The Cassidy Paper infamously claims to show that chiropractic doesn’t cause strokes, but the data doesn’t actually support that conclusion

PainSci » bibliography » Cassidy et al 2008
updated Apr 16, 2025

Tags: chiropractic, harms, quackery, neck pain, headache, classics, manual therapy, professions, treatment, spine, anatomy, pain problems, head/neck, spine pain, head, research, scientific medicine, pro

Three pages on PainSci cite Cassidy 2008: 1. What Happened To My Barber? 2. The Chiropractic Controversies 3. Does Spinal Manipulation Work?

PainSci notes on Cassidy 2008:

Since its publication, “the Cassidy paper” has been the defensive citation used by chiropractors to respond to accusations that neck adjustments involve a risk of stroke, and therefore should not be conducted without proven benefit and informed consent. The abstract seems to strongly exonerate chiropractors: “We found no evidence of excess risk of VBA stroke associated with chiropractic care compared to primary care.”

“But abstracts are like movie trailers,” as Dr. Mark Crislip wrote. “They give a flavor of the movie, but often leave out many important plot devices and characters. … If you were to read this article in its entirety, you would not be so sanguine about the safety of chiropractic.” He goes on to explain exactly why in one of the earliest popular posts on ScienceBasedMedicine.org, Chiropractic and Stroke: Evaluation of One Paper.

What chiropractors take from the paper is that the strokes that follow cervical spinal manipulation were already in progress, causing the symptoms that inspired the appointment. That is possible, but it is only one possibility, and the paper’s abstract emphasizes this hypothesis like it’s a conclusion (and most people stop reading when they are told what they want to hear). That hypothesis is not supported by the data Cassidy et al. pulled together (which has some key flaws). And there are other ways to explain the data. Dr. Crislip’s conclusion “from reading the paper in its entirety, rather than the abstract”:

A population that should not have a stroke, the young, has a marked increase association with stroke 24 hours after visiting a chiropractor and that given the rarity of a vertebral artery dissection as a cause of stroke in the elderly, the elderly is not a group that one could easily find an increase in stroke after chiropractor visit.

original abstract †Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

STUDY DESIGN: Population-based, case-control and case-crossover study.

OBJECTIVE: To investigate associations between chiropractic visits and vertebrobasilar artery (VBA) stroke and to contrast this with primary care physician (PCP) visits and VBA stroke.

SUMMARY OF BACKGROUND DATA: Chiropractic care is popular for neck pain and headache, but may increase the risk for VBA dissection and stroke. Neck pain and headache are common symptoms of VBA dissection, which commonly precedes VBA stroke.

METHODS: Cases included eligible incident VBA strokes admitted to Ontario hospitals from April 1, 1993 to March 31, 2002. Four controls were age and gender matched to each case. Case and control exposures to chiropractors and PCPs were determined from health billing records in the year before the stroke date. In the case-crossover analysis, cases acted as their own controls.

RESULTS: There were 818 VBA strokes hospitalized in a population of more than 100 million person-years. In those aged <45 years, cases were about three times more likely to see a chiropractor or a PCP before their stroke than controls. Results were similar in the case control and case crossover analyses. There was no increased association between chiropractic visits and VBA stroke in those older than 45 years. Positive associations were found between PCP visits and VBA stroke in all age groups. Practitioner visits billed for headache and neck complaints were highly associated with subsequent VBA stroke.

CONCLUSION: VBA stroke is a very rare event in the population. The increased risks of VBA stroke associated with chiropractic and PCP visits is likely due to patients with headache and neck pain from VBA dissection seeking care before their stroke. We found no evidence of excess risk of VBA stroke associated with chiropractic care compared to primary care.

original abstract †Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

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