CMT & Stroke Risk: Myth vs. Fact

By now, most of you have probably heard that the American Heart Association recently published a statement regarding the association between cervical dissection (CD) and cervical manipulative therapy (CMT). The Aug. 7, 2014 statement, accompanied by a white paper on the topic, was endorsed by both the American Association of Neurological Surgeons and the Congress of Neurological Surgeons. The statement reads as follows:

"CD is an important cause of ischemic stroke in young and middle-aged patients. CD is most prevalent in the upper cervical spine and can involve the internal carotid artery or vertebral artery. Although current biomechanical evidence is insufficient to establish the claim that CMT causes CD, clinical reports suggest that mechanical forces play a role in a considerable number of CDs. Most population controlled studies have found an association between CMT and VAD stroke in young patients. Although the incidence of CMT-associated CD in patients who have previously received CMT is not well established and probably low, practitioners should strongly consider the possibility of CD as a presenting symptom, and patients should be informed of the statistical association between CD and CMT prior to undergoing manipulation of the cervical spine."¹

Based upon our own initial strong response to this statement, we want to take a moment to remind everyone about the most recent article we wrote for this column, in which we talked about the need for doctors of chiropractic to be evidence-based in our responses to derogatory comments and/or articles written about the profession.² Key recommendations that apply to the present conservation included the following:

• Take a day or two to calm down before getting out your pen!
• Ask yourself, what is the author's claim and is it supported by data?
• Consider whether you have data to support a counterclaim.
• Assess your response as critically as you have assessed the original article.

The AHA statement presents a perfect opportunity to put this advice into practice. We have already calmed down, so the next step is to review the author's claims and ask if each claim is supported by data (i.e., true or false) and whether there is data to support a counterclaim.

Analyzing the AHA Statement

"CD is an important cause of ischemic stroke in young and middle-aged patients." This statement is supported by the data. Although rare, epidemiological studies estimate the incidence of internal carotid artery dissection at approximately 2.6/100,000 and vertebral artery dissection at approximately .97/100,00.³

"CD is most prevalent in the upper cervical spine and can involve the internal carotid artery or vertebral artery." This statement is well-supported by data. Internal carotid artery dissection is more common than vertebral artery dissection.⁴

"Although current biomechanical evidence is insufficient to establish the claim that CMT causes CD..." This statement is misleading, at best. It is not that current biomechanical evidence is insufficient to establish the claim, but rather that every study designed to address this issue has shown that it is very unlikely that CMT causes CD.

In the AHA white paper associated with their statement, several studies are referenced that evaluated CMT and cervical biomechanics. A small number of imaging studies have examined arterial blood flow to see if the strain placed on the arteries during a cervical adjustment decreases blood flow at a level that would cause arterial dissection.^5-7 Not a single study indicated this is the case. An additional study on cadavers also showed that pressure on the vertebral arteries resulting from CMT is not sufficient to cause a dissection.⁸

"Clinical reports suggest that mechanical forces play a role in a considerable number of CDs." This statement should not be considered evidence-based for several reasons. First, clinical or case reports are considered to be a relatively low level of evidence,⁹ and for very good reason. It is not possible to establish a causal relationship from clinical reports. Second, we know from the biomechanical research referenced above that the mechanical forces associated with CMT are unlikely to be the cause of CDs. Finally, the phrase considerable number is disappointingly vague and therefore essentially meaningless.

"Most population-controlled studies have found an association between CMT and VAD stroke in young patients." This statement is supported by the data.¹⁰

"Although the incidence of CMT-associated CD in patients who have previously received CMT is not well established..." We do not have really good data on how many people who experience a CD previously received CMT. At the same time, it is misleading to include the phrase CD-associated, as it implies causation. As stated in Wikipedia, "a correlation [or in this case, an association] between two variables does not necessarily imply that one causes the other."

In fact, the most definitive study to date, published in Spine by Cassidy, et al.,¹⁰ found that while there is an association between CMT and CD, a similar association exists between a visit to a family-practice medical physician and CD. In other words, a person is as likely to have gone to a medical doctor as a doctor of chiropractic in the week before experiencing a CD. Since MDs are not performing CMT, a logical hypothesis is that people with symptoms of a CD seek treatment from a health care provider.

