Subarachnoid hemorrhage (SAH) typically presents to emergency departments with acute (peaking in 1h) non-traumatic headaches. SAH is present in 7% of this patient group, and is associated with a 25-50% six-month mortality, and a 30% risk of permanent neurological deficit.¹ Prompt diagnosis is key, as patients with normal neurological findings have the most to lose from a missed diagnosis.

The Ottawa Subarachnoid Rule applies to alert (GCS 15) patients >15 years old who present to the emergency department within 14 days of the onset of an acute non-traumatic headache, defined as reaching maximum intensity within 1h. Their neurological examination must be normal.

Investigate if any of the following high-risk variables are present:

• Age > 40 y
• Neck pain or stiffness
• Witnessed loss of consciousness
• Onset during exertion
• Thunderclap headache (instantly peaking pain)
• Limited neck flexion on examination (inability to touch chin to chest or lift head 8 cm off the bed)

If one or more features are present, this rule is 100% (95% CI, 98.6%-100%) sensitive and 17.8% (95% CI, 16.6%-19.1%) specific for the diagnosis of SAH. If no high-risk variables are present, there is no need investigate the patient for SAH (unless you have a very high clinical suspicion for SAH)

Note: The Ottawa Subarachnoid Rule should not be applied to patients with new neurologic deficits, previous aneurysms, SAH, or brain tumors who are likely at high-risk, and require imaging. The rule excludes patients with a history of recurrent headaches (>3 episodes over the >6 months), which are likely benign and require no imaging. These types of patients were excluded from the rules’ development.

• Investigations include a plain (unenhanced) CT scan of the head (if available), and may require lumbar puncture and/or cerebral angiography.
• When scanned within 6h of headache onset, modern third generation CT scans are 100% sensitive (95% CI, 97.0%-100.0%) for the detection of SAH.² when interpreted by a radiologist. If the CT is negative no other investigations are required.
• These results are not generalizable to resident or emergency physician interpretations of the CT scan.
• Blood with hemoglobin <90g/L does not light up well on CT scan. These patients should receive further investigations even in their CT is negative (LP or angiography).
• If you have a very high index of suspicion (e.g. multiple first-degree relatives with SAH), you may wish to pursue further investigations even if the head CT is reported as negative.
• After 6h from headache onset, CT scanning is only 85.7% sensitive. Therefore, patients with delayed imaging and a negative scan require further investigations (LP).

• If any of the criteria for investigations above are met and there is no access to a CT scanner, the patient should be evaluated clinically for potential increased intracranial pressure and other risks of complications from an LP.
• If a detailed neurologic examination is normal, there are no signs of papilledema on examination, there is no bleeding disorder or use of anticoagulants, and there is no history of intracranial pathology, it is reasonable to perform a lumbar puncture to rule out SAH.

• All aneurysmal SAHs were identified by the presence of xanthochromia on visual inspection of the cerebrospinal fluid (example image) or the presence of >2000 x 10⁶ red blood cells/L in the cerebrospinal fluid (sensitivity 100%; 95% CI 74.7%-100.0%).³ In other words, if there is no xanthochromia and the red blood cell count is <2000 x 10⁶ red blood cells/L in the cerebrospinal fluid, the patient is highly unlikely to have suffered an aneurysmal SAH.
• If the LP is positive or if clinical suspicion is high, the next diagnostic step is cerebral angiography to identify cerebral aneurysms.

All patients with an SAH diagnosed with CT or positive LP will need angiography. CT angiography is very sensitive for aneurysms over 2mm. However, up to 4% of the population harbor a cerebral aneurysm of which the vast majority will never cause the patient any ill effects. Therefore, current practice is to do cerebral angiography only after other tests have confirmed or are highly suspicious for SAH.

• CT or MRI angiography should be used for patients who present >6hrs from headache onset who:
  • fail to have a successful lumbar puncture (i.e. contraindicated due to a bleeding disorder or signs of increased intracranial pressure, or failure to obtain cerebrospinal fluid)
  • have high-risk cerebrospinal fluid results (xanthochromia or >2000 RBCs x 10⁶/L) OR
  • have a very concerning story with an LP result which is not completely normal.

1. Perry JJ, Stiell IG, Sivilotti MLA, et al. Clinical Decision Rules to Rule Out Subarachnoid Hemorrhage for Acute Headache. JAMA 2013;310(12):1248-55. doi: 10.1001/jama.2013.278018

   
   

2. Perry JJ, Stiell IG, Sivilotti MLA, 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):1-10. doi: 10.1136/bmj.d4277

   
   

3. Perry JJ, Alyahya B, Sivilotti MLA, et al. Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study. BMJ 2015;350(h568):1-7. doi: 10.1136/bmj.h568