A CDR to determine risk for sub-arachnoid hemorrhage (SAH) was derived and has been externally validated in a single study. The CDR’s purpose was to identify those at high risk for SAH and included those with acute non-traumatic headaches that reached maximal intensity within one hour and who had normal neurologic exams. Of note, the rule has many inclusion and exclusion criteria that the ED physician must be familiar with and was only derived for patients 16 years or older. The study authors note that the CDR is to identify patients with SAH; it is not an acute headache rule. In the validation study, of over 5,000 ED visits with acute headache, only 9% of those met inclusion criteria. Also, clinical gestalt again plays a role as the authors suggest not to apply the CDR to those who are ultra-high risk with a pre-test probability for SAH of > 50%.
The Ottawa SAH Rule was 100% sensitive but did not lead to reduction of testing vs. current practice. The authors state that the value of the Ottawa SAH Rule would be to standardize physician practice in order to avoid the relatively high rate of missed sub-arachnoid hemorrhages.