Insights from Dr. Jeffrey Perry, Creator of the Ottawa Subarachnoid Hemorrhage Rule

Subarachnoid hemorrhage, if undiagnosed, can have devastating consequences. While headache is a common presenting complaint in emergency departments, only about 1% of these patients are diagnosed with SAH. The Ottawa SAH Rule helps rule out SAH with 100% sensitivity, to better identify which patients do and do not need further workup. We talked with Dr. Jeffrey Perry, first author of the Ottawa SAH Rule derivation study.


Dr. Jeffrey J. Perry

How did you develop the Ottawa SAH Rule? Was there a particular patient or clinical experience you had?

Two things: One was the apparent subjectivity I noticed as a resident in evaluating patients for SAH, where the criteria for which patients we would investigate seemed to be very different. Some of the patients I thought were very low risk, other physicians would want to still investigate them for SAH, including doing a CT, which didn’t bother me too much, but then they would go on to do an LP, which is very uncomfortable, and time-consuming, and it seemed to contribute to already very prevalent ED overcrowding. So that was the clinical side of things.

Research-wise, there was a survey done of both American and Canadian physicians looking at areas where they were most interested in developing clinical decision rules, and determining which patients were at risk for SAH was one of the most-requested.

Do you know of any cases where the rule has been misapplied or misused?

When we did our prospective validation study, we found the physicians misinterpreted the rule about 4% of the time. None of those resulted in NOT investigating patients with SAH, but most of the time it was over-investigating. It appeared to be neck pain or stiffness on history that was prompting physicians to over-investigate, as well as the thunderclap headache. If it wasn’t instantly peaking, even evolving over the course of minutes, it wasn’t meant to be deemed high risk. Only if it was instantly peaking, and not progressing after the onset, did it meet that criteria. I’m happy to see that that designation is in your calculator.

Do you have any comment on the low specificity of the rule? Obviously, for a true rule-out tool, sensitivity is more important, and the Ottawa SAH Rule is 100% sensitive, but the specificity quoted is 15%. What are the implications of that, if any?

We struggled as investigators in looking at how sensitive it needed to be. Obviously, the more sensitive it is, the less specific, and we felt that given the potential catastrophic consequence of missing SAH, we thought that it was still more important to ensure that the rule was as close to 100% sensitive as possible, even though it may have sacrificed some of the specificity. Population studies have shown that up to 1 in 20 patients with SAH are actually missed on their initial presentation. These numbers are much higher in rural and remote areas than at tertiary care centers, which are highly academic and tend to investigate patients a lot more, so their miss rate is a lot lower. We felt that, given that physicians are still missing some SAHs, that it was important to make sure it was sensitive, knowing that it does translate into a relatively low specificity. Given that physicians are investigating most patients, even more so in the States, we don’t expect to increase the rate of investigation and still may translate into a very small reduction, but our aim is to make sure the patients who are being investigated are the right patients so we’re not missing any cases.

You’ve hit on an important point of resource utilization, which Canada seems to be more cognizant of given the different economic structures in healthcare. Do you know of any cost benefit or impact analysis of using the rule?

There has been an analysis of the theoretical impact of costs in Ontario, and most of the savings are from reducing the potential miss rate, which the previous population-based study demonstrated, that based on provincial data, 1 in 20 patients were missed, meaning they were diagnosed with something other than SAH, and then re-presented up to 7 days later. Based on that baseline miss rate, applying the rule would still translate into significant cost savings on the population, even though it doesn’t directly decrease the rate of investigations by much. Now, that may not be as applicable in a center where they’re already investigating a lot of the patients, though there may be some decrease in the investigation rate and translate into some cost savings. But, there’s no cost analysis of those kinds of centers where the investigation rate is nearly 100%.

Our group has researched the so-called “Six Hour Rule,” where patients who have a CT scan within 6 hours of headache onset were studied. Our studies have shown that most of those patients don’t need to go on to have any further testing. That would also lower the LP rate or the follow-up CT rate, depending how you investigate further. That, too, would lower the cost. This cost analysis was done independent of us.

How do you use the Ottawa SAH Rule in your own practice?

First of all, I’m happy your calculator shows which patients to apply the rule to: you want to make sure that it’s even appropriate to apply the rule. Is it a patient with a new headache that is atraumatic, peaks within an hour, and is not a patient with established recurrent headache? And also make sure that the patient doesn’t have a history of previous SAH or known cerebral aneurysm. If the patient is appropriate, you apply the six criteria, and if they have any one of those, then they are deemed high risk.

If the patient is high risk and needs investigation, we look at the time since the onset of the headache. If it’s less than six hours, we get a CT, and if the CT is negative, then for most patients at that point we stop investigating and we have a discussion with the patient. If there’s some kind of extenuating situation, like the patient has a first-degree relative who died of a SAH, that patient may still want to go on and have further testing, but for the vast majority of our patients, we suggest stopping at that point and let the patient know that the risk of a bleeding aneurysm is less than 1%, and to try to figure out if it was any less than that we’d have to do a lumbar puncture, and then we describe the lumbar puncture, and have shared decision making with the patient. I say if it was me I would stop, but if they’re still worried about it and want to do further testing, we can do that.

In my experience, all but one patient has declined further testing. For the one patient who wanted the LP, it was negative, but they were more reassured. Certainly for most patients you can stop further testing at that point. For those patients who do undergo LP, we have criteria to determine the difference between traumatic tap and a true SAH, so we then apply those criteria to the LP results to determine if any further testing like cerebral angiography is needed.



Jeffrey J. Perry, MD, is a professor in the department of emergency medicine at the University of Ottawa in Ottawa, Canada. His research interests include clinical decision rules, subarachnoid hemorrhage, and systematic review and meta-analysis. Dr. Perry is also a senior scientist with the Ottawa Hospital Research Institute and Research Chair of Emergency Neurological Research.

To view Dr. Jeffrey J. Perry’s publications, visit PubMed