This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
A rule for the worst headache of your life
Subarachnoid hemorrhage, bleeding around the brain usually from a ruptured aneurysm, accounts for only a small fraction of headaches seen in the emergency department, but missing it can be fatal. The Ottawa subarachnoid hemorrhage rule, published by Perry and colleagues in 2013, helps decide which patients with an acute headache need investigation.
Who it applies to
Alert patients aged 15 or older with a new, severe, non-traumatic headache that reached maximum intensity within 1 hour. It does not apply to patients with:
- new neurological deficits
- a previous aneurysm, subarachnoid hemorrhage or brain tumor
- recurrent headaches similar to this one, 3 or more over at least 6 months
The six criteria
Investigate for subarachnoid hemorrhage if any one is present:
- Age 40 or older
- Neck pain or stiffness
- Witnessed loss of consciousness
- Onset during exertion
- Thunderclap headache, with instantly peaking pain
- Limited neck flexion on examination
Interpreting the result
If none of the criteria is present, subarachnoid hemorrhage can be ruled out. In the validation study, the rule identified every patient with a hemorrhage, a sensitivity of 100%, but its specificity was only about 15%. Most patients with a positive rule do not have a bleed; they simply cannot be cleared without tests.
If any criterion is present, the usual next step is a non-contrast CT of the head. When performed within 6 hours of onset on a modern scanner and read by an experienced radiologist, a normal CT is very reassuring; later, a lumbar puncture or CT angiography may be needed.
Limits
The rule supports clinical judgment; it does not replace it. A patient who appears unwell, or whose story is atypical, should be investigated whatever the result.