Ottawa SAH Rule: the six findings and how they combine

The Ottawa SAH Rule checks six findings in an alert adult with a sudden headache: age 40 or older, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache (instantly peaking pain), and limited neck flexion on examination. If any one of the six is present, the rule is positive. If none is present, it's negative. A 40-year-old with no other finding is positive; a 39-year-old with no other finding is negative.

This page doesn't give disposition advice. It shows how the six findings combine. What to do with a positive or negative rule depends on the clinical context.

Not a medical device. Not a diagnosis or dosing recommendation.

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Emergency calculators
Perry JJ et al. · 2013
The Ottawa SAH Rule logic
Worked examples
Ottawa SAH Rule reference values
Frequently asked questions

What is the Ottawa SAH Rule?

The Ottawa SAH Rule is a clinical decision rule for acute headache, published by Jeffrey J. Perry, Ian G. Stiell, Marco L. A. Sivilotti and colleagues in JAMA in 2013. SAH stands for subarachnoid hemorrhage, bleeding into the space around the brain. It's also called the Ottawa subarachnoid hemorrhage rule or the Perry SAH rule.

The 2013 abstract explains the background: "Three clinical decision rules were previously derived to identify patients with headache requiring investigations to rule out subarachnoid hemorrhage." The study set out "to assess the accuracy, reliability, acceptability, and potential refinement" of those rules in a new group of patients.

Magent's calculator returns a true/false result in a field named investigation_indicated. This page explains how that result is worked out. It doesn't say what to do with it.

The Ottawa SAH Rule logic

rule positive = (age >= 40)
             OR neck_pain_or_stiffness
             OR witnessed_loc
             OR onset_during_exertion
             OR thunderclap_headache
             OR limited_neck_flexion

Exact expression the calculator uses:

Investigation indicated iff age >= 40 or neck_pain_or_stiffness or witnessed_loc or onset_during_exertion or thunderclap_headache or limited_neck_flexion

There are no points. Any single finding makes the rule positive.

The six findings

Finding	Input	Met when
Age 40 or older	`age` (18–120)	40 or more; 39 isn't
Neck pain or stiffness	`neck_pain_or_stiffness` (true/false)	true
Witnessed loss of consciousness	`witnessed_loc` (true/false)	true
Onset during exertion	`onset_during_exertion` (true/false)	true
Thunderclap headache (instantly peaking pain)	`thunderclap_headache` (true/false)	true
Limited neck flexion on examination	`limited_neck_flexion` (true/false)	true

How to apply the rule

Check the patient group. Magent's record describes the published group as alert, neurologically intact adults with a nontraumatic headache peaking within 1 hour. Magent doesn't check this; the caller does.
Age. 40 or older meets the first finding. 39 doesn't.
The five yes/no findings. Neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, limited neck flexion.
Combine. Any finding met: positive. None met: negative.

The four findings plus two

The abstract describes the rule's build-up. A rule with four findings (age 40 or older, neck pain or stiffness, witnessed loss of consciousness, onset during exertion) came first. "Adding 'thunderclap headache' (ie, instantly peaking pain) and 'limited neck flexion on examination' resulted in the Ottawa SAH Rule." Magent's calculator uses all six.

What the calculator returns

Magent's calculator returns the true/false investigation_indicated result, the age it used, and a row for each finding showing whether it was met.

Worked examples

These use Magent's own expected values for the calculator. Unless stated, age is 30 and every yes/no finding is false.

Example 1: nothing present

Age 30 isn't 40 or older.
All five yes/no findings are false.
Rule negative (investigation_indicated false).

Example 2: age 40 vs 39

Age 40 → met: rule positive.
Age 39 → not met: rule negative.

Example 3: neck pain or stiffness alone

Age 30 → not met.
Neck pain or stiffness → met.
Rule positive.

Example 4: thunderclap headache alone

Age 30 → not met.
Thunderclap headache → met.
Rule positive.

