Ottawa SAH Rule for Headache Evaluation
Identifies alert, non-traumatic headache patients who are at risk for subarachnoid haemorrhage (SAH) and require investigation. ALL criteria must be absent to rule out SAH.
How to use & interpret
The Ottawa subarachnoid haemorrhage (SAH) rule helps decide which alert patients with an acute, non-traumatic headache that reaches maximum intensity within one hour need investigation for SAH. It applies only to patients aged ≥15 with new severe headache and no neurological deficit.
Investigation is warranted if ANY of six features is present: age ≥40, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, 'thunderclap' (instantly peaking) headache, or limited neck flexion on examination. The rule is highly sensitive (designed not to miss SAH) but not specific, so a positive rule means investigate (CT ± lumbar puncture), not that SAH is present.
Score interpretation
No Ottawa criteria met. SAH very unlikely (NPV ~99.4%).
→ SAH investigation not required. Consider alternative diagnoses: migraine, tension headache, CSF leak, CVT. Discharge with safety-net advice.
One or more Ottawa criteria met. Sensitivity ~100% for SAH.
→ Urgent CT head (non-contrast). If CT negative and ≥ 6h from headache onset: LP for xanthochromia (or CT angiography per local protocol). Neurology review. If SAH confirmed: urgent neurosurgery.
Interpretation bands for the Ottawa SAH Rule. Apply clinical judgement and local guidance.
Frequently asked questions
Can the Ottawa SAH rule be used to rule in subarachnoid haemorrhage?
No. It is a high-sensitivity rule-out tool for a specific low-risk presentation. A positive rule mandates investigation; it does not confirm SAH, and it does not apply to patients with deficits, reduced GCS or trauma.
References
- Perry JJ, et al. Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study. BMJ. 2011;343:d4277.
- Perry JJ, et al. Clinical decision rules to rule out subarachnoid hemorrhage for acute headache. JAMA. 2013;310(12):1248–1255.
Related
Curated clinical cross-links plus same-class fallbacks.
- Cetuximab · Anti-EGFR Monoclonal Antibody — Head and Neck Cancer
- Cisplatin · Platinum Chemotherapy — Head and Neck Cancer
- Methotrexate · Chemotherapy / Immunosuppressant — Head and Neck / Granulomatosis
- Pembrolizumab (Head and Neck SCC) · PD-1 Inhibitor
- Nivolumab (Head and Neck SCC) · PD-1 Inhibitor
- Tranexamic Acid (Surgical / Trauma Haemorrhage) · Antifibrinolytic (Haemostatic)
- Acute Stroke / TIA Assessment · NICE NG128; RCP Stroke Guidelines 2023
- Status Epilepticus (Adults) · NICE CG137; ESEM guidelines; RCP Neurology Guidelines
- Suspected Subarachnoid Haemorrhage · NICE NG228; RCEM 2023; AHA/ASA 2023
- Adult Head Injury · NICE NG232 (2023)
- Bell's Palsy / Facial Nerve Palsy · ENT UK 2017; AAN
- Vertigo Workup · ENT UK; NICE CKS
Featured in these MRCEM clinical pathways
The Ottawa SAH Rule is covered in detail — with RCEM/NICE evidence base, indications and pitfalls — in the following exam-focused pathways on our sister siteReviseMRCEM.
MRCEM Primary / Intermediate / OSCE candidates: each pathway includes exam-style questions, RCEM/NICE citations, and FAQ summaries.
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.