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The Ottawa Subarachnoid Hemorrhage (SAH) Rule is a clinical decision tool developed to help emergency physicians determine which patients presenting with acute headache require further investigation to rule out subarachnoid hemorrhage. Developed by Dr. Jeffrey Perry and colleagues in 2013 through a prospective study of 2,131 neurologically intact patients with acute non-traumatic headache, this rule aims to identify all cases of SAH while reducing unnecessary CT scans and lumbar punctures.
SAH is a potentially devastating condition, most commonly caused by ruptured intracranial aneurysm, with a mortality rate of approximately 40% and significant morbidity among survivors. The classic presentation is a sudden, severe 'worst headache of my life,' but SAH can present with varying headache severity. Missing the diagnosis is one of the most common causes of malpractice litigation in emergency medicine, making sensitive screening tools essential.
The Ottawa SAH Rule evaluates six clinical criteria: age 40 or older, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache (instant peak intensity), and limited neck flexion on examination. If any of these six criteria are present, further investigation (CT head followed by lumbar puncture if CT is negative, or CTA) is recommended. In the original derivation study, the rule demonstrated 100% sensitivity for SAH, meaning no cases were missed when the rule was applied correctly.
The rule applies to alert (GCS 15) patients presenting with a new severe non-traumatic headache that reached maximum intensity within one hour and is not recurrent in nature. It does not apply to patients with focal neurological deficits, prior SAH, known brain tumors, or chronic recurrent headaches. This calculator helps emergency physicians rapidly apply the Ottawa SAH Rule to support clinical decision-making for acute headache presentations.
The Ottawa SAH Rule evaluates six criteria in patients with acute severe headache (GCS 15, new headache, maximum intensity within 1 hour):
If any criterion is present, further investigation is recommended (CT head ± lumbar puncture or CT angiography). If no criteria are present, SAH is very unlikely, and the patient may be managed conservatively with appropriate follow-up instructions.
Sensitivity of the rule is 100% for SAH in the derivation cohort, with specificity of approximately 15%.
If any criterion is positive, the rule recommends investigation for SAH, typically starting with non-contrast CT head (which detects >98% of SAH within 6 hours) followed by lumbar puncture if CT is negative, or CT angiography as an alternative. If no criteria are positive, the probability of SAH is extremely low (0% in the derivation study), and the patient can generally be reassured and discharged with appropriate headache management and return precautions.
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No criteria met — SAH extremely unlikely. Conservative management with return precautions appropriate.
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Multiple criteria positive — urgent investigation for SAH required (CT head ± LP or CTA).
The Ottawa SAH Rule is a 6-criterion clinical decision tool that helps emergency physicians determine which patients with acute severe headache require investigation to rule out subarachnoid hemorrhage.
The rule applies to alert (GCS 15) patients with a new severe non-traumatic headache that peaked within 1 hour. It does not apply to patients with focal deficits, known SAH history, brain tumors, or chronic recurrent headaches.
The rule has 100% sensitivity for SAH in the derivation cohort, meaning no cases of SAH were missed when all six criteria were negative. This makes it a very safe rule-out tool.
Non-contrast CT head is the first-line investigation. If performed within 6 hours and negative, some protocols accept this as sufficient. If CT is negative after 6 hours, lumbar puncture or CT angiography is recommended.
A thunderclap headache is defined as a headache that reaches its maximum intensity at the instant of onset, like a clap of thunder. It is the hallmark presentation of SAH but can also be caused by other conditions.
The specificity is relatively low (approximately 15%), meaning many patients who do not have SAH will still trigger the rule. This is by design — high sensitivity is prioritized over specificity for this potentially fatal condition.
A CT performed within 6 hours of headache onset with modern scanners has sensitivity approaching 100% for SAH. After 6 hours, sensitivity decreases, and additional testing (LP or CTA) is recommended if clinical suspicion remains.
No. The Ottawa SAH Rule is a screening tool to assist clinical judgment. If clinical suspicion for SAH is high based on the overall presentation, investigation should proceed regardless of the rule result.
SAH from ruptured aneurysm becomes increasingly common after age 40, with peak incidence between ages 40 and 60. Including age as a criterion increases sensitivity for this high-risk demographic.
While ruptured aneurysm causes approximately 80% of non-traumatic SAH, other causes include arteriovenous malformations, perimesencephalic hemorrhage (benign), vasculitis, coagulation disorders, and drug use (especially cocaine).
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