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Ontario EDs misdiagnose 1 in 20 ruptured brain aneurysms


A new Institute for Clinical Evaluative Sciences (ICES) study has found that about one in 20 patients with a ruptured brain aneurysm (a type of stroke known as subarachnoid hemorrhage or SAH) are misdiagnosed when they visit an Ontario emergency department.

“SAH is a rare but serious condition that physicians try to diagnose quickly because it frequently leads to neurological impairment and death. However, patients’ symptoms can vary from a simple headache to obvious neurological problems, and the condition is not common, so emergency physicians encounter it infrequently even over many years of practice. For all these reasons, the diagnosis can be challenging, and we know it is sometimes missed,” said Dr. Michael Schull, ICES scientist and senior author of the study.

“We know little about population rates of the misdiagnosis of SAH, and whether there are hospital factors that contribute to it.”

To study this issue further, ICES investigators tracked persons admitted with a non-traumatic SAH to all Ontario hospitals between April 2002 and March 2005. They examined the association between hospital factors such as teaching status, the number of SAH seen annually in an ED, and access to a computed tomography (CT) scanner (a key diagnostic test used to diagnose SAH). SAH was defined as missed if the patient had an ED visit related to the SAH, but where the diagnosis was not made, in the 14 days before hospital admission.

Of the 1,507 patients diagnosed with SAH during the study period, 5.4% had a missed diagnosis. Patients who presented to non-teaching hospitals were more than twice as likely to have a missed SAH as those at teaching sites, and those who were triaged as low urgency had a 2.7-fold increase in the risk of misdiagnosis. The annual volume of SAHs seen in an ED and on-site access to a CT scanner did not affect the risk of misdiagnosis in an ED.

“Our study found a lower misdiagnosis rate than in previous studies, which is good news; however, missing five per cent of patients is still too many given the gravity of this condition,” said Dr. Schull.

“Our results also showed that the type of hospital a patient visits is an important risk factor for misdiagnosis, but why this is the case remains unclear. It does not appear to relate to the frequency of the condition in the ED or to access to CT scanners. In other words, reducing the misdiagnosis risk may have less to do with better access to technology, and more to do with human factors, such as medical staff training and experience, the availability of consultants, or differences in diagnostic protocols.”

The study, “Missed diagnosis of subarachnoid hemorrhage in the emergency department”, is in the April 2007 issue of the journal Stroke.

Author affiliations: ICES (all authors); Clinical Epidemiology Unit and Department of Emergency Services (Dr. Schull), Sunnybrook Health Sciences Centre; Department of Medicine (Dr. Schull) and Department of Health Policy, Management and Evaluation (all authors), University of Toronto.

ICES is an independent, non-profit organization that uses population-based health information to produce knowledge on a broad range of healthcare issues. Our unbiased evidence provides measures of health system performance, a clearer understanding of the shifting healthcare needs of Ontarians, and a stimulus for discussion of practical solutions to optimize scarce resources. ICES knowledge is highly regarded in Canada and abroad, and is widely used by government, hospitals, planners, and practitioners to make decisions about care delivery and to develop policy.


  • Julie Dowdie
  • Media Relations Officer, ICES
  • (416) 480-4780 or cell (416) 432-8143


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