SGEM - October 27th, 2015
Background: Headaches represent around 2% of all emergency department visits. Of these presentations 1-3% turn out to be a subarachnoid hemorrhage (SAH) (Edlow, Vermeulen, Perry, Morgenstern).
About 5% of SAH are misdiagnosed on the first emergency department assessment (Vermeulen). This is partly because 50% of SAH present with no neurologic deficit (Weir).
Dr. Jeff Perry and his team have tried to create a clinical decision tool to rule out SAH for acute headaches (SGEM#48). The Ottawa SAH Tool contains six variables to decide if a CT scan is necessary.
Applying the tool could decrease the miss rate of SAH from about five percent down to almost zero with a slight increase in utilization. However, the tool needs further evaluation in implementation studies before it is ready for “prime time”.
Traditional methods of working up a SAH has been non-contrast CT followed by a lumbar puncture (LP). Dr. David Newman has questioned this dogma on his SMART-EM podcast. He suggested LPs are not always needed after a negative CT scan.
Dr. Newman calculated the number needed to LP to identify one SAH for which an intervention was indicated to be 700, prompting the question “are you part of the ‘700 Club’?” Should any of us be part of the 700 Club?
Clinical Question: In emergency department acute headache patients, how frequently does LP diagnose SAH after unremarkable CT scan?
Author’s Conclusions: “In patients presenting to the emergency department with acute, non-traumatic severe headache, LP to diagnose or exclude 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 LP following non-diagnostic or normal non-contrast CT brain imaging.”
Number Needed to Tap (NNTap) of 250 to diagnose one aneurysm not picked up on CT scan.
SGEM Bottom Line: In this study, one patient would be diagnosed with SAH out of every 250 patients receiving a LP who presented to the emergency department with a headache that did not have their bleed identified on CT scan.
Comment on author’s conclusion compared to SGEM Conclusion:
- In the setting of acute, non-traumatic headache presenting to the ED, a multi-slice CT (16-64 slice) that does not demonstrate radiographic evidence of SAH is likely sufficient to rule-out a SAH in most patients. In fact, the number needed to tap (NNTap) to detect one aneurysm in this CT-negative population is 250 using a spectrophotometric definition of abnormal LP, which constitutes a significant amount of patient risk and discomfort, expense to healthcare payers, and delays in care for others awaiting treatment with no benefit for 249/250 patients.
- Also, an additional 33% of CT-negative/LP equivocal patients with positive, inconclusive, or un-interpretable CSF results would still require additional imaging (CTA or MRA) to truly rule out SAH. Although an accurate, reliable, well-validated clinical decision instrument with a convincing impact analysis would be a useful adjunct to clinical gestalt, such an instrument does not currently exist and was not tested in this study.
- Future studies that evaluate the role of LP in CT-negative headache patients with suspected SAH should prospectively assess outcomes similarly in all patients to avoid differential verification bias, while reporting on both visual/spectrophotometric CSF bilirubin and overall CSF RBC diagnostic accuracy (including likelihood ratios and interval likelihood ratios).
http://thesgem.com/2015/11/sgem134-listen-to-what-the-british-doctors-say-about-lps-post-ct-for-sah/