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Supplementary Research Studies

Examining Vertebrobasilar Artery Stroke in Two Canadian Provinces

Boyle, Eleanor PhD*†; Côté, Pierre DC, PhD*†‡§¶; Grier, Alexander R. DC, MBA; Cassidy, J David DC, PhD, DrMedSc*†‡§

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doi: 10.1097/BRS.0b013e31816454e0
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There are few published reports of the incidence rate of vertebrobasilar artery (VBA) dissection and stroke in the general population. One quality study suggests that it is very rare.1 In Olmsted County and Rochester, MN, Lee et al, reported that the age- and sex-adjusted incidence rate of VBA dissection-related stroke was 0.97 per 100,000 person-years in Olmstead and 1.12 per 100,000 person-years in Rochester for the period 1987 to 2003.

Understanding the incidence of VBA stroke is important for several reasons. First, it is a potentially life-threatening condition. Second, it may result in significant morbidity. Finally, VBA stroke is viewed as a serious potentially adverse event of manual therapy to the cervical spine. For example, 2 deaths in Canada from VBA dissection and stroke after chiropractic care in the 1990s attracted much media attention; and a call by some neurologists to avoid neck manipulation for acute neck pain.2 The first case occurred in Ontario in 1996 and the second one in Saskatchewan in 1998. The inquest for the death in Saskatchewan occurred in 1998. In Ontario, there were calls for an inquest occurring in 1996 and 2000, which were both denied.3 The Ontario inquest occurred in 2002.

Our first objective was to describe the incidence rate of VBA stroke (cumulative, age-specific, and sex-specific incidence rate) in the Canadian provinces of Ontario and Saskatchewan between April 1, 1993 and March 31, 2002. Our second objective was to determine from an ecological perspective whether the annual incidence of VBA paralleled the rate of annual chiropractic utilization in Saskatchewan from January 1, 1993 to December 31, 2004 and in Ontario from April 1, 1993 to March 31, 2002.

Materials and Methods

Study Design

We conducted population-based ecological study of individuals living in Ontario or Saskatchewan and who were registered with their respective provincial health insurance plan.

Study Population

Canada has a universal health care system whereby all necessary physician and hospital services are provided to the individual with no deductible or copayment. However, the type and amount of allied health care services covered within the universal health care system differs by province. In Saskatchewan, chiropractors were paid a negotiated fee per service by Saskatchewan Health and individuals seeking care from chiropractors pay a copayment, unless they are eligible for low-income-related supplementary benefits, in which case there is no cost at point of service for chiropractic treatment. There are no limits on the number of allowed services. In Ontario, there was a limit on the annual number of billable chiropractic services and a copayment was allowed. Since December 1, 2004, chiropractic services are no longer covered by the Ontario universal health care system.

Between 1993 and 2004, the population of Saskatchewan was stable at approximately 1,000,000 residents.4 In 2000, the median age of the population was 35.9 years and 50.3% were women. During the same period, the population of Ontario grew from approximately 10 to 12 million residents. In 2000, the median age of the Ontario population was 36.5 years and 50.6% were women.

All hospitals in Ontario submit data to the discharge abstract database (DAD) which is maintained by the Canadian Institute of Health Information. All hospital discharges for acute inpatient, chronic inpatient, and rehabilitation admissions as well as same day surgeries are captured in DAD. The historical database holds records from the 1981/82 fiscal year to the present. The DAD was used for Ontario case ascertainment.

Saskatchewan case ascertainment was based on Saskatchewan Health’s hospital separation files, which date back to 1970. The files include all hospitalization for Saskatchewan Health beneficiaries in Saskatchewan, out-of-province, and out-of country hospitals.

Ascertainment of VBA Cases

To be eligible, individuals must have had 2 years of health care coverage in the province of Ontario or in the province of Saskatchewan before the incident stroke.


All individuals admitted to an acute care hospital with a discharge diagnosis of an incident vertebrobasilar occlusion or vertebrobasilar stenosis stroke [International Classification of Diseases, Ninth edition (ICD-9CM) 433.0 and 433.2] between April 1, 1993 and March 31, 2002 were included as cases. Individuals who had any type of stroke (ICD-9 433.0, 433.2, 434, 436, 433.1, 433.3, 433.8, 433.9, 430, 431, 432, and 437.1), transient cerebral ischemia (ICD-9 435), or late effects of cerebrovascular diseases (ICD-9 438) before their VBA discharge diagnosis from April 1, 1991 to March 30, 2002 were not considered an incident VBA case.


