Calc Function

    • Calcs that help predict probability of a diseaseDiagnosis
    • Subcategory of 'Diagnosis' designed to be very sensitiveRule Out
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    Ottawa Subarachnoid Hemorrhage (SAH) Rule for Headache Evaluation

    Rules out SAH in patients with headache.


    This rule has very specific inclusion and exclusion criteria that must be followed closely for appropriate application:

    Only apply in: Alert patients ≥15 years old, new severe atraumatic headache, maximum intensity within 1 hour.

    Cannot be used in: Patients with new neurologic deficits, prior aneurysms, prior SAH or known brain tumors; also cannot be used in patients with recurrent headaches (≥3 headache episodes within 6 months).

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    • Consider SAH workup in patients with ANY positive criteria, but as with other rule-out decision aids, just because a patient fails the rule does not require that all patients are then evaluated for SAH, given its very low specificity.
    • May consider avoiding further SAH-specific workup in patients with all negative criteria.


    • In patients with any positive criteria by the Ottawa SAH Rule (i.e., cannot rule out SAH), workup for SAH typically begins with includes noncontrast CT head. Consider lumbar puncture (LP) and/or cerebral angiography if clinical suspicion remains.
    • By completing this study, the authors were also able to provide insight into the appropriate workup for patients with possible SAH. They recommend:
      • Non-contrast CT scan within 6 hours of headache onset is sufficient to rule out SAH in most patients
      • If a patient is deemed to be particularly high-risk, LP should be performed
        • If there is no visual xanthochromia and tube 4 of the LP has <2000 x 106/L, SAH is ruled out unless 'ultra high risk'
        • If the patient is 'ultra high risk,' CT angiography (CTA) can be performed to evaluate for cerebral aneurysm. Neurosurgical consultation may be particularly helpful in these 'ultra high risk' patients.
      • CT angiography can be helpful in significant time delay between presentation and initial headache (ex: headache last week).
    • Neurology and neurosurgical consultation should be obtained in patients with suspected or confirmed SAH.

    Critical Actions

    Patients who are ruled out for SAH may still have other causes for headache that require workup or intervention, and differential diagnosis should be broad.

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