Subarachnoid hemorrhage is a classic emergency medicine diagnosis: rare, deadly, and easiest to miss in its early stages when there’s the greatest chance of intervention. A third of SAH die immediately, a third have longterm neurological consquences, and a third make a good recovery–and it’s this last group that has the highest miss rate and the greatest benefit of intervention. SAH are stratified by presenting mental status, which correlates with mortality, according to the Hunt and Hess score: grade 1(headache only) 70% survival; grade 2 (headache, neck stiffness) 60% survival; grade 3 (drowsy) 50% survival; grade 4 (stupour, hemiparesis) 20% survival; grade 5 (coma) 10% survival.
Here are a few papers from the literature on misdiagnosis, the increasing sensitivity of CT, and the potential of a new clinical decision rule.
Misdiagnosis of Symptomatic Cerebral Aneurysm: Prevalence and Correlation With Outcome at Four Institutions. Stroke 1996;27:1558-1563. This retrospective review of 217 patients shows the miss rate from 20 years ago: 25% misdiagnosed at initial evaluation, of which 85% were initially grade 1-2. Nearly half of those misdiagnosed deteriorated or rebled before definitive treatment. The misdiagnosis, in descending order of frequency: viral meningitis, migraine, headache NYD, cerebral infarction, sinus headache, hypertension headache, hypertensive ICH, psychogenic, cluster headache, trauma. Some SAH were missed because the CT was misread or was negative because of the time elapsed between symptom onset and patient presentation; but most SAH were missed because of failure to consider the diagnosis and order a CT. 95% of diagnosed SAH were done on the basis of CT and 4% on the basis of CTA after a negative CT (including 2% which had a negative LP).
Initial Misdiagnosis and Outcome After Subarachnoid Hemorrhage. JAMA 2004 Feb;291(7). In this retrospective study of 482 SAH from a decade ago, 12% were initially misdiagnosed, including 19% of those with normal mental status at first contact; 39% of misses suffered neurological complications. Again, migraine or tension headache was the most common misdiagnosis (36%), followed by viral, MSK pain, sinusitis, hypertension and meningitis. Again, failure to obtain a CT was the most common diagnostic error (73%), followed by misinterpretation of CT or LP (16%) and LP not performed after CT (7%).
Now with modern CT scans there’s increasing debate about the role of LP after a negative CT. At least for those who present soon after symptom onset, CT scans are nearing 100% sensitivity: Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis. Stroke 2016 Mar;47(3):750-5. This analysis of 5 articles including 8907 patients found a modern non-contrast CT scan performed within 6 hours of symptom onset to have a sensitivity of 0.987 for detection of SAH. The utility of the LP also has to be balanced against its side effects of post-LP headache and potential false positive results: An Observational Study of 2,248 Patients Presenting with Headache, Suggestive of Subarachnoid Hemorrhage, That Received a Lumbar Puncture Following a Normal Computed Tomography of the Head. Acad Emerg Med 2016: of the 2,248 patients, 92 had “positive LP” of which only 10% (0.04% of the total number) had a aneurysm–giving a number needed to LP of 250.
The greatest risk of missing SAH is not patients who have a CT but not an LP, it’s patients who don’t get a CT. So the main question is who to CT, and who can safely avoid CT: Clinical decision rules to rule out subarachnoid hemorrhage in acute headache. JAMA 2013 Sept 25;310(12):1248-55. “Among patients presenting to the emergency department with acute nontraumatic headache that reached maximal intensity within 1 hour and who had normal neurologic examination findings, the Ottawa SAH Rule was highly sensitive (100% sensitivity, 15% specific) for identifying subarachnoid hemorrhage.” This has since been externally validated with the same 100% sensitivity but a lower specificity of 7.6%: External validation of the Ottawa subarachnoid hemorrhage clinical decision rule in patients with acute headache. Am J Emerg Med 2015 Feb;33(2): 244-249. We could also think of it as a rule out criteria for a subset of patients with acute headache who can avoid CT: age 40, neck pain, still/limited flextion, loc, exertional, thunderclap. Like the PERC rule this identifies the young patients without red flags who can safely avoid radiation +/- LP, but doesn’t help for those 40 or older; it also doesn’t consider the minority of high risk patients for whom CT alone is unsufficient to rule out SAH.
Here’s my mnemonic for the OSAH rule: if any of these factors are present consider SAH and get a “CT HEAD”:
- Hurt neck
- Age 40
- Decrease flexion