All Saskatchewan Health beneficiaries admitted to hospital with a discharge diagnosis of an incident vertebrobasilar occlusion or vertebrobasilar stenosis stroke (ICD-9 433.0 and 433.2) during January 1, 1993 to December 31, 2004 were included as cases. From January 1, 1976 to March 31, 2002, the ICD 9 codes were used to code the diagnoses after which ICD-10CA codes were used. Under the ICD-10CA classification, VBA was defined as I65.0xx and I65.1xx where “x” represents all subcategories associated with that specific code. Individuals who had a previous hospital admission from January 1, 1970 to December 30, 2004 for any type of stroke (ICD-9 433.0, 433.2, 434, 436, 433.1, 433.3, 433.8, 433.9, 430, 431, 432, and 437.1; ICD-10 I65.1xx, I65.0xx, G46.0-G46.2xx,,, G46.4-G46.7xx, I64.xx, I65.2xx, I65.3xx, I65.8xx, I65.9xx,,, and, transient cerebral ischemia (ICD-9 435; ICD-10 or late effects of cerebrovascular diseases (ICD-9 438; ICD-10 I69.x) were excluded.

Chiropractic Utilization Rates

Two different chiropractic utilization rates were calculated for each province. The first rate was based on the number of patients who had an encounter with at least one chiropractor during the fiscal year. If an individual saw more than one chiropractor during the fiscal year, the individual was only counted once. The second rate was based on the total number of services provided by chiropractors during the fiscal year. We assumed that the type of services and in particular, the proportion of cervical manipulations conducted by a chiropractor for any given year would remain constant over time. The number of chiropractors practicing in a given year was based on the number of chiropractors registered with the province. This would lead to an underestimate of the utilization rates because there are registered chiropractics that are no longer in practice or had treated very few patients in the year, or had moved to another province.

Data from the Ontario Ministry of Health and Long-Term Care provided to the Ontario Chiropractic Association was used to determine chiropractic utilization rates. The annual number of patients per 100,000 population per chiropractor and the annual number of services per 10,000 population per chiropractor was calculated for Ontario. Radiograph services provided by chiropractors were excluded in Ontario. In Ontario, chiropractors cannot order radiographs from medical clinics reimbursed by the provincial health care system. Instead, chiropractors use their own radiograph machine, refer patients to chiropractic radiology clinics, or recommend that patients consult with their family physician for a radiograph.

Using data provided to the Chiropractic Association of Saskatchewan by the Medical Services Branch of Saskatchewan Health, we calculated the annual number of services per 1000 population per chiropractor (provided to the authors by the Chiropractic Association of Saskatchewan 2007).5 The services included services received outside of the province; however, these services were usually provided to Saskatchewan residents who live in one of the border towns of Saskatchewan such as Flin Flon, Manitoba and Lloydminister, Alberta. Three percent to 4% of chiropractic services were provided to Saskatchewan residents outside the province. In Saskatchewan, chiropractors are allowed to request radiographs from medical clinics. These radiograph services were not included in the calculation for the total number of services. We did include radiograph services which occurred in the chiropractor offices using privately owned machines. Less than 1% of all chiropractic services provided to Saskatchewan residents can be attributable to radiographs provided in chiropractor offices. The annual number of patients per 10,000 population per chiropractor was calculated using the same data source.

Statistical Analysis

The cumulative nonstandardized, sex-specific, and age-specific incidence rates of VBA stroke per 100,000 person-years were calculated for each province for the period April 1, 1993 to March 31, 2002. In addition, the annual incidence rate of VBA stroke per 100,000 person-years was calculated in Ontario for the period April 1, 1993 to March 31, 2002 and in Saskatchewan for the period January 1, 1993 to December 31, 2004.

Statistics Canada’s yearly population estimates were used for Ontario and Saskatchewan as denominators.4 Individuals under the age of 2 were excluded from the denominator because the numerator only included individuals who had at least 2 years of health coverage.


Overall, the incidence nonstandardized rate of VBA strokes for the 2 provinces was similar (0.855 per 100,000 person-years and 0.750 per 100,000 person-years for Saskatchewan and Ontario, respectively) (Table 1). The incidence rate was higher for men than for women and was higher for individuals aged 45 years and above than for individuals aged less than 45 years.

Table 1:
The Crude, Sex- and Age-Specific Incidence Rates for the Provinces of Ontario and Saskatchewan

The yearly incidence rates of VBA for Saskatchewan and for Ontario were similar from 1993 to 1996 (Figures 1 and 2). However, there was a 360% increase in the incidence rate in Saskatchewan from 1998 and 2000. In 2002 to 2003, the incidence rate gradually declined to its pre-1998 level after which it started to climb again in 2004. There was a small peak in the incidence rate for Ontario after 1999 after which it fell back to its pre-1999 level.

