108 D izziness is a common complaint in the emergency depart- ment (ED). It accounts for 2.5% of all ED visits in the United States, 1 and the number of visits doubled between 1995 and 2009. 2 Vertigo is a subcategory of dizziness that has both peripheral (inner ear) and central (brain) causes. 3,4 Although most vertigo is peripheral and benign, ≈3% of patients who come to the ED for vertigo will be diagnosed with a stroke during that ED visit. 5 Strokes that cause vertigo usually origi- nate from the cerebellum and brain stem; these stroke types are associated with particularly high morbidity and mortality. 6 Identifying the small group of patients with serious illness among all vertiginous patients may be challenging because the signs and symptoms are similar in some cases of periph- eral and central vertigo. 3,7 Standard teaching in emergency medicine is that a thorough history and examination, which includes a neurological examination, can effectively rule out central causes of vertigo 8,9 ; the neurological literature substantiates this approach 4,7,10 If the neurological examina- tion is abnormal, magnetic resonance imaging (MRI) with diffusion weighted imaging is the test of choice to visualize the posterior fossa. 10,11 Even the initial MRI, however, is not 100% sensitive. 12 Recently, the HINTS test (Head Impulse Nystagmus Test of Skew) has been proposed for the detection of cerebellum and brain stem strokes; however, it was derived in a patient group with a different pretest probability than those seen in the ED (72% had a stroke versus 3% in ED vertigo patients); testing was performed by a neuro-opthomologist, and it has not been externally validated. 13 A third diagnostic test option is computed tomography (CT). Unlike MRI, CT imaging is easy to obtain in most EDs; however, noncontrast head CT has been shown to have low sensitivity for detect- ing ischemic strokes overall, 12,14 and several small studies have suggested that the sensitivity is even worse in patients with Background and Purpose—The purpose of this study was to determine the proportion of emergency department (ED) patients with a diagnosis of peripheral vertigo who received computed tomography (CT) head imaging in the ED and to examine whether strokes were missed using CT imaging. Methods—This population-based retrospective cohort study assessed patients who were discharged from an ED in Ontario, Canada, with a diagnosis of peripheral vertigo, April 2006 to March 2011. Patients who received CT imaging (exposed) were matched by propensity score methods to patients who did not (unexposed). If performed, CT imaging was presumed to be negative for stroke because brain stem/cerebellar stroke would result in hospitalization. We compared the incidence of stroke within 30, 90, and 365 days subsequent to ED discharge between groups, to determine whether the exposed group had a higher frequency of early strokes than the matched unexposed group. Results—Among 41 794 qualifying patients, 8596 (20.6%) received ED head CT imaging, and 99.8% of these patients were able to be matched to a control. Among exposed patients, 25 (0.29%) were hospitalized for stroke within 30 days when compared with 11 (0.13%) among matched nonexposed patients. The relative risk of a 30- and 90-day stroke among exposed versus unexposed patients was 2.27 (95% confidence interval, 1.12–4.62) and 1.94 (95% confidence interval, 1.10–3.43), respectively. There was no difference between groups at 1 year. Strokes occurred at a median of 32.0 days (interquartile range, 4.0–33.0 days) in exposed patients, compared with 105 days (interquartile range, 11.5–204.5) in unexposed patients. Conclusions—One fifth of patients diagnosed with peripheral vertigo in Ontario received imaging that is not recommended in guidelines, and that imaging was associated with missed strokes. (Stroke. 2015;46:108-113. DOI: 10.1161/STROKEAHA. 114.007087.) Key Words: computed tomography, x-ray ◼ emergency service, hospital ◼ stroke ◼ vertigo Missed Strokes Using Computed Tomography Imaging in Patients With Vertigo Population-Based Cohort Study Keerat Grewal, MD; Peter C. Austin, PhD; Moira K. Kapral, MD, MSc; Hong Lu, PhD; Clare L. Atzema, MD, MSc Received September 14, 2014; final revision received October 21, 2014; accepted October 28, 2014. From the Division of Emergency Medicine, Department of Medicine, University of Toronto, Toronto, Ontario, Canada (K.G., C.L.A.); Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada (P.C.A., C.L.A.); University Health Network, Toronto, Ontario, Canada (M.K.K.); and the Institute for Clinical Evaluative Sciences, Toronto, Ontario, Canada (P.C.A., M.K.K., H.L., C.L.A.). The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.114. 007087/-/DC1. Correspondence to Clare Atzema, MD, MSc, 2075 Bayview Ave, G146, Toronto, Ontario, Canada M4N3M5. E-mail clare.atzema@ices.on.ca © 2014 American Heart Association, Inc. Stroke is available at http://stroke.ahajournals.org DOI: 10.1161/STROKEAHA.114.007087 by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from by guest on May 19, 2016 http://stroke.ahajournals.org/ Downloaded from