LETTERS TO THE EDITOR

902

b. Number of patients with ‘‘CAD’’ related to the vertebral artery in the VA electronic health record: 141 1 7 5 148. c. Proportion of all cases that are atraumatic CAD related to the vertebral artery: 148 of 388 5 .3814433. d. We can calculate the annual number of vertebrobasilar stroke cases in Cai et al, by multiplying the number of identified cases with dissection using the same codes as Cassidy et al, (433.0 and 433.2) by the proportion computed in c. 327.4 3 .3814433 5 124.88/year.  Incidence of cases with vertebral artery in Cai et al 5 124.88/15,779,020 veterans/year 5 .791/ 100,000 person-years. Therefore, the incidence of stroke involving the vertebral artery in the VA population is similar to what was reported by us in Ontario (.750/100,000 persons-years) and Saskatchewan (.855/100,000 persons-years). This is also very similar to the incidence of vertebral artery dissection (.97 per 100,000 population [95% confidence interval, .52-1.4]) reported by Lee et al, from their study in the city of Rochester, MN, and Olmsted County, MN. In our view, the study of misclassification in administrative data is important and we applaud efforts in this respect. However, Cai et al, have not provided a useful analysis in this respect. Furthermore, their study fails to provide further insight into the debate on chiropractic care and stroke.

John David Cassidy, PhD, DrMedSc Division of Epidemiology Dalla Lana School of Public Health University of Toronto Toronto, Ontario E-mail: [email protected] Pierre C^ ote, DC, PhD Faculty of Health Sciences University of Ontario Institute of Technology Oshawa, Ontario, Canada http://dx.doi.org/10.1016/j.jstrokecerebrovasdis.2014.11.034

References 1. Cai X, Razmara A, Paulus JK, et al. Case misclassification in studies of spinal manipulation and arterial dissection. J Stroke Cerebrovasc Dis 2014;23:2031-2035. 2. Cassidy JD, Boyle E, Cote P, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a populationbased case-control and case-crossover study. Spine 2008; 33(4 Suppl):S176-S183. 3. Boyle E, C^ ote P, Grier AR, et al. Examining vertebrobasilar artery stroke in two Canadian provinces. Spine 2008; 33(4 Suppl):S170-S175.

Response to Cassidy and Cote Dear Editor, We thank Drs Cassidy and Cote for their letter in response to our article ‘‘Case Misclassification..’’ It appears as if the methods and impact of our study was misinterpreted and we appreciate the opportunity to offer clarification. Our primary aim was very simple—to determine the positive predictive value (PPV) for identifying cervical artery dissection (CAD) by using the search strategy used in the studies by Rothwell and Cassidy. The earlier studies purport to negate the causal association between cervical manipulative therapy (CMT) and CAD by demonstrating a similar association between visits to primary care physicians (PCPs) before being identified as a ‘‘case.’’ We agree that there is no plausible casual mechanism that would explain an association between PCPs and dissection. PCPs do not cause dissections. Reverse causation bias would be a reasonable explanation. However, this line of reasoning is only sound if their ‘‘cases’’ had CADs. Our study suggests that almost none of them did. As they recognize, case identification by ICD9 codes is problematic. They are aware that by choosing codes specific for an anatomic location (posterior cerebral circulation) they will have included patients with many different vascular pathologies—atherosclerosis, small-vessel disease, embolic occlusions, vasculitis, infections, and others. Any disease state that results in posterior circulation stenosis, transient ischemia, or infarct would have been included in their case definition. One cannot make causal inferences regarding CAD, or exclude such relationships, by studying patients without the disease. For example, it would be misleading to make any claims about dissection based on a study of patients with cardiogenic stroke due to atrial fibrillation. The diseases, and so the natural histories, are different. Our study sought to quantify the misclassification that was present in their population, that is, of their cases, how many actually had dissections? We hypothesized that (1) the degree of misclassification would be great because CAD prevalence amongst all stroke patients (including those restricted to the posterior circulation) is relatively low, and (2) misclassification would differ by age because dissection prevalence is higher amongst young stroke patients. We started with their case identification strategy and then reviewed individual medical records to identify as many CADs as we could. Anything that increases CAD identification within the population would increase the PPV of their strategy and so support their approach. They are curiously critical of our decision to include dissection-specific codes when, in fact, by doing so, we biased our results in their favor. During the study period in Ontario, dissection codes were not in use. Therefore,

LETTERS TO THE EDITOR

the posterior circulation codes would have been the appropriate ones for coders to apply to patients with CAD. However, in the Veterans Administration system, dissection codes were available and so many patients with CAD would not have been identified by the Rothwell/Cassidy posterior circulation codes because coders would have used the more specific ones. By including them, we enriched the population with possible CAD cases, which led to an improvement in the PPV of their ICD9-based definition. Had we excluded them, the PPV overall would have been even lower than the 10.5% that we found. Cassidy and Cote suggest that we have neither provided a useful analysis of their article nor contributed to the debate on CMT and CAD. We disagree. Our analysis shows clearly that their article investigated a population of stroke patients almost none of whom had CAD—this was especially true in those older than 45 years. It is therefore misleading and inaccurate to claim that their article ‘‘indicates that a patient is as likely to have seen a medical doctor (MD) as a doctor of chiropractic (DC) in the week before experiencing a vertebral artery dissection [emphasis added]’’—as was done by the American Chiropractic Association who cite this as the ‘‘best scientific evidence available.1’’ Their study was not about dissection and so

903

if it is the ‘‘best’’ science then the argument is on a rocky foundation. They studied the wrong patients and their sensitivity analyses do not address this issue. The misclassification that we claim is not stroke versus no stroke (which they explore in their discussion) but stroke from dissection versus stroke from other causes. The study by Cassidy examined a population of patients with conventional (nondissection related) strokes. Such patients have numerous vascular risk factors and are more closely associated with their PCPs than a control, nonstroke population. It is that finding that is neither novel nor relevant to the CMT debate.

David E. Thaler, MD, PhD Department of Neurology Tufts Medical Center Boston, Massachusetts E-mail: [email protected] http://dx.doi.org/10.1016/j.jstrokecerebrovasdis.2015.01.015

Reference 1. Available at: http://www.acatoday.org/pdf/AHAWeak nesses_TalkingPoints.pdf. Accessed Jan 22, 2015.