Figure 1:
Annual incidence rate per 100,000 person-years for Saskatchewan, the number of patients per 10,000 population per chiropractor and the number of encounters per 1000 population per chiropractor. Chiropractic utilization data are based on fiscal year rather than calendar year.
Figure 2:
Annual incidence rate per 100,000 person-years for Ontario, the number of patients per 100,000 population per Chiropractor and the number of encounters per 10,000 population per Chiropractor. Chiropractic utilization data are based on fiscal year rather than calendar year. †For the 2002 incidence rate was calculated based on the number of cases in the first quarter multiplied by 4.

The annual number of patients per 10,000 per registered chiropractor population for Saskatchewan was stable between 1993 and 1997. The number of individuals using chiropractic dropped by 12% between 1996 and 1999 (Figure 1). In 1993/94 on average, each registered chiropractor saw approximately 734 discrete patients. In 2000/01, this number had decreased to approximately 683 patients per registered chiropractor. However, the annual number of services per 1000 population per chiropractor did not change at the same rate. Rather, we observed a small decrease in 1998 followed by a stabilization that approximates the pre-1998 rates thereafter. The sharp increase and subsequent drop in the VBA incidence rate for Saskatchewan was not associated with a similar increase in chiropractic utilization (Figure 1).

The pattern of chiropractic utilization was substantially different in Ontario than in Saskatchewan (Figure 2). The rate of chiropractic utilization was smaller in Ontario than in Saskatchewan. We observed a steady decrease in chiropractic utilization from 1993 to 2002. In 1993/94 on average, each chiropractor saw approximately 535 discrete patients. In 2000/01, this number had decreased to approximately 444 patients per chiropractor.


We measured the incidence rate of VBA stroke in 2 Canadian provinces. Overall, the incidence was similar in Ontario and Saskatchewan ranging from 7.5 to 8.6 per 1,000,000 person-years, respectively. In both provinces, there was a spike in the annual incidence rate of VBA in 2000. The annual incidence rates ranged from 0.48 to 1.05 per 1,000,000 person-years in Ontario and the rate ranged from 0.39 to 1.79 per 1,000,000 person-years in Saskatchewan. Chiropractic utilization differed between the 2 provinces. In Ontario, there was a steady decrease in the utilization rate overtime whereas, Saskatchewan experienced fairly stable rates.

The incidence rates reported for Saskatchewan in our study are artificially increased because of the spike in the annual incidence from 1999 to 2002. Despite this, the incidence rates reported in our study are slightly lower than the rates reported by Lee et al, in Olmstead County, Minnesota, and Rochester, Minnesota for the period 1995 to 2003.1 The hospitals in the state of Minnesota may have had greater availability and accessibility to diagnostic imaging than the provinces in Canada which may have resulted in increased reporting of VBA. In Canada, VBA may have been misclassified as another type of stroke because of the problems associated with accurately diagnosing this type of stroke.6

A large increase in the annual incidence rate of VBA stroke was found in Saskatchewan and a smaller increase in Ontario after 1999. This increase subsequently dropped in 2002 for Ontario and in 2003 for Saskatchewan. Such a large population increase in the incidence rate of noninfectious and rare health problems is puzzling. Four possible hypotheses were considered to explain this large increase in incidence: (1) changes in diagnostic methods; (2) changes in diagnostic coding; (3) a dramatic increase in the prevalence of a risk factor; and (4) change in diagnostic/reporting behaviors.

The first 3 hypotheses unlikely account for the increase and the subsequent decrease in the VBA incidence rate. Firstly, between 1998 and 2000, there were no significant technological improvements in the diagnostic methods or procedures used to diagnose VBA strokes that could account for observed increase in incidence. Secondly, there were no changes in the diagnostic codes used to report VBA strokes during the study period. The introduction of the ICD-10-CA codes for the provinces occurred slightly later. It was first introduced for some hospitals in Saskatchewan on April 1, 2001 and all Saskatchewan hospitals were using it by April 1, 2002. Ontario hospitals were using ICD-10-CA in 2002. Thirdly, an increase in the prevalence of risk factors for stroke could not subsequently explain the drop in the VBA incidence rate after the 2000 spike. The fourth hypothesis may be the most plausible. The 2 highly publicized deaths after chiropractic neck manipulations may have influenced the diagnostic and reporting behaviors of physicians resulting in a reporting bias.2 A reporting bias occurs when patients are examined differently depending on the exposures they had before their stroke. In this case, it is possible that physicians may have focused their diagnostic investigation on VBA stroke in patients who had recently reported seeing a chiropractor before their hospital admission. This could result in more cases being diagnosed than what would have been diagnosed if the physicians were following their usual standard of care. Another possibility is that ischemic strokes that were considered “probable,” or “unlikely” VBA stokes before 1998 to 2000 may have been coded as VBA strokes during that period.