Ottawa SAH Rule reference values

These are Magent's expected values for the calculator. At age 30 with no other finding, the rule is negative. Age 40 alone makes it positive; age 39 doesn't. At age 30, any one of neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache or limited neck flexion makes it positive.

History of the Ottawa SAH Rule

The rule comes from Jeffrey J. Perry, Ian G. Stiell, Marco L. A. Sivilotti, Michael J. Bullard and colleagues, Clinical decision rules to rule out subarachnoid hemorrhage for acute headache, JAMA, 2013 (volume 310, issue 12, pages 1248–1255).

The study was a "Multicenter cohort study conducted at 10 university-affiliated Canadian tertiary care emergency departments." It enrolled "2131 adults with a headache peaking within 1 hour and no neurologic deficits." Physicians "completed data forms after assessing eligible patients prior to investigations."

The study defined subarachnoid hemorrhage as "(1) subarachnoid blood on computed tomography scan; (2) xanthochromia in cerebrospinal fluid; or (3) red blood cells in the final tube of cerebrospinal fluid, with positive angiography findings."

The abstract's conclusion limits the findings to "patients with these specific clinical characteristics" and says they "require additional evaluation in implementation studies before the rule is applied in routine clinical care."

When it's used

Magent's record describes the published group: alert, neurologically intact adults with a nontraumatic headache that peaks within 1 hour. The abstract describes the same group.

Where it's used

The 2013 study took place in 10 Canadian emergency departments.

Who uses it

In the 2013 study, emergency physicians recorded the findings on data forms before any investigations.

Limits of the calculation

It combines six findings. It doesn't diagnose subarachnoid hemorrhage and doesn't examine anyone.
Limited neck flexion is an examination finding the caller supplies.
Magent doesn't take a GCS or a time-to-peak; the caller applies the patient group.
It isn't the Hunt-Hess grade or the Fisher grade.
This page doesn't say what to do when the rule is positive or negative.

Clinical decisions belong to a qualified clinician who knows the patient.

ICH Score is a points score for a confirmed intracerebral hemorrhage, not a rule for a headache before any scan.
Glasgow Coma Scale scores eye, verbal and motor responses. The Ottawa SAH Rule's group is alert patients, and Magent's rule doesn't take a GCS.

Glossary

Subarachnoid hemorrhage (SAH): bleeding into the space between the brain and the thin tissues covering it.
Thunderclap headache: a headache that reaches full intensity instantly, as the rule defines it.
Neck flexion: bending the head forward, chin toward chest.
Xanthochromia: yellow discoloration of cerebrospinal fluid.
Cerebrospinal fluid: the fluid around the brain and spinal cord.

Common mistakes

Missing age 40. 40 or older meets the finding; it isn't "over 40."
Treating it as a score. There are no points. Any one finding makes the rule positive.
Using it outside the published group. The study group was alert adults with no neurologic deficits and a headache peaking within 1 hour.
Using the four-finding version. The Ottawa SAH Rule has six findings.

Source

Perry JJ, Stiell IG, Sivilotti MLA, et al. Clinical decision rules to rule out subarachnoid hemorrhage for acute headache. JAMA. 2013;310(12):1248-1255. DOI: 10.1001/jama.2013.278018

Frequently asked questions

What are the Ottawa SAH Rule criteria?

Age 40 or older, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache and limited neck flexion on examination.

What does a positive Ottawa SAH Rule mean?

At least one of the six findings is present. This page doesn't give disposition advice about what follows.

Does age 40 count in the Ottawa SAH Rule?

Yes. The finding is age 40 or older.

What is a thunderclap headache in the Ottawa SAH Rule?

Pain that peaks instantly, per the abstract's wording ("instantly peaking pain").

Who developed the Ottawa SAH Rule?

Jeffrey Perry, Ian Stiell and colleagues, in a 2013 paper in JAMA.