SPINE (Phila Pa 1976) 2008 (Feb 15); 33 (4 Suppl): S176–183 ~ FULL TEXT
Cassidy JD, Boyle E, Côté P, He Y, Hogg-Johnson S, Silver FL, Bondy SJ
Centre of Research Expertise for Improved Disability Outcomes,
University Health Network Rehabilitation Solutions,
Toronto Western Hospital,
Toronto, ON, Canada.
The United Nations and the World Health Organization created the Neck Pain Task Force in 2002 as part of the larger “Bone and Joint Decade” project. The findings of this task force was published a supplemental issue of the prestigeous Spine Journal in 2008.
Their findings resulted from a six-year review of more than 31,000 research citations, and a subsequent analysis of over 1,000 studies. The multi-disciplinary report, involving specialists from 19 different areas of expertise, and from eight different collaborating universities in four countries, is widely regarded as one of the most extensive reports on the subject of neck pain ever developed, and it offers the most current perspective on the scientific evidence related to the care and management of neck pain.
One of these research groups concentrated their efforts studying the purported relationship between spinal manipulation and Vertebrobasilar Stroke. In conjunction with the release of the Neck Pain Task Force report, Dr. Cassidy and co-workers published the following article.
This populations-based, case-control, and case-crossover study employed rigorous epidemiological methodology to establish, as best as possible, controls between the two populations studied, i.e.
patients of chiropractors and
patients of primary care providers (PCP).
Out of over one million patient years (the length of time the total number of patients had been under care), 818 hospitalized vertebrobasilar artery stroke patients were identified. In the patients below the age of 45, those experiencing a stroke were three times more likely to have visited either their chiropractor or their PCP than the control patients. Those over 45 showed no increased association between experiencing a stroke and having seen their chiropractor than those experiencing a stroke and having seen their PCP.
The authors concluded:
Vertebrobasilar artery stroke is a rare event in the population.
There is an association between vertebrobasilar artery stroke and chiropractic visits in those under 45 years of age.
There is also an association between vertebrobasilar artery stroke and use of primary care physician visits in all age groups.
We found no evidence of excess risk of VBA stroke associated chiropractic care.
The increased risks of vertebrobasilar artery stroke associated with chiropractic and physician visits is likely explained by patients with vertebrobasilar dissection-related neck pain and headache consulting both chiropractors and primary care physicians before their VBA stroke.
While the findings of this study cast a much more positive light on the chiropractic profession’s use of cervical spinal manipulation than previous studies that have tended to vilify the chiropractor, it must be noted that all treatments, including cervical manipulation, have some inherent risk. The chiropractic profession continues its support of on-going research to determine the full extent of such risk related to cervical manipulation and ways to reduce that risk.
It should also be noted that chiropractic physicians are some of the most well-trained and highly-skilled practitioners of manual medicine, specifically in the use of cervical spine manipulation. The thorough training received by doctors of chiropractic equips individual practitioners in the art and science of diagnosing spine related problems. There are multiple methods doctors of chiropractic can utilize in manipulating or mobilizing troublesome cervical spines, ranging from very low-force to even non-force techniques to the more standard high-velocity, low-amplitude manipulation. Within the profession there are even non-thrust methods that can be applied in special situations.
In this evidence-based health care delivery system that we are now all a part, the work of the Neck Pain Task Force and the work of Cassidy and colleagues provide valued contributions to the overall framework of evidence that guides all rational practitioners in how to provide the best care to their patients.
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 chiropractic care compared to primary care.
From the FULL TEXT Article
Neck pain is a common problem associated with considerable comorbidity, disability, and cost to society. [1–5] In North America, the clinical management of back pain is provided mainly by medical physicians, physical therapists and chiropractors.  Approximately 12% of American and Canadian adults seek chiropractic care annually and 80% of these visits result in spinal manipulation. [7, 8] When compared to those seeking medical care for back pain, Canadian chiropractic patients tend to be younger and have higher socioeconomic status and fewer health problems. [6, 8] In Ontario, the average number of chiropractic visits per episode of care was 10 (median 6) in 1985 through 1991.  Several systematic reviews and our best-evidence synthesis suggest that manual therapy can benefit neck pain, but the trials are too small to evaluate the risk of rare complications. [9–13]
Two deaths in Canada from vertebral artery dissection and stroke following chiropractic care in the 1990s attracted much media attention and a call by some neurologists to avoid neck manipulation for acute neck pain.  There have been many published case reports linking neck manipulation to vertebral artery dissection and stroke.  The prevailing theory is that extension and/or rotation of the neck can damage the vertebral artery, particularly within the foramen transversarium at the C1–C2 level. Activities leading to sudden or sustained rotation and extension of the neck have been implicated, included motor vehicle collision, shoulder checking while driving, sports, lifting, working overhead, falls, sneezing, and coughing.  However, most cases of extracranial vertebral arterial dissection are thought to occur spontaneously, and other factors such as connective tissue disorders, migraine, hypertension, infection, levels of plasma homocysteine, vessel abnormalities, atherosclerosis, central venous catherization, cervical spine surgery, cervical percutaneous nerve blocks, radiation therapy and diagnostic cerebral angiography have been identified as possible risk factors. [17–21]
The true incidence of vertebrobasilar dissection is unknown, since many cases are probably asymptomatic, or the dissection produces mild symptoms.  Confirming the diagnosis requires a high index of suspicion and good vascular imaging. The cases that are most likely to be diagnosed are those that result in stroke. [19, 22] Ischemic stroke occurs when a thrombus develops intraluminally and embolizes to more distal arteries, or less commonly, when the dissection extends distally into the intracranial vertebral artery, obliterating branching vessels.  The best incidence estimate comes from Olmstead county, where vertebral artery dissection causing stroke affected 0.97 residents per 100,000 population between 1987 and 2003. 
