Chiropractic care and the risk of vertebrobasilar stroke

Research published today has found no significant association between chiropractic visits and vertebrobasilar stroke. In this guest post, Stephen Perle from University of Bridgeport discusses the findings and their limitations.

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The association between vertebrobasilar artery system (VBA) stroke and cervical spinal manipulative therapy (C-SMT) is a controversial topic and it evokes strong emotions in some.

Damage to the VBA system usually leads to major disability or death. Vertebrobasilar stroke carries a mortality rate of more than 85%. Because it involves the brainstem and cerebellum, most survivors have multisystem dysfunction, such as quadriplegia or hemiplegia, ataxia, dysphagia, dysarthria, gaze abnormalities, and cranial neuropathies.

VBA cases are rare, which means that more is unknown than is known.  The knowledge vacuum magnifies the value attached to anecdotes, which are prone to the logical fallacy of post hoc ergo propter hoc (after this, therefore because of this).

The literature has many case studies documenting VBA stroke following violent neck movements that apply unusual forces to the neck. So the biological plausibility for VBA stroke following forceful neck manipulation is reasonable.

However, from a research point of view the condition’s small incidence means that inevitably the most commonly used method to establish causation is the case control study. Case control studies are usually retrospective and are known for their bias including recall bias.

In a study published today, Kosloff and colleagues have analyzed the largest health insurance data set (both commercial insurance and Medicare Advantage (MA) plans) used to investigate the association between chiropractic visits and stroke.

Data from approximately 5% of the US population, over 39 million persons from 49 of the 50 US states (only North Dakota was excluded) were used.  Three years’ worth of data were searched to find cases, which were all patients admitted to an acute care hospital with VBA occlusion and stenosis strokes (chosen by ICD-9 codes).

Four age and gender matched controls were randomly selected. Exposures were encounters with either a chiropractor or a primary care physician (in the US a medical doctor who is typically an internist or family practitioner) prior to the VBA stroke.

The cases included 1,159 VBA strokes in the commercial health plan and 670 in the MA plan. Consistent with previous research, no significant association was found between chiropractic visits and VBA strokes in the older population. However, contrary to other case control studies Kosloff and colleagues also found no association between chiropractic visits and VBA strokes.

The authors acknowledge certain limitations of their study due to the nature of insurance claims data. These data do not code for what specific treatment was rendered or immediate responses to treatment. Thus it is not known if chiropractic manipulation was performed during any office visit and if there was any immediate adverse response. Further, the accuracy of the VBA stroke diagnoses is unknown. Finally as the authors note there is a loss of “contextual information surrounding clinical encounters between chiropractors and Primary Care Physicians and their patients.”  This limits the knowledge of other known risk factors.

The authors’ conclusions are correctly reserved given the limitations. The dataset does not find a significant association between chiropractic manipulation and VBA stroke, thus adding weight to the view that chiropractic care is an unlikely cause. But this finding does not exclude the possibility that chiropractic manipulation might have some role in causation.

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David E Thaler

Cassidy and colleagues, studying a Canadian population, showed an association between posterior circulation stroke and spinal manipulative therapy (SMT) and between posterior circulation stroke and visits to primary care physicians (1). Kosloff et al have repeated the Cassidy study in an American population so to generalize the original Canadian observations. We have shown that the ICD-9 codes used by Cassidy to define cases have a positive predictive value for cervical arterial dissection (CAD) of only around 11% when compared to neurologist review of the medical record and neuroimaging (2). A small fraction of the “cases” identified by Cassidy’s method had the actual disease of interest (CAD, with or without stroke). Instead they likely had posterior circulation disease of a more common sort (lacunes, atherosclerosis, embolism). Cassidy’s case-identification strategy induced significant outcome misclassification that biased estimates of the SMT-CAD association towards the null.
Kosloff and colleagues have made the same mistake but then compounded the error by systematically excluding patients with dissection from their population. In Ontario, at the time of the Cassidy study, ICD-9 codes specific for dissection (443.XX) were not in use. Patients in Canada, clinically diagnosed with dissection, would have been coded with a posterior circulation code included in the Cassidy study, and so would have met inclusion criteria for his study. In the US, the dissection-specific codes are in widespread use. Patients with clinical CAD diagnoses would have been most accurately coded with a 443.XX code and not with the anatomically-based posterior circulation codes. Kosloff did not include the dissection codes as part of the case definition and so patients with CAD were systematically excluded. It follows that the positive predictive value of the Kosloff CAD identification strategy is likely to be even lower than that which was observed by Cassidy.
Prior studies suggest vascular risk factors are not associated with CAD risk (3). That Kosloff’s subjects had non-CAD-related stroke is suggested by the higher prevalence of cardiovascular risk factors in the case group (Table 3). The depletion of dissections from the Kosloff population also explains why no association between SMT and case-status was observed even in the younger (<45y) group – a finding which has been consistently seen in other studies, including Cassidy’s, with statistically and clinically significant odds ratios.
Kosloff et al define their cases as “vertebrobasilar stroke” but then draw inferences about CAD. This is an error in logic that ignores the heterogeneity of stroke subtypes. In addition to outcome misclassification, epidemiologic studies of SMT as a trigger of CAD face other methodological challenges – low CAD incidence rates, confounding, reverse causation, misclassification of SMT exposure (including recall bias), and selection bias. Rigorously designed studies that address these hurdles are needed to provide quality evidence to resolve this critical public health question. Unfortunately, Kosloff’s study does not provide such data.
REFERENCES
(1) Cassidy JD, Boyle E, Cote P, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population based case-control and case-crossover study. Spine 2008; 33(4 Suppl):S176-S183.
(2) Cai X, Razmara A, Paulus JK, Switkowski K, Fariborz PJ, Goryachev SD, D’Avolio L, Feldmann E, Thaler DE. Case Misclassification in Studies of Spinal Manipulation and Arterial Dissection. Journal of Stroke and Cerebrovascular Diseases, 2014, 23(8): 2031-2035, doi: 10.1016/j.jstrokecerebrovasdis.2014.03.007
(3) Sidney M. Rubinstein, Saskia M. Peerdeman, Maurits W. van Tulder, Ingrid Riphagen, and Scott Haldeman. A Systematic Review of the Risk Factors for Cervical Artery Dissection. Stroke. 2005; 36:1575-1580, doi:10.1161/01.STR.0000169919.73219.30

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Nigus Tyler

It’s already proven that there is no significant association between chiropractic visits and vertebrobasilar stroke. Stephen Perle do a massive research about this report. Chiropractic care is there to help to eases the back pain at the same time there is no risk from other parts of the body.

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