DALLAS – The revised Ottawa Subarachnoid Hemorrhage Rule is now ready for prime-time use in EDs.
The final, tweaked version of the rule sailed through a large new prospective validation study, exhibiting 100% sensitivity and 14% specificity for the detection of this high-mortality headache, Dr. Jeffrey J. Perry reported at the annual meeting of the Society of Academic Emergency Medicine*.
Dr. Jeffrey Perry
"This tool provides a method to standardize who is investigated in an attempt to minimize the chances of missed diagnosis, given that we know from previous studies that 1 in 20 patients with subarachnoid hemorrhage are missed at the time of their first ED [emergency department] visit," explained Dr. Perry, an emergency physician at the Ottawa Hospital Research Institute and the University of Ottawa.
"The Ottawa Rule will not result in a large reduction in investigations in Canada, but the investigations will be more standardized. I suspect perhaps in the United States, where there’s often a lower threshold to investigate, there may actually be some reduction in investigations, but that hasn’t been assessed," Dr. Perry observed.
An earlier version of the Ottawa Subarachnoid Headache Rule was previously tested in a study conducted at 10 Canadian tertiary-care EDs. In that study, published last year (JAMA 2013;310:1248-55), the rule showed 98.5% sensitivity and 27.5% specificity for subarachnoid hemorrhage (SAH). Dr. Perry and his colleagues felt that 98.5% sensitivity just wasn’t good enough for a condition with 50% mortality and permanent neurologic deficits in 42% of survivors. So they added two additional elements to the rule: "thunderclap headache with instantly peaking pain," and "limited neck flexion on examination." But by changing the Ottawa SAH Rule, it became necessary to conduct a new prospective validation study of the revised rule in a new patient cohort. That’s the study that Dr. Perry presented at SAEM 2014.
The study took place in six Canadian university-affiliated EDs. It included 1,140 patients eligible for application of the rule; that is, they were alert, neurologically intact adults presenting with a new acute nontraumatic headache peaking within 1 hour. They typically reported a pain score of 10 out of a possible 10.
SAH was defined on the basis of subarachnoid blood on a CT scan, RBCs in the final tube of cerebrospinal fluid, or xanthochromia in the cerebrospinal fluid. The SAH diagnostic criteria were met by 64 patients, or 5.6%.
All 64 patients with SAH were identified by the Ottawa SAH Rule as being at high risk for the condition. So were 922 patients who proved not to have SAH. None of the 154 patients identified as low risk by the rule turned out to have SAH. This translated to a rule sensitivity of 100% and specificity of 14.3%. Application of the rule would lead to investigation using CT in 86.5% of patients who presented to the ED with acute headache.
The combined populations of this validation study and the earlier JAMA study totaled 3,271 patients, 196 of whom turned out to have SAH. When the revised rule was applied to the combined cohort, it demonstrated 100% sensitivity and 16.7% specificity for SAH.
Up to 4.5% of all ED visits are for headache. Only 1% of these headaches are SAHs. In the new Ottawa Rule validation study, the largest final diagnostic categories were migraine, which accounted for nearly 20% of the headaches, and benign headache, which was present in 53% and consisted mostly of tension headaches and cluster headaches. Other than SAH, serious headaches were relatively rare, with meningitis the final diagnosis in 0.3% of patients, TIA/stroke in 0.4%, and brain tumor in 0.3%.
The validation study was funded by the Canadian Institutes of Health Research. Dr. Perry reported having no conflicts of interest.
Correction, 6/23/14: An earlier version of this article misstated the name of the Society of Academic Emergency Medicine.