To date there have been two case-control studies of stroke following neck manipulation. Rothwell et al used Ontario health data to compare 582 cases of VBA stroke to 2328 age and sex-matched controls.  For those aged <45 years, cases were five times more likely than controls to have visited a chiropractor within 1 week of VBA stroke. Smith et al studied 51 patients with cervical artery dissection and ischemic stroke or transient ischemic attack (TIA) and compared them to 100 control patients suffering from other strokes not caused by dissections.  Cases and controls came from two academic stroke centers in the United States and were matched on age and sex. They found no significant association between neck manipulation and ischemic stroke or TIA. However, a subgroup analysis showed that the 25 cases with vertebral artery dissection were six times more likely to have consulted a chiropractor within 30 days before their stroke than the controls.
Finally, because patients with vertebrobasilar artery dissection commonly present with headache and neck pain,  it is possible that patients seek chiropractic care for these symptoms and that the subsequent VBA stroke occurs spontaneously, implying that the association between chiropractic care and VBA stroke is not causal. [23, 26] Since patients also seek medical care for headache and neck pain, any association between primary care physician (PCP) visits and VBA stroke could be attributed to seeking care for the symptoms of vertebral artery dissection.
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.
Our study advances knowledge about the association between chiropractic care and VBA stroke in two respects. First, our case control results agree with past case control studies that found an association between chiropractic care and vertebral artery dissection and VBA stroke. [24, 25] Second, our case crossover results confirm these findings using a stronger research design with better control of confounding variables. The case-crossover design controls for time independent confounding factors, both known and unknown, which could affect the risk of VBA stroke. This is important since smoking, obesity, undiagnosed hypertension, some connective tissue disorders and other important risk factors for dissection and VBA stroke are unlikely to be recorded in administrative databases.
We also found strong associations between PCP visits and subsequent VBA stroke. A plausible explanation for this is that patients with head and neck pain due to vertebral artery dissection seek care for these symptoms, which precede more than 80% of VBA strokes.  Since it is unlikely that PCPs cause stroke while caring for these patients, we can assume that the observed association between recent PCP care and VBA stroke represents the background risk associated with patients seeking care for dissection-related symptoms leading to VBA stroke. Because the association between chiropractic visits and VBA stroke is not greater than the association between PCP visits and VBA stroke, there is no excess risk of VBA stroke from chiropractic care.
Our study has several strengths and limitations. The study base includes an entire population over a 9–year period representing 109,020,875 person-years of observation. Despite this, we found only 818 VBA strokes, which limited our ability to compute some estimates and bootstrap confidence intervals. In particular, our age stratified analyses are based on small numbers of exposed cases and controls (Table 2). Further stratification by diagnostic codes for headache and neck pain related visits imposed even greater difficulty with these estimates. However, there are few databases that can link incident VBA strokes with chiropractic and PCP visits in a large enough population to undertake a study of such a rare event.
A major limitation of using health administrative data are misclassification bias, and the possibility of bias in assignment of VBA-related diagnoses, which has previously been raised in this context.  Liu et al have shown that ICD-9 hospital discharge codes for stroke have a poor positive predictive value when compared to chart review.  Furthermore, not all VBA strokes are secondary to vertebral artery dissection and administrative databases do not provide the clinical detail to determine the specific cause. To investigate this bias, we did a sensitivity analysis using different positive predictive values for stroke diagnosis (ranging from 0.2 to 0.8). Assuming nondifferential misclassification of chiropractic and PCP cases, our analysis showed attenuation of the estimates towards the null with lower positive predictive values, but the conclusions did not change (i.e., associations remained positive and significant—data not shown). The reliability and validity of the codes to classify headache and cervical visits to chiropractors and PCPs is not known.
It is also possible that patients presenting to hospital with neurologic symptoms who have recently seen a chiropractor might be subjected to a more vigorous diagnostic workup focused on VBA stroke (i.e., differential misclassification).  In this case, the predictive values of the stroke codes would be greater for cases that had seen a chiropractor and our results would underestimate the association between PCP care and VBA stroke.
A major strength of our study is that exposures were measured independently of case definition and handled identically across cases and controls. However, there was some overlap between chiropractic care and PCP care. In the month before their stroke, only 16 (2.0%) of our cases had seen only a chiropractor, while 20 (2.4%) had seen both a chiropractor and PCP, and 417 (51.0%) had just seen only a PCP. We were not able to run a subgroup analysis on the small number of cases that just saw a chiropractor. However, subgroup analysis on the PCP cases (n = 782) that did not visit a chiropractors during the 1 month before their stroke did not change the conclusions (data not shown).
Our results should be interpreted cautiously and placed into clinical perspective. We have not ruled out neck manipulation as a potential cause of some VBA strokes. On the other hand, it is unlikely to be a major cause of these rare events. Our results suggest that the association between chiropractic care and VBA stroke found in previous studies is likely explained by presenting symptoms attributable to vertebral artery dissection. It might also be possible that chiropractic manipulation, or even simple range of motion examination by any practitioner, could result in a thromboembolic event in a patient with a pre-existing vertebral dissection. Unfortunately, there is no acceptable screening procedure to identify patients with neck pain at risk of VBA stroke.  These events are so rare and difficult to diagnose that future studies would need to be multicentered and have unbiased ascertainment of all potential exposures. Given our current state of knowledge, the decision of how to treat patients with neck pain and/or headache should be driven by effectiveness and patient preference. 
Our population-based case-control and case-crossover study shows an association between chiropractic visits and VBA strokes. However, 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.