How it's calculated
Applies only to alert patients (GCS 15) aged 15 or older with a new, severe, non-traumatic headache reaching maximum intensity within 1 hour. Any single positive finding out of six triggers further investigation.
Criteria
| Criterion |
|---|
| Age ≥ 40 years |
| Neck pain or stiffness |
| Witnessed loss of consciousness |
| Onset during exertion |
| Thunderclap headache (instantly peaking) |
| Limited neck flexion on examination |
Clinical use
- Used in the emergency department to decide which patients with acute, non-traumatic headache need further investigation (CT head ± lumbar puncture) for subarachnoid haemorrhage.
- Designed to be highly sensitive (close to 100% in the validation cohort) — it is meant to safely rule out the need for further work-up when entirely negative, not to confirm SAH when positive.
- Only validated for use in alert patients (GCS 15) with a headache that reached maximum intensity within 1 hour — it doesn't apply to more gradual-onset or chronic headache presentations.
Frequently asked questions
What is a 'thunderclap headache'?
A headache that reaches maximum intensity almost instantly (classically described as reaching peak severity within seconds to a minute) — a classic feature of subarachnoid haemorrhage, though it has other causes too.
Does a negative Ottawa SAH Rule completely rule out subarachnoid haemorrhage?
It's designed to be highly sensitive, meaning very few true SAH cases would be missed if none of the six criteria are present — but as with any clinical decision rule, it should be applied to the population it was validated in and combined with clinical judgement.
Who should this rule not be used in?
Patients with GCS below 15, a headache that developed gradually (not reaching maximum intensity within 1 hour), recurrent headaches with a similar pattern to prior benign headaches, or new neurological deficits (which by themselves may warrant imaging regardless of this rule).
References
- Perry JJ, et al. Clinical Decision Rules to Rule Out Subarachnoid Hemorrhage for Acute Headache. JAMA. 2013;310(12):1248-1255.
- Perry JJ, et al. High risk clinical characteristics for subarachnoid haemorrhage in patients with acute headache: prospective cohort study. BMJ. 2010.