Research Review by Dr. Michael Haneline©

Date:

Jan. 2008

Study Title:

Risk of Vertebrobasilar Stroke and Chiropractic Care Results of a Population-Based Case-Control and Case-Crossover Study

Authors:

Cassidy JD et al.

Authors’ Affiliations:
  • Centre of Research Expertise for Improved Disability Outcomes, University Health Network Rehabilitation Solutions, Toronto Western Hospital, and the Division of Heath Care and Outcomes Research, Toronto Western Research Institute
  • Department of Public Health Sciences, Management and Evaluation, University of Toronto
  • Department of Health Policy, Management and Evaluation, University of Toronto

Publication Information:

Spine, 2008; 33(4S): S176–S183.

Summary:

Neck pain is a widespread problem that is treated most commonly by medical physicians, physical therapists and chiropractors. Approximately 12% of adults in Canada and the United States (US) visit chiropractors annually, although Canadian chiropractic patients tend to be younger and have fewer health problems than those in the US.

Numerous clinical trials and a few systematic reviews have been carried out which point to the effectiveness of manual therapy, but too few subjects have been included to assess the risk of rare adverse events.

Vertebrobasilar artery (VBA) stroke is a rare neurological event that has been reported to occur very infrequently following cervical manipulation and other neck-related activities, as well as spontaneously. Reports of VBA stroke in association with chiropractic care have mainly been derived from case reports and a few case-control studies. Researchers have therefore questioned whether the relationship is causal, primarily because most of the patients in these studies presented for care complaining of neck pain and headache, which are very common early symptoms of VBA dissection that can lead to VBA stroke.

The purpose of this study was to test the assumption that the reason VBA stroke patients have a higher rate of chiropractic care preceding the event is because they seek care already having VBA-related symptoms. To do this, the authors compared rates among neck pain patients who received chiropractic care versus those who received care by primary care physicians (PCPs). If the rates of care preceding VBA stroke were high in both groups, it would lend support to the assumption.

To strengthen the study’s findings, two different research designs were utilized (population-based case-control and case-crossover designs), both using the same cases. Cases and controls were derived from the same source population, which included all residents of Ontario covered by the publicly funded Ontario Health Insurance Plan (OHIP) over the 9 year period extending from 1993 to 2002. All patients who were hospitalized with VBA strokes during this period were included as cases. For each case, four age and sex-matched controls were randomly selected from the Registered Persons Database. The case-crossover study made comparisons using four control periods of time that were randomly chosen from the year before the date of the VBA stroke.

Cases were considered to be exposed if they had reimbursed ambulatory encounters with chiropractors or PCPs for a neck-related disorder during the one-year period before the index date from the OHIP database.

Pertinent Results:

The study included 818 cases and 3164 matched controls. The mean age of all subjects was 63 years and 63% were male. There was a higher proportion of comorbid conditions (e.g. hypertension, heart disease, and diabetes) among cases.

When considering the overall groups within 30 days of the index date:
  • 4% of cases and controls had visited a chiropractor
  • 53% of cases and 30% of controls had visited a PCP
For subjects who were under 45 years of age, within 7 days of the index date:
  • 7.8% of cases had consulted a chiropractor, compared to 3.4% of controls
  • 24.5% of cases had consulted a PCP, compared to 6.6% of controls
  • overall, there was an increased association between chiropractic visits and VBA stroke regardless of the hazard period
For subjects who were 45 years of age and older:
  • there was no association between chiropractic visits and VBA stroke
There was an increased risk of VBA stroke regardless of the hazard period or the age of the subject in those who visited a PCP in the month prior to the index date.

The results of the case-control and case-crossover analyses were very similar.

Strong associations were observed between PCP visits and subsequent VBA stroke, which the authors suggested was probably related to patients with head and neck pain from vertebral artery dissection who obtained care for these symptoms. This association represents an underlying intrinsic probability of patients with vertebral artery dissection, especially when one considers that it is not plausible that PCP care would be capable of causing a VBA stroke.

The association between chiropractic visits and VBA stroke was not found to be larger than the association between PCP visits and VBA stroke. The authors therefore concluded that there is no excess risk of VBA stroke from chiropractic care.

Limitations to this study include:
  • the use of health administrative data, which may lead to misclassification bias.
  • Not separating VBA strokes into categories of etiology, which is especially important in this context, since dissection-related strokes are mainly what have been ascribed to chiropractic care. The inclusion of cases with VBA strokes due to atherosclerotic thrombi, for instance, would tend to confound the findings of this study. The authors investigated the potential for this bias to influence the study’s conclusions using a sensitivity analysis and did not think it would.
  • Patients who have recently seen a chiropractor and then present to a hospital with neurologic symptoms might receive a more thorough examination focused on VBA stroke. Thus revealing more chiropractic-related cases than PCP patients with subsequent neurological symptoms.

Conclusions & Practical Application:

There is an association between VBA stroke and chiropractic visits in patients under 45 years of age. A similar association exists between VBA stroke and PCP visits in patients from all age groups. Therefore, the results of this study provide no evidence of excess risk of VBA stroke associated with chiropractic care.

The increased risk of VBA stroke that was found to be associated with both chiropractic and PCP visits is probably related to patients seeking treatment of VBA dissection-related neck pain and headache from these providers prior to their VBA stroke.

This study does a very good job of answering the long-standing question about why patients appear to have a higher rate of VBA stroke following chiropractic care, but because strokes have been reported to occur immediately following cervical manipulation, it still appears that manipulation is capable of triggering a stroke in a patient with existing VBA dissection. It is not known from the Cassidy et al study, however, what proportion of reported VBA strokes occurred under these conditions, yet given the data reported for all cases of VBA, it appears to be extremely rare. This would likely mean that any sudden or extreme neck movement in these patients (manipulation or some activities of daily living) is at least theoretically risky after dissection has occurred.

Previous literature has indicated that there are no valid pre-manipulative tests to screen for potential VBA stoke. Practitioners should therefore attempt to identify patients who present with active dissections by being alert for signs of VBA insufficiency, especially in those with a sudden onset of new and unusual headache and neck pain. Neurological signs and symptoms associated with VBA insufficiency include fainting spells, dizziness, diplopia, dysarthria, dysphagia, ataxic gait, nausea, numbness, and nystagmus. Patients with frank neurological manifestations of VBA insufficiency should not receive cervical manipulation, but should be referred for appropriate medical care, sometimes via emergency transport.