Under the assumption that chiropractic manipulation to the cervical spine is a risk factor for VBA stroke, it is possible that the increase in incidence may have been related to an increase in the utilization of chiropractic services. However, 2 observations do not support this hypothesis. First, we found that the incidence rate of VBA strokes was similar in Saskatchewan and Ontario even though chiropractic utilization was 10 times higher in Saskatchewan than in Ontario. Second, the sharp increase in the rate of VBA stroke occurred despite a decrease in chiropractic utilization in Ontario. In Saskatchewan, the sharp increase in the incidence of VBA strokes occurred whereas the chiropractic utilization remained fairly stable. These findings are in agreement with the results of Cassidy et al who found that there was no significant added risk associated with antecedent exposure to a chiropractor before VBA compared against the risk associated with exposure to a primary care physician in the ambulatory setting.7

Provincial payments of chiropractic services differed between the 2 provinces. Residents of Saskatchewan may have been required to pay additional fees above the government share for their chiropractic care unless covered by supplementary health benefits, and there are no limits to the number of encounters they may have in a given fiscal year. Ontario residents also had to make a copayment, but the provincial government placed a cap on the number of visits an individual may make, after which patients were responsible for the entire fee and the visit may not be captured in the administrative database. The number of visits was capped at 22 per year, until the fiscal year of 1999/2000 when it was capped at $150 per year (i.e., around 15 visits). This cap in coverage would have resulted in the under-reporting of the number of chiropractic services in Ontario. This may partly explain the drop in utilization rate in 1999.

This study is ecological in nature and is therefore liable to ecological bias. Therefore, we cannot draw inferences at the individual level on the association between chiropractic utilization and the risk of VBA stroke. We are unable to determine the exact cause of the increase in the annual incidence rate during the latter part of the 90s. We can only hypothesize that it may have resulted from the increased awareness of the “potential” risk of stroke associated with chiropractic care.

Another limitation of our study is that many strokes are likely misclassified in the administrative data.6 This may explain the differences in the incidence rates with the American study.1

The strength of our study is that the administrative database used in the analysis captures all discharge diagnoses for all residents of the 2 provinces. It also involved using over 10 million person-years of data from Saskatchewan and slightly over 109 million person-years of data from Ontario to calculate the cumulative incidence rate.


The annual incidence rate of VBA strokes was fairly stable in Ontario and Saskatchewan between 1993 and 2004, except for 2000 when a sharp increase was observed. The increase in the incidence rate of VBA stroke could not be explained by a proportional increase in exposure to chiropractors at the ecological level of analysis. It may have been a reporting bias, influenced by media attention resulting from a coroner’s inquest into a death from VBA stroke after chiropractic care.

Key Points

  • The annual incidence rate of VBA strokes for both provinces exhibited a spike in 2000.
  • In Ontario, chiropractic utilization gradually decreased between 1993 and 2002.
  • In Saskatchewan, chiropractic utilization dropped in 1998 and remained stable thereafter.
  • At the ecological level, chiropractic utilization was not associated with the increased incidence rate of VBA.


The authors would like to acknowledge the members of the Decade of the Bone and Joint 2000–2010 Task Force on Neck Pain and its Associate Disorders for their advice on the manuscript. They also would like to acknowledge Dr. Carlo Ammendolia and Dr. Bob Haig who provided them with the Ontario chiropractic-related data and information on the billing restrictions.


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5.Saskatchewan Health Medical Services Branch. Services, Payments, Patients and Doctors by Service Code, by Type of Practitioner. Complete Chiro-Opto Program. Internal Documents prepared quarterly and annually—1990–91 to 2006–07. Provided to the authors by the Chiropractic Association of Saskatchewan 2007.
6.Liu L, Reeder B, Shuaib A, et al. Validity of stroke diagnosis on hospital discharge records in Saskatchewan, Canada: implications for stroke surveillance. Cerebrovasc Dis 1999;9:224–30.
7.Cassidy JD, Boyle E, Côté P, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population-based case control and case-crossover study. Spine 2008;33(Suppl)S176–183.

incidence; vertebrobasilar artery stroke; population-based study; bias; chiropractic utilization

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