CDR Blog

Preamble

A 35-year-old woman presents to the emergency department with the “worst headache” of her life. What are important questions to ask on history? Not sure if you should get your preceptor right away? Worried that this might be a subarachnoid hemorrhage? Our podcast discusses the Ottawa Subarachnoid Hemorrhage Rule in an emergency medicine setting. This powerful clinical decision-making tool aids in the exclusion of a frightening diagnosis and, in combination with clinical judgment, is used by medical professionals across Canada in the assessment of acute headaches.

Anatomy

To start with a little bit of anatomy, the subarachnoid space means the space between the arachnoid mater and the pia mater, which are two of three meninges or layers that cover up the brain and spinal cord. In order of most superficial to deep, the three meninges go dura mater, arachnoid mater, and pia mater. The subarachnoid space actually gets its name from the spider-web-like appearance of the arachnoid mater. The subarachnoid space is normally filled with cerebrospinal fluid (CSF), which acts as a cushion to protect the brain. This fluid is produced by the ventricles of the brain and is in a constant cycle of being absorbed and replenished.

Subarachnoid Hemorrhages Explained

So, in a Subarachnoid hemorrhage (SAH), head trauma, arteriovenous malformation or a ruptured aneurysm could all lead to bleeding into this space. Now the brain itself isn’t actually use to anything but CSF existing in this space, which leads to two main issues: 1) this results in decreased blood flow to the part of the brain distal to the bleed which leads to cerebral ischemia. 2) On the other hand, collection of blood uncontrollably into the subarachnoid space can also lead to increased intracranial pressure and possible brain herniation. These complications create a slew of symptoms, however the presentation can be highly varied and inconsistent with only ~10% of all those presenting with a SAH having the traditional ‘thunderclap’ headache. Because of this varied presentation, and the high mortality rate associated with this condition, a clinical decision rule was created by the University of Ottawa. This rule helps clinicians rule out SAH in patients presenting with acute, non-traumatic headache.

Applying the Ottawa Subarachnoid Hemorrhage Rule

Before using this tool, clinicians need to understand when to apply this clinical decision rule (CDR). The inclusion criteria include alert patients > 15yrs old, new severe atraumatic headache, and maximum intensity reached within 1 hour. Exclusion criteria includes patients with new neurological deficits, prior aneurysm, prior SAH, known brain tumors, or chronic recurrent headaches. The tool rules out SAH and has 6 criteria. If any of these criteria apply to the patient, then the clinician cannot rule out a SAH and will have to pursue additional diagnostic work-up including non-contrast CT head, and possible lumbar puncture. The criteria are as follows:

o Age > or = 40

o Neck pain or stiffness

o Witnessed loss of consciousness

o Onset during exertion

o Thunderclap headache peaking pain within 1 second

o Limited neck flexion on exam

Evidence to support the Ottawa Subarachnoid Hemorrhage Rule

So now just to touch on a little bit of evidence regarding this tool. A prospective multicenter cohort study involving 6 tertiary Canadian EDs over 4 years, determined that this tool has a sensitivity of 100%, with a specificity of only around 15%. This means that the tool cannot be used to make the diagnosis, but it can be used to rule out SAH if all the criteria are met. Additionally, since the tool has this low specificity, many patients are subject to work-ups that don’t identify SAH. However, a good screening tool is based on its ability to rule things out, which is why this is less of a concern.

1. Beck-Esmay J. Validation of the Ottawa Subarachnoid Hemorrhage Rule in Patients With Acute Headache. REBEL EM. 2018 Sep.

2. Kairys N, Das JM, Garg M. Acute Subarachnoid Hemorrhage. StatPearls Publishing. 2021 Jan.

3. Perry JJ. Ottawa Subarachnoid Hemorrhage Rule. Emergency Medicine Practice. 2019 Feb.

4. Perry JJ, Sivilotti MLA, Émond M, et al. Prospective Implementation of the Ottawa Subarachnoid Hemorrhage Rule and 6-Hour Computed Tomography Rule. Stroke. 2020;51:424-430.

5. Perry JJ, Stiell IG, Sivilotti ML, et al. High risk clinical characteristics for subarachnoid haemorrhage in patients with acute headache: prospective cohort study. BMJ. 2010;341:c5204.