Cassidy JD, Boyle E, Côté P, He Y, Hogg-Johnson S, Silver FL, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population-based case-control and case-crossover study. Spine. 2008 Feb 15;33(4 Suppl):S176–83.
Cassidy et al find there is an ASSOCIATION between chiropractic care and stroke as have other studies found before it, however this study also looked at the association between stroke and seeking care with a medical physician. They found that the odds of having a stroke after seeing a DC and after seeing a MD were identical.
Why? Because patients are showing up in their medical practitioners office with a stroke in progress. Now what happens if you die in the MD's office Vs. the DC's office? There is a natural bias against the DC because his office visits are supposed to be "routine" where as an MD is supposed to see "Life and Death". Hence the bias against Chiropractic in these unfortunate cases.
Here are some further studies showing unfortunately that many of these patients with strokes are not diagnosed.
Newman-Toker DE, Moy E, Valente E, Coffey R, Hines AL. Missed diagnosis of stroke in the emergency department: a cross-sectional analysis of a large population-based sample. Diagnosis. 2014;1(2).
Murphy DR. Current understanding of the relationship between cervical manipulation and stroke: what does it mean for the chiropractic profession? ChiroprOsteopat. 2010 ed. 2010;18:22.
Furthermore Biomechanical research has shown that cervical manipulation does not remotely come close to straining the arteries any amount that would cause harm and that ROUTINE DAILY activities puts a greater stretch on the arteries than manipulation:
1. Symons B, Herzog W. Cervical artery dissection: a biomechanical perspective. J Can Chiropr Assoc. 2013 Dec;57(4):276–8.
2. Herzog W, Tang C, Leonard T. Internal Carotid Artery Strains During High-Speed, Low-Amplitude Spinal Manipulations of the Neck. J Manipulative PhysiolTher.2012 Nov 6.
3. Herzog W, Leonard TR, Symons B, Tang C, Wuest S. Vertebral artery strains during high-speed, low amplitude cervical spinal manipulation. J ElectromyogrKinesiol.2012 Apr 5.
4. Symons B, Wuest S, Leonard T, Herzog W. Biomechanical characterization of cervical spinal manipulation in living subjects and cadavers. J ElectromyogrKinesiol. 2012 Mar 6.
5. Herzog W. Response to letter to editor by Drs. Haynes and Vincent. J ElectromyogrKinesiol. 2012;22(6):1018.
6. Wuest S, Symons B, Leonard T, Herzog W. Preliminary report: biomechanics of vertebral artery segments C1-C6 during cervical spinal manipulation. J Manipulative PhysiolTher. 2010 May;33(4):273–8.
7. Austin N, DiFrancesco LM, Herzog W. Microstructural damage in arterial tissue exposed to repeated tensile strains. J Manipulative PhysiolTher. 2010 Jan;33(1):14–9.
8. Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative PhysiolTher. 2002 Oct;25(8):504-10.
And finally another series of studies on dogs with man made injuries to the vertebral arteries found that manipulation was incapable of expanding the size of those injuries. This suggests that manipulation doesn’t worsen a dissection.
1. Kawchuk GN, Jhangri GS, Hurwitz EL, Wynd S, Haldeman S, Hill MD. The relation between the spatial distribution of vertebral artery compromise and exposure to cervical manipulation. J Neurol. 2008 Mar;255(3):371–7.
2. Wynd S, Anderson T, Kawchuk GN. Effect of cervical spine manipulation on a pre-existing vascular lesion within the canine vertebral artery.Cerebrovasc Dis. 2008;26(3):304–9.
3. Kawchuk GN, Wynd S, Anderson T. Defining the effect of cervical manipulation on vertebral artery integrity: establishment of an animal model. J Manipulative PhysiolTher. 2004 Nov-Dec;27(9):539–46.
So the research shows that people are likely having dissection events heading into their practitioners office, that the manipulation by the practitioner can not cause dissection on a healthy individual, and that an event in progress is not made worse by cervical manipulation.