The Ottawa Subarachnoid Haemorrhage (SAH) Rule is a high-sensitivity clinical decision rule to identify which neurologically intact adults with new severe non-traumatic headache need investigation for possible SAH.

The rule was derived in a prospective multicentre cohort of 2131 adults presenting to Canadian emergency departments with acute non-traumatic headache that reached maximal intensity within 1 hour and a normal neurological examination. 132 patients (6.2%) had SAH, defined by subarachnoid blood on CT, xanthochromia in cerebrospinal fluid, or red blood cells in cerebrospinal fluid with positive angiography.
The final Ottawa SAH Rule incorporates six high-risk features (age ≥40 years, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, and limited neck flexion on examination). In the derivation study it achieved 100% sensitivity (95% CI 97.2–100.0) and 15.3% specificity for SAH.
A subsequent multicentre validation and pooled analysis of 3874 patients with acute headache reported 100% sensitivity (95% CI 98.4–100) with specificity around 16%, confirming the rule’s role as a high-sensitivity, low-specificity tool to support investigation decisions rather than to avoid imaging in clearly concerning presentations.

Practical limitations

The rule is intended for alert patients (GCS 15) older than 15 years with new severe non-traumatic headache peaking within 1 hour, and was not validated in patients with new neurological deficits, papilloedema, known cerebral aneurysm, prior SAH, brain tumour, hydrocephalus, or confirmed SAH before assessment.
Patients with established recurrent similar headaches (≥3 episodes of the same character and intensity over >6 months) were excluded from the derivation studies, so the rule should not be applied to typical migraine or other recurrent primary headache syndromes.
Interobserver reliability is only moderate for some components (thunderclap headache, neck pain or stiffness, and limited neck flexion), so careful history and examination are required when classifying these features.
The rule supports decisions about CT brain and further testing (lumbar puncture or vascular imaging) for suspected SAH. A negative rule in an eligible patient makes SAH very unlikely, but it does not exclude other serious secondary causes of headache.

Ottawa Subarachnoid Haemorrhage (SAH) Rule for Headache Evaluation