"...and probably low..." There is no probably about it: The incidence of CMT-associated CD in patients who have previously received CMT is very, very low. Approximately 8 percent of U.S. adults receive spinal manipulation every year.¹¹ We don't know exactly how many of these patients receive cervical manipulation, but it is safe to say it is a "considerable number" (in the millions). The relatively high prevalence of CMT use must be contrasted with the very low incidence of CD. Cassidy, et al., identified 1,818 strokes in 100-million person years.¹⁰ A more recent study was able to confirm 388 CDs within a population of 16 million.¹²

"...practitioners should strongly consider the possibility of CD as a presenting symptom..." This is absolutely true. Two common symptoms are unilateral neck pain and headache.⁴

"...and patients should be informed of the statistical association between CD and CMT prior to undergoing manipulation of the cervical spine." By virtue of our chiropractic license, we, as is true for all health care providers, have an obligation to inform patients of any known risk associated with any procedure we perform. However, health care practitioners also have an obligation to ensure the information they present to patients in the context of informed consent is based upon the best evidence available.

Currently, the best basic science evidence available indicates that the strains placed on the vertebral artery during CMT are unlikely to cause a stroke, and the best clinical evidence available shows that a person is as likely to have seen a primary care medical physician as a doctor of chiropractic in the seven days prior to experiencing a CD.

A Missed Opportunity

As we critically assess our response, it is our opinion that the AHA statement mixes scientific facts with half-truths and misleading statements, leading people to ultimately arrive at the erroneous conclusion that it has been established CMT causes CD. If you look at the newspaper and blog headlines generated by this statement (e.g., "How Safe Are the Vigorous Neck Manipulations Done by Chiropractors?"¹³ and "Chiropractic Manipulation of Neck: Stroke Risk?"¹⁴), you realize this is precisely what happened when the statement was released.

The AHA white paper lists several events that are associated with CD. These include major and minor cervical trauma, use of oral contraceptives, sporting activities, stretching the neck, some neck movements, violent coughing or vomiting, and visiting a health care provider who administers spinal manipulation. Yet for some reason, the AHA chose to focus its statement on the single association within that list for which there is the strongest evidence against a causal relationship.

By concentrating exclusively on the purported risk of CD following CMT, the AHA has missed an invaluable opportunity to educate both patients and practitioners of the incidence, warning signs, and broad range of factors associated with CD. We challenge the chiropractic profession to step up to the plate and take on this important task.

References

1. Biller J, Sacco R, Albuquerque F, et al. AHA/ASA Scientific Statement. Cervical Arterial Dissections and Association With Cervical Manipulative Therapy: A Statement for Healthcare Professionals From the American Heart Association / American Stroke Association, 2014.
2. Lawrence D, Goertz C. "How to Respond When the Media Criticizes Chiropractic: Do's and Don'ts." Dynamic Chiropractic, July 1, 2014.
3. Lee VH1, Brown RD Jr, Mandrekar JN, Mokri B. Incidence and outcome of cervical artery dissection: a population-based study. Neurology, 2006 Nov 28;67(10):1809-12.
4. Schievink W, Mokri B, O'Fallon W. Recurrent spontaneous cervical-artery dissection. N Engl J Med, 1994;330:393-397.
5. Creighton D, Kondratek M, Krauss J, et al. Ultrasound analysis of the vertebral artery during non-thrust cervical translatoric spinal manipulation. J Man Manip Ther, 2011 May;19(2):84–90.
6. Haynes M, Milne N. Color duplex sonographic findings in human vertebral arteries during cervical rotation. J Clin Ultrasound, 2001 Jan;29(1):14-24.
7. Bowler N, Shamley D, Davies R. The effect of a simulated manipulation position on internal carotid and vertebral artery blood flow in healthy individuals. Man Ther, 2011 Feb;16:87–93.
8. Symons BP1, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manip Physiol Ther, 2002 Oct;25(8):504-10.
9. Aslam S, Georgiev H, Mehta K, Kumar A. Matching research design to clinical research questions. Indian J Sex Transm Dis, 2012 Jan-Jun;33(1):49-53.
10. Cassidy JD, Boyle E, Côté P, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population-based case-control and case-crossover study. Spine, 2009 Feb;32(2 Suppl):S201-8.
11. Barnes PM1, Bloom B, Nahin RL. Complementary and alternative medicine use among adults and children: United States, 2007. Natl Health Stat Report, 2008 Dec 10;(12):1-23.
12. Cai X, Razmara A, Paulus K, et al. Case Misclassification in studies of spinal manipulation and arterial dissection. J Stroke Cerebrovasc Diseases, 2014.
13. Berger S. "How Safe Are the Vigorous Neck Manipulations Done by Chiropractors?" The Washington Post, Jan. 6, 2014.
14. Thompson D. "Chiropractic Manipulation of Neck: Stroke Risk?" WebMD, Aug. 7, 2014.

Editor's note: This article is the latest contribution to Drs. Goertz and Lawrence's column, "Research: Not Just for Scientists Anymore." The column includes an affiliated blog where DCs can post questions for the authors.