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Dynamic Chiropractic Canada – November 1, 2008, Vol. 01, Issue 02
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Evaluating Vertebrobasilar Artery Stroke Risk

By Shawn Thistle, DC, BKin (hons), CSCS

Editor's note: Dr. Shawn Thistle is the editor and occasional author of this column. All content is provided courtesy of Dr. Thistle's Research Review Service. This issue's research summary was written by Dr. Michael Haneline of Palmer Chiropractic College West.

The Study

Title: Examining Vertebrobasilar Artery Stroke in Two Canadian Provinces
Authors: Boyle E, Cote P, Grier AR, Cassidy JD
Authors' Affiliations: The Centre of Research Expertise for Improved Disability Outcomes (CREIDO), University Health Network Rehabilitation Solutions, Toronto Western Hospital Division of Heath Care and Outcomes Research, Toronto Western Research Institute, Toronto, Ontario
Publication Information: Spine, 2008;33(4S):S170-5; discussion, 2379.


Vertebrobasilar artery (VBA) dissection and stroke occur infrequently in the general population. A recent study by Lee, et al.,1 reported that the incidence rate of VBA dissection-related stroke in Rochester, Minn., was only 1.12 per 100,000 person-years. An accurate estimate of the incidence VBA stroke is important because the condition is potentially life-threatening and has been reported to occur following cervical spine manual therapy. The present study had two main objectives:

  1. To determine the annual incidence of hospitalized VBA stroke, as well as the rate of chiropractic utilization, in Saskatchewan and Ontario between 1993 and 2004.
  2. To determine whether the incidence of VBA stroke paralleled the rate of chiropractic utilization at an ecological level (i.e., when considering the whole population) during the same time period.

This was a population-based ecological study that involved residents of Ontario or Saskatchewan who were registered with their respective provincial health insurance plan and had coverage for two years. Eligible cases were defined as follows:

  • All people in Ontario admitted to an acute care hospital with a discharge diagnosis of vertebrobasilar occlusion or vertebrobasilar stenosis stroke.
  • All Saskatchewan Health beneficiaries admitted to a hospital with a discharge diagnosis of vertebrobasilar occlusion or vertebrobasilar stenosis stroke.

Chiropractic utilization rates were calculated based on two methods: the number of patients who had an encounter with at least one chiropractor during the fiscal year; or the total number of services provided by chiropractors during the fiscal year.

Key Findings

  • The incidence nonstandardized rate of VBA strokes for Saskatchewan was 0.855 per 100,000 person-years and 0.750 per 100,000 person-years for Ontario.
  • The yearly incidence rates of VBA strokes were similar in both provinces from 1993 to 1996, except there was a 360 percent increase in the incidence rate in Saskatchewan from 1998 and 2000, followed by a gradual decline during 2002 to 2003 to its pre-1998 level, and there was a smaller increase in the incidence rate in Ontario in 1999 which rapidly returned to its pre-1999 level.
  • The number of people using chiropractic in Saskatchewan actually decreased 12 percent between 1996 and 1999. Thus, there was not an increase in chiropractic utilization to match the sharp increase and subsequent drop in the incidence rate of VBA stroke in Saskatchewan.
  • Although not as dramatic of a contrast, there was a steady decrease in chiropractic utilization in Ontario from 1993 to 2002 that also did not correspond to the increased incidence of VBA stroke that was observed in 1999.

Conclusions and Practical Application

The authors offered four possible hypotheses to explain the sizeable increase in the incidence of VBA stroke that was observed in 1999: changes in diagnostic methods, changes in diagnostic coding, a dramatic increase in the prevalence of a risk factor, and a change in diagnostic/reporting behaviors. The fourth hypothesis was considered to be the most plausible and thought to be attributable to two deaths that occurred in Canada following chiropractic neck manipulations. The deaths were highly publicized in the media and may have influenced the way physicians diagnosed and reported VBA strokes, which resulted in a reporting bias. Later in the paper, the authors hypothesized that the increased incidence of VBA strokes during this time period may have been due to an increased awareness of the potential risk of stroke associated with chiropractic care.

The authors also suggested that the sudden increase in the incidence of VBA strokes plausibly could have been related to an increased utilization of chiropractic services. However, they mentioned several factors that cast doubt on this hypothesis: 

  • The incidence of VBA strokes was similar in both provinces, while chiropractic utilization was 10 times higher in Saskatchewan vs. Ontario during the same time period.
  • There was a decrease in chiropractic utilization in Ontario and fairly constant utilization in Saskatchewan around the time of the increase in the incidence of VBA stroke.

Study Limitations

  1. There might have been an underreporting of the number of chiropractic services in Ontario because of reimbursement caps, which leads patients to obtain care outside the government plan.
  2. The potential for ecological bias, whereby inferences cannot be drawn at the individual level on the association between chiropractic utilization and the risk of VBA stroke because the study only considered population data.
  3. Misclassification of strokes in the administrative data that was used.

This study's strongest finding is that the incidence of VBA stroke was not correspondingly higher in Saskatchewan as compared to Ontario during the study period, even though the rate of chiropractic utilization was 10 times higher in Saskatchewan. If cervical manipulation were a primary cause of VBA stroke, there should have been a corresponding increase in the incidence of the condition in the population in which the rate of manipulation was higher.

The authors pointed out that the sudden spike in the incidence rate of VBA stroke was not explained by a proportional increase in chiropractic utilization, which does not really provide evidence for or against the relationship between chiropractic utilization and an increased incidence rate of VBA. Instead, the increased incidence rate was most likely explained by the previously mentioned reporting bias caused by the media attention surrounding the deaths after chiropractic care that occurred in Canada prior to the spike.


  1. Lee VH, et al. Incidence and outcome of cervical artery dissection: a population-based study.Neurology, 2006;67(10):1809-12.

Dr. Shawn Thistle is founder and president of the Research Review Service (, from which all content for this and other articles by Dr. Thistle is derived. Research Review Service posts approximately 60 reviews like this each year and currently has a database of more than 250 reviews. Dr. Thistle graduated from the Canadian Memorial Chiropractic College, where he has been a faculty member since 2004. He holds an honours degree in kinesiology (McMaster) and a certificate in contemporary medical acupuncture. He is also fully ART-certified and is a certified strength and conditioning specialist. Dr. Thistle practices full time at Shape Health and Wellness Centre in Toronto.

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