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    Ottawa Subarachnoid Hemorrhage (SAH) Rule for Headache Evaluation

    Rules out SAH in patients with headache.
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    INSTRUCTIONS

    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.

    Do not use in: Patients with new neurologic deficits, prior aneurysm, prior SAH, known brain tumors, or chronic recurrent headaches (≥3 headaches of the same character and intensity for >6 months).

    When to Use
    Pearls/Pitfalls
    Why Use

    Patients with headache who are neurologically intact.

    • The Ottawa SAH Rule was developed for use in an emergency department setting in patients presenting with acute headache.
    • The rule is 100% sensitive for SAH (i.e., a rule-out tool).
    • Should only be used in patients who are alert and oriented, presenting within 14 days of headache, without history of head trauma or fall in the past 7 days.
    • It can NOT be used in patients with new neurologic deficits, previous history of headache syndrome, or intracranial lesions (see Evidence for full exclusion criteria).
    • Specificity is low (15%), and so it should not be used to diagnose SAH, even in patients in whom all criteria are positive. 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.
    • Ruling out SAH is challenging in patients presenting with headache but no neurologic deficits.
    • SAH is rare and accounts for only ~1% of ED patients presenting with headache (Vermeulen 1990), and missed diagnoses have potentially devastating results.
    • Conversely, a tool that reliably rules out SAH is useful to avoid unnecessary workup (more tests do not always clarify the diagnosis).
    • Lumbar puncture (LP) is often performed as the confirmatory test if noncontrast CT is negative but clinical suspicion still high. LP is painful and carries risk of bleeding and headache worse than the original presenting headache.
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    Result:

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    Next Steps
    Evidence
    Creator Insights

    Advice

    • 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.

    Management

    • 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 <2,000 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 (e.g. 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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    Dr. Jeffrey J. Perry
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    Dr. Ian Stiell
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