Cervical Spine HVLA and risk of stroke

I recently posted this cartoon around various social media platforms. It riled a few Chiropractors, with good reason too. I’d like to qualify my position by stating that the HVLA to the cervical spine isn’t necessarily any less safe when a Chiropractor performs it, compared to say, a Physio, or some other manual therapist. I suspect there would be a great deal of variation across and within professions in manipulation “style”, as well as variation in factors such as direction and intensity of force applied.

I put all that to side though, because the main purpose of this cartoon was to demonstrate that a particular piece of research that is often held up by Chiropractors as proof that neck manipulation is safe, contains some signs of bias with regards to data interpretation. This appears to influence the conclusion of the paper, which is a particularly appealing conclusion to those who advocate for HVLA of the neck. The same criticism could be leveled at any other manual therapist who holds up this particular study as a defence of the safety of HVLA thrusts to the cervical spine.

The paper in question is this one. RISK OF VERTEBROBASILAR STROKE AND
CHIROPRACTIC CARE: Results of a Population-Based Case-Control and Case-Crossover Study. 2008. J. David Cassidy, DC et al. This paper was critiqued by Mark Crislip on Science Based Medicine. I found it useful but I struggled with some of the statistical jargon. So I went back to the original article, have brushed up on my understanding of case control studies, crossover studies and odd ratios… and have done my best to explain, in simple terms, what this study was attempting to demonstrate, how the data produced might be differently interpreted, and how this might lead to a different conclusion than the published one. The purpose of my cartoon was really to stimulate some debate. The purpose of this post is to help me and any readers understand the paucity of cause/effect data for a rare event like a stroke from a cervical spine manipulation… and to understand how, in the absence of such cause/effect evidence, we need to look to an assessment of risks and benefits during clinical decision making. More specifically, the purpose is to point out that this paper by Cassidy et al (2008), can not and should not be held up as evidence that that HVLA thrusts to the cervical spine carry a negligible risk that is outweighed by purported benefits.

In the closing paragraph of the introduction of the paper, Cassidy et al (2008) write;

“The purpose of this study is to investigate the association between chiropractic care and VBA stroke and compare it to the association between recent PCP care and VBA stroke using two epidemiological designs. Evidence that chiropractic care increases the risk of VBA stroke would be present if the measured association between chiropractic visits and VBA stroke exceeds the association between PCP visits and VBA strokes.”

Basically, they took a whole bunch of medical records of people who had a VBA stroke, and then had a look at whether or not they had been to see a Chiropractor or a General Practitioner (GP) in the period of time preceding the stroke. The presumption is that a visit to the doctor would never cause a stroke (since a GP would not be providing HVLA thrusts to patient’s necks). Therefore, if Chiropractic treatment was causing VBA strokes, the analysis of these medical records should reveal a higher association between the incidence of VBA stroke and the utilisation of Chiropractic services compared with the association between VBA stroke and the utilisation of GP services (which have a zero risk of stroke via HVLA thrust to the neck- since GPs don’t carry out such treatment).

It is a bit like taking a bunch of boxers with bruised faces, and comparing the association between having participated in a boxing match and having a bruised face, with the association between visiting a GP and having a bruised face. What the authors essentially argue is that if the association between having a bruised face and being in a boxing match, is not greater than the association between having a bruised face and visiting your doctor, then we should not conclude that those who participate in boxing are at a high risk for getting a bruised face i.e we should conclude that boxing doesn’t cause face bruising, but rather, whatever does cause the face bruising, drives boxers to either participate in boxing, or visit a GP.

I don’t think it is difficult to understand that if / when an association between a bruised face and visiting your doctor is equal to the association between participating in a boxing match and having a bruised face, we cannot start claiming that boxing and visiting your doctor are equally risky for getting a face bruise. And so it is with VBA strokes and the association between visiting a GP or Chiropractor. An equivalent association between a stroke and visit to the Chiropractor, and the association between a stroke and a visit to a GP, is not a reasonable basis upon which to claim that there is a common cause of stroke that precedes, and drives patients to seek care from either a Chiropractor (or any manual therapist) or a GP. This is exactly the assumption that Cassidy et al (2008) make in their paper. Their conclusion;

“we found a similar association between primary care physician visits and VBA stroke. This suggests that patients with undiagnosed vertebral artery dissection are seeking clinical care for headache and neck pain before having a VBA stroke”.

is based on an assumption that isn’t supported by the data produced by their work. It is a hypothesis to explain the data. We can’t conclude from this data that associations between VBA stroke and chiropractic visits, and VBA strokes and GP visits are likely due to patients with headache and neck pain from VBA dissection seeking care before their stroke.

An alternative interpretation of the data, is provided by Crislip in his critique of this article.

“Like the authors, it is neither proven nor disproved by the data, but I think has more plausibility than their explanation.

There is a baseline number of VBA strokes in a population. When patients have serious symptoms, they tend to preferentially seek care with MD’s rather than alternative providers. The group patients who sought care from their primary providers were indeed having incipient VBA strokes. It is why they went to the doctor.

The other group, who were not having an incipient VBA stroke (suggested by the codes for chiropractor visits) had neck manipulation and VBA strokes were induced in this population to a the rate equal to or exceeding the rate of the PMD group.

The study could be interpreted that visits to a chiropractor for neck manipulation induce strokes at a rate equal to or greater than the control population ie those presenting with stroke.

Again: a hypothesis. It explains the data as well as the authors, and with more plausibility when viewed in the light of anatomy and physical forces to the neck, other uncontrolled clinical observations :), and prior epidemiologic studies of the association for manipulation and stroke.”

The real risk of stroke from a cervical spine HVLA is not measurable from this paper. It stands to reason that this paper should not be held up as a defence of the safety of HVLA thrust procedures to the cervical spine. It also stands to reason that Crislip’s hypothesis is, as he wrote, neither proved nor disproved by the data.

So what is the risk? And is it worth it? What benefit do we get from an HVLA to the neck? Personally, I have had my neck cracked many times over the years. Often I’ve had instant relief of acute neck pain. That instant relief is a powerful incentive for both patient and treatment provider. There may even be a scenario where a person, knowing the risk, still wants to take the chance on getting the instant relief, so that they can, I don’t know, be right for a big sports performance, or make an important flight, or something. As far as the risk of stroke from a HVLA to the neck goes, an explanation of the benefits against the risks is key. The only benefit I can ever see is that the patient might feel instantly better, as opposed to feeling better a few days later (in the case of an acute mechanical strain to the neck). To me that benefit isn’t worth it. I no longer offer HVLA thrusts to the cervical spine. I no longer see how the benefits can ever be seen to outweigh the risk of stroke, even if it is small. I also find it frustrating that, while the jury is still out on the risk of a stroke from a HVLA to the cervical spine, the default position for many health providers is to simply continue offering it.

There is also a persistent argument from advocates that the expert manipulator is a safe manipulator, compared to the novice. To this I always wonder, who is the poor guy who gets to be the guinea pig while a novice therapists “learns” how to “safely” crack a neck? Should we send our student manual therapists to the jails to practice on criminals? How do we decide who to expose to an unnecessary risk of stroke?

There are other risks to consider too. They are perhaps more subtle, and relate to the entrenched culture of paternalism, medicalization and loss of self efficacy in the delivery of manual therapy. Reducing the use of theatrical interventions such as HVLAs might help address the other risks. But that is a discussion for another day.


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