kingdom with an acute headache suspicious for SAH who have had a nondiagnostic NCHCT and a LP performed. Between 2006-2011.


LP after normal NCHCT


Primary: The rate of diagnosis of SAH by LP after negative NCHCT (defined by spectrophotometric detection of bilirubin, not by CSF RBC count or xanthochromia).


Multicenter, observational retrospective chart review


Patients who did not receive LP for CSF analysis and patients who had an LP as part of work-up for meningitis.

Primary Results

Primary Results:

  • 2,248 patients met inclusion criteria and received LPs:
    • 92 were positive (4.8%)
    • 1,507 were negative (75.8%)
    • 299 were inconclusive (15%)
    • 350 were uninterpretable (15.6%)
  • All 92 patients that had positive LPs underwent CTA or MRA. 9.3% of these had vascular abnormalities (eight aneurysms and one carotid cavernous fistula). This represents 0.45% of all patients that received LPs
  • Of the 299 inconclusive LPs, no vascular abnormalities were found
  • Of the 350 uninterpretable LPs, two aneurysms were found

Critical Findings:

  • 0.47% (9/2248) of all patients receiving LPs after negative head CTs were found to have vascular abnormalities on further imaging
  • This equates to performing 204 LPs in order to identify one patient who will benefit from neurosurgical intervention


  • This is the largest retrospective study to date that focuses on patients presenting to the ED with acute severe headache who receive LP following a negative CT imaging of the head
  • The clinical problem is well defined
  • The study population represents the appropriate target population, emergency department patients with concerning headaches


  • Spectrophotometry was used to evaluate CSF, which is typical practice in the United Kingdom but not in 99% of North American hospitals where typically CSF RBC counts and visualization of xanthochromia are used. This is important because classically spectrophotometry has been thought to have a higher sensitivity than visualization of xanthochromia for detection of bilirubin in but this has also come into question recently (Chu 2014).
  • There was no data collection on patients with uninterpretable or inconclusive LPs. Uninterpretable LPs were only evaluated at 2/6 sites. Only 5/28 (18%) and 17/56 (30%) had further imaging so unrecognized aneurysms may have occurred. This means the sensitivity reported in the study may be lower than calculated. A prospective study would have had the benefit of following up on these patients to ensure negative LP patients were not really false negatives.
  • There was very little data provided on specifics of chart review. It is unknown who performed the chart review. The authors reported there was no formal training of the data abstractors and that it was difficult to maintain standardization in data abstraction. The abstractors were not blinded to the study hypothesis and inter-rater reliability was not quantified.
  • There is no data provided on timing to CT or LP. This is relevant because CT becomes less sensitive over time for SAH and technically spectrophotometry should only be performed on CSF 12 hours after headache onset since it takes 12 hours for the formation of CSF bilirubin.
  • CT technology has evolved between 2006 – 2011 when the study was conducted and now. Therefore sensitivity of CT diagnosing SAH now in 2015 is higher than reported in this study and the number of LPs needed to diagnose one SAH is now even higher.

Author's Conclusions

“In patients presenting to the emergency department with acute severe headache, lumbar puncture to rule in or out SAH after negative head CT has a very low diagnostic yield due to low prevalence of the disease and uninterpretable or inconclusive samples. A clinical decision rule may improve diagnostic yield by selecting patients requiring further evaluation with a lumbar puncture following nondiagnostic or normal noncontrast CT brain imaging.”

Our Conclusions

In this study, in acute non-traumatic headache patients presenting to the ED suspicious of SAH, 204 LPs needed to be performed on head CT negative patients in order to identify one aneurysm. 31% of the LPs performed were uninterpretable or inconclusive. If we take into account the inherent risks of performing LPs, the number of false positives and subsequent testing this brings, healthcare cost and delays, the utility of performing LPs in the diagnosis of SAH in NCHCT negative patients is brought into question.
However, because no validated clinical decision rule currently exists to assist providers in deciding which head CT negative patients to LP, based on ASA/AHA guidelines, ED providers should still perform LPs on those patients that they think are at high-risk for SAH based on clinical gestalt if the patient’s initial NCHCT is negative.

Potential Impact To Current Practice

Potentially raises the threshold for deciding whether or not to perform an LP on a patient with a negative head CT that is being worked up for SAH.

Bottom Line

In this study, in acute non-traumatic headache patients presenting to the ED suspicious of SAH, 204 LPs needed to be performed on head CT negative patients in order to identify one aneurysm.

Read More

The SGEM #134: Listen, to what the British Doctors Say about LPs post CT for SAH


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