Validation of the Ottawa Subarachnoid Hemorrhage Rule in Patients With Acute Headache

Background: The assessment and management of the acute onset headache from the perspective of the emergency department is a point of contention and discussion commonly faced by emergency providers.  The Ottawa Subarachnoid Hemorrhage Rule is a clinical decision making instrument that was created to help identify patients who need further workup beyond a basic history and physical exam. It does not define the extent of workup required, specifically whether or not a CT versus CT and LP are required to rule out a subarachnoid hemorrhage. In a 2010, Perry et al (1) published results from a prospective cohort study which attempted to formulate a collection of sensitive, high risk characteristics that could identify patients who require workup for subarachnoid hemorrhage. The three separate collections of high-risk features were all found to be highly sensitive (100% sensitivity with 95% CI) and so further investigation was found to be warranted. An additional prospective cohort by Perry et al (2) was designed to further assess the sensitivity, specificity, and overall applicability of these 3 decision making rules to identify patients who require subarachnoid hemorrhage workup. The initial results of this study showed one of the clinical decision making instruments to have a superior sensitivity of 98.5% (95% CI, 94.6%-99.6%). The rule was then redefined to include “thunderclap” headache and limited neck flexion on exam, and then reassessed utilizing a recursive partitioning analysis in order to obtain 100% sensitivity. The Perry et al 2017 (3) study was designed to validate the collection of high-risk characteristics this group has identified as warranting possible workup for subarachnoid hemorrhage.

Article: Perry et al. Validation of the Ottawa Subarachnoid Hemorrhage Rule in Patients With Acute Headache. CMAJ. 2017 Nov 13;189(45):E1379-E1385. doi: 10.1503/cmaj.170072. PMID: 29133539

The risk factors that were identified to make up the Ottawa Subarachnoid Hemorrhage Rule:

  • Age ≥ 40
  • Neck pain or stiffness
  • Witnessed loss of consciousness
  • Onset during exertion
  • “Thunderclap headache” (defined as instantly peaking pain)
  • Limited neck flexion on examination (defined as inability to touch chin to chest or raise head 3 cm off the bed if supine)

For whom should this rule be applied?

  • Patients who are ≥ 15 years of age
  • Severe, atraumatic, headache of peak intensity within 1 hour
  • No history of intracranial tumors
  • No history of chronic headaches (≥3 headache of similar character and intensity in prior 6 months)
  • No history of previous subarachnoid hemorrhage or aneurysm
  • No new neurologic deficits

These risk factors set the confines of a patient population who is not already at high risk for spontaneous subarachnoid hemorrhage, and who may benefit from a clinical decision instrument that sets them as so low risk for having a subarachnoid hemorrhage that no further workup is necessary.

Clinical Question: Is the sensitivity of the Ottawa subarachnoid hemorrhage clinical decision making instrument adequate for clinical application?


  • Prospective, multicenter cohort study
  • 6 tertiary Canadian EDs that were used in the 2010 and 2013 studies to validate the Ottawa Subarachnoid Hemorrhage Rule
  • 4 year span of data collection
  • Each hospital had previously participated in the derivation phase of this larger study
  • Patients 15 years of age or older with non-traumatic headache with maximal intensity within 1 hour were asked to enroll in the study
  • In addition, a chart review from this time frame was performed and patients meeting criteria were enrolled as a subset called “missed eligible”.

Exclusion Criteria:

  • GCS <15
  • Head trauma within 7 days
  • More than 14 days of headache
  • History of chronic headaches, defined as 3 or more headaches of same character and intensity over a 6 month period
  • History of recent workup for subarachnoid hemorrhage
  • Papilledema on fundoscopic exam
  • New neurologic deficits on exam
  • History of cerebral aneurysm, subarachnoid hemorrhage, hydrocephalus, or ventricular shunt
  • Referred from another facility with confirmed SAH
  • Returned for reassessment of same headache after already having had both CT and LP


  • Subarachnoid hemorrhage was defined as:
    • Subarachnoid blood visible on non-contrast CT scan of the head
    • Xanthochromia of the cerebrospinal fluid
    • The presence of > 1 × 106 erythrocytes/L in the 4th tube of cerebrospinal fluid collected during a lumbar puncture with aneurysm or arteriovenous malformation confirmed on cerebral angiography
  • Patients with a normal non contrast CT of the head and lumbar puncture were defined as ruled out for having a subarachnoid hemorrhage.

Clinical Bottom Line and Critical Results:

  • N = 1153 enrolled with acute headache
  • 67 patients (5.8% of the population) diagnosed with a subarachnoid hemorrhage
  • 590 missed eligible patients identified via medical record review, including 33 patients (5.6%) with subarachnoid hemorrhage.
  • Sensitivity of 100% (95% CI 94.6%-100%)
  • Specificity of 13.6% (95% CI 13.1%-15.8%)
  • (+) LR: 1.16
  • (-) LR: 0
  • When pooled with refined previous validation cohort
    • N = 3874
    • Sensitivity of 100% (95% CI 98.4%-100%)
    • Specificity of 15.9% (95% CI 14.8%-17.1%)
  • Comparison of potential effect of Ottawa SAH decision making instrument on neuroimaging:
    • Actual neuroimaging of the cohort was 88.0% with additional 1.0% undergoing LP without CT
    • Overall rate of investigation of 89%
    • Rate of investigation when Ottawa rules followed would have been 84.3%
    • Potentially saving 4.7% of the population an invasive workup

