Ottawa subarachnoid haemorrhage rule
Ruling out subarachnoid haemorrhage in alert adults with acute non-traumatic headache
Sources last checked: 2026-11-03
Ottawa subarachnoid haemorrhage rule
Ruling out subarachnoid haemorrhage in alert adults with acute non-traumatic headache
Only for alert (GCS 15), neurologically intact adults with a new, severe, non-traumatic headache that reached maximum intensity within 1 hour. Not for patients with new neurological deficits, papilloedema, previous aneurysm, SAH, brain tumour, ventricular shunt or hydrocephalus, or an established pattern of ≥ 3 similar headaches over ≥ 6 months.
Interpretation
| Rule not applicable | The patient is outside the population the rule was derived and validated in, so the rule makes no statement for this patient. |
|---|---|
| Rule negative — SAH ruled out by the rule | None of the six criteria present. Sensitivity was 100 % in the 2013 JAMA cohort (95 % CI 97.2–100 %) and in the 2017 CMAJ validation (95 % CI 94.6–100 %). |
| Rule positive — SAH cannot be ruled out by the rule | At least one criterion present. The rule is a rule-out tool with low specificity (15.3 % in 2013, 13.6 % in 2017); a positive rule means further investigation for SAH, not that SAH is likely. |
What is the Ottawa subarachnoid haemorrhage rule?
The Ottawa SAH rule helps rule out subarachnoid haemorrhage in alert adults whose new, severe, non-traumatic headache peaked within one hour. Perry and colleagues refined it from earlier candidate rules in 2,131 patients at 10 Canadian emergency departments (JAMA 2013) and validated it in 1,153 further patients (CMAJ 2017). Sensitivity was 100 % in both cohorts, at the cost of low specificity.
When to use it
- Alert, neurologically intact adults with acute non-traumatic headache that reached maximum intensity within one hour.
- Deciding whether a patient needs any investigation for SAH at all.
- Documenting a structured assessment of 'worst headache' presentations.
When not to use it
- New neurological deficits, papilloedema or reduced consciousness — these patients need investigation regardless.
- Known aneurysm, previous SAH, brain tumour, ventricular shunt or hydrocephalus.
- Recurrent headaches of the same character (≥ 3 episodes over ≥ 6 months) or recent head trauma.
How it is calculated
Investigate for SAH if ANY of: age ≥ 40 years; neck pain or stiffness; witnessed loss of consciousness; onset during exertion; thunderclap headache (instantly peaking); limited neck flexion on examination. Only for alert patients with a new severe non-traumatic headache that peaked within 1 hour of onset and no new neurological deficit.
Clinical pearls and limitations
- Only for alert (GCS 15), neurologically intact adults with a new, severe, non-traumatic headache that reached maximum intensity within 1 hour. Not for patients with new neurological deficits, papilloedema, previous aneurysm, SAH, brain tumour, ventricular shunt or hydrocephalus, or an established pattern of ≥ 3 similar headaches over ≥ 6 months.
- The rule is designed to rule out, not to rule in: specificity was only 15.3 % (2013) and 13.6 % (2017).
- Age ≥ 40 alone makes the rule positive, so most older headache patients will still need investigation.
- 'Thunderclap' here means pain that peaked instantly, as opposed to over minutes.
- Neck pain or stiffness is a symptom; limited neck flexion is an examination finding — score both separately.
- In the 2017 validation neuroimaging rates stayed about 87 %, so the rule mainly protects against missed SAH rather than reducing scans.
Frequently asked questions
What are the six criteria of the Ottawa SAH rule?
Age ≥ 40 years, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap (instantly peaking) headache, and limited neck flexion on examination.
Is the Ottawa SAH rule validated?
Yes. After the 2013 JAMA cohort, a prospective validation in six Canadian emergency departments (CMAJ 2017) again found 100 % sensitivity (95 % CI 94.6–100 %).
Does a positive Ottawa SAH rule mean the patient has SAH?
No. Most rule-positive patients do not have SAH; a positive rule only means that SAH cannot be excluded without further investigation.
Who should the Ottawa SAH rule not be used for?
Patients with new neurological deficits, prior aneurysm, SAH or brain tumour, a shunt or hydrocephalus, papilloedema, GCS < 15, head trauma, or a long-standing pattern of similar headaches.
Evidence and guidelines
- Primary source: Perry JJ, Stiell IG, Sivilotti ML et al. Clinical decision rules to rule out subarachnoid hemorrhage for acute headache. JAMA 2013;310(12):1248-1255.
- Perry JJ, Sivilotti MLA, Sutherland J et al. Validation of the Ottawa Subarachnoid Hemorrhage Rule in patients with acute headache. CMAJ 2017;189(45):E1379-E1385.
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