The Ottawa SAH rule has very specific inclusion and exclusion criteria that must be followed closely for appropriate application:
Why to Use
It is challenging to rule out SAH in patients who present with headache and no neurologic deficits. SAH is rare, accounting for approximately 1% of patients presenting to the ED with headache (Vermeulen 1990), but missed diagnoses ave potentially devastating results. A tool that reliably rules out SAH is useful to avoid unnecessary workups.
Lumbar puncture is often performed as the confirmatory test if a noncontrast head CT scan is negative but the clinical suspicion for SAH remains high. Lumbar puncture is painful and carries the risk of bleeding and of headache that may be worse than the original presenting headache.
When to Use
Use the Ottawa SAH rule in patients aged ≥ 15 years who present with headache and are neurologically intact.
In patients who have any positive criteria for the Ottawa SAH rule (ie, SAH cannot be ruled out), workup for SAH typically begins with a noncontrast head CT. Consider lumbar puncture and/or cerebral angiography if clinical suspicion remains. In their 2013 validation study, Perry et al provided insight into the appropriate workup for patients with possible SAH.
Neurology and neurosurgical consultation should be obtained for patients with suspected or confirmed SAH.
Abbreviations: CT, computed tomography; ED, emergency department; SAH, subarachnoid hemorrhage..
Consider workup for SAH in patients who have any positive criteria; however, given the low specificity of the rule, not every patient who fails the rule will require workup for SAH. In patients for whom all criteria are negative, consider avoiding further SAH-specific workup.
Patients in whom SAH has been ruled out may still have other causes of headache that require workup or intervention. The differential diagnosis should be broad.
The first iteration of what is now known as the Ottawa SAH rule was derived by Perry et al in 2010. The study prospectively enrolled 1999 patients with headache who were from 5 Canadian tertiary care centers; 130 of these patients had confirmed SAH. Sixteen variables were identified as predictive for SAH (13 on history and 3 on physical examination). Recursive partitioning was used to identify combinations of these variables and create the 3 separate decision rules with the highest sensitivity for SAH.
Perry et al (2013) prospectively validated these findings in a study of 2131 patients at 10 sites, using the following inclusion and exclusion criteria for enrollment:
The variables were again run through recursive partitioning and the final Ottawa SAH rule was found to be 100% sensitive for SAH (95% confidence interval [CI], 25.6%-29.5%). Specificity was 15.3% (95% CI, 13.8%-16.9%).
Not all patients in the validation study underwent a full workup with CT scan and lumbar puncture (80% had a CT scan and 45% had lumbar puncture). The patients who were discharged without undergo-ing a CT scan and lumbar puncture were assessed using a follow-up tool that included structured telephone interviews and medical records review.
The authors acknowledged that some patients with small nonaneurysmal SAH may have been missed.
Bellolio et al (2015) also externally validated the Ottawa SAH rule by retrospectively applying it to 454 patients who presented to the ED with headache. Sensitivity was 100% (95% CI, 62.9%-100%) but specificity was lower than in the validation by Perry et al (7.6%, 95% CI 5.4%-10.6%), so the authors concluded that the rule’s clinical use may be limited.
According to the hierarchy of evidence for clinical decision rules that was developed by McGinn et al (2000), the Ottawa SAH rule is a level 2 clinical decision rule, with established accuracy in at least 1 large prospective study, but no impact analysis completed as of yet.
Jeffrey J. Perry, MD, MSc
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David Zodda, MD, FACEP; Gabrielle Procopio, PharmD, BCPS; Amit Gupta, MD
Mert Erogul, MD; Steven A. Godwin, MD, FACEP
February 1, 2019
March 1, 2022