Interpretation of Results:

  • All cases of subarachnoid hemorrhage within the cohort were identified when the Ottawa SAH decision making instrument was applied
  • Validated tool that could help to standardize headache and SAH work ups
  • Assist in avoiding unnecessary CT and LP


  • Asks a clinically important question about a diagnosis that is difficult to make
  • Physicians scored the Ottawa Subarachnoid Decision making instrument prior to investigations were started
  • Large sample size
  • Patient population from multiple tertiary care centers
  • No upper limit of age for inclusion criteria


  • Each hospital had previously participated in the derivation phase of the study meaning they had comfort with application of the Ottawa SAH Decision Rule, which may not be the case at other centers.
  • 1 in 3 cases were missed during initial clinical assessment enrollment. In these cases, the presence or absence of elements of the rules had to be attained via chart review. However, these cases performed identically to the group that had the rule applied in the ED.
  • Lower limit of sensitivity within the confines of the utilized confidence interval is 94.6 which may still be too high a risk for some physicians to be willing to take when ruling out a SAH.


  • A high degree of sensitivity is required for the formulation of a clinical decision making instrument, especially one looking to limit the workups of such a clinically significant diagnosis. That being said, the specificity of this instrument is very low which implies that while no subarachnoid hemorrhages are being missed, many work ups are being performed on patients without this diagnosis in order to assure such a high sensitivity.
  • Are small, nonaneurysmal, subarachnoid hemorrhages being more commonly identified with this study even though no interventions were necessary? SAH requiring intervention may have been a more meaningful outcome to measure.
  • This clinical decision making instrument also does not address the extent of workup required to rule out a subarachnoid hemorrhage. Given the number of risk factors acknowledged during ED assessment, does this have any bearing on whether or not a CT and an LP must be performed to rule out SAH, even if the onset of symptoms is within a certain time frame? Might a CT head and less than two risk factors be enough to rule out a SAH, regardless of time frame of headache?

Author’s Conclusion:

We found that the Ottawa SAH Rule had excellent sensitivity for identifying subarachnoid hemorrhage in a new consecutive cohort of patients with acute headache. Patients who are neurologically intact with a new rapidly peaking headache and who lack each of the 6 elements of the rule do not need further investigation to rule out subarachnoid hemorrhage. Instead, other diagnoses should be considered and managed accordingly in these patients.

Our Conclusion:

The validation study of the Ottawa Subarachnoid Rule, in conjunction with the previously mentioned studies that lead to the formation of this rule, provides substantial, promising evidence towards the application of this screening tool in daily practice of emergency medicine. However, the evidence thus far is based all in one country, at institutions where the rule was first designed, and in similar clinical settings. This should all be taken into careful consideration when utilizing it to clinical practice and implies that while the evidence is promising, it still may not be ready for primetime application. This rule, when applied in conjunction with a history and clinical examination, can be utilized to support the decision of a provider to either work up or clinically rule out a subarachnoid hemorrhage. The sensitivity provided by this validation study is both impressive and comforting to emergency physicians who choose to discharge patients who fall within the confines of rule and exclusion criteria.  However, this is a one sided rule, implying that while a negative rule may reasonably exclude an SAH, a positive rule does not necessarily mandate further workup. It should also be noted that in order to attain such sensitivity, the rule provided is such that the specificity it remarkably low which may lead to an increased number of invasive workups. Thus, if a patient is unable to be ruled out with this clinical decision making instrument, clinical assessment and shared decision making with the patient should be utilized to assess further need for workup.

Practical Application:

  • This is a highly sensitive screening tool for the assessment of patients in the Emergency Department with acute onset headaches.

Potential to Impact Current Practice:

  • The high sensitivity of this clinical decision instrument comes at the price of a relatively low specificity. This will naturally lead to more negative workups but at the benefit of catching nearly all subarachnoid hemorrhages within the confines of the patient population and exclusion criteria previously discussed.

Clinical Bottom Line:

The Ottawa Subarachnoid Hemorrhage Rule is a high sensitivity screening tool that may be applied to the aforementioned patient population, in conjunction with a complete history and physical exam, in order to clinically rule out a subarachnoid hemorrhage. However as a one sided rule, with poor specificity, if a patient is not ruled out, clinical decision making and shared decision making must be used to determine which patients would need further workup.

Guest Post By:
Anthony DeVivo, DO
Emergency Medicine Chief Resident
Mount Sinai St. Luke’s – West.
Twitter: @anthony_devivo


  1. Perry, JJ et al. High Risk Clinical Characteristics for Subarachnoid Haemorrhage in Patients with Acute Headache: Prospective Cohort Study. BMJ 2010. PMID: 21030443
  2. Perry, JJ et al. Clinical Decision Rules to Rule Out Subarachnoid Hemorrhage for Acute Headache. JAMA 2013. PMID: 24065011.
  3. Perry JJ et al. Validation of the Ottawa Subarachnoid Hemorrhage Rule in Patients with Acute Headache. CMAJ 2017. PMID: 29133539.

Post Peer Reviewed By: Jenny Beck-Esmay, MD (Twitter: @jbeckesmay ) and Salim R. Rezaie, MD (Twitter: @srrezaie)

Cite this article as: Jenny Beck-Esmay, MD, "Validation of the Ottawa Subarachnoid Hemorrhage Rule in Patients With Acute Headache", REBEL EM blog, September 24, 2018. Available at:

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