Research Review By Dr. Michael Haneline©


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Date Posted:

September 2015

Study Title:

Chiropractic care and the risk of vertebrobasilar stroke: results of a case–control study in U.S. commercial and Medicare Advantage populations


Kosloff T, Elton D, Tao J & Bannister W

Author's Affiliations:

Optum Health – Clinical Programs at United Health Group, Minnesota, USA.

Publication Information:

Chiropractic & Manual Therapies 2015; 23: 19.

Background Information:

Neck pain and headache (including migraine) are very common conditions that prompt millions of people to seek health care every year and persons with these conditions often utilize chiropractic care. Indeed, a 2003 survey of chiropractors reported that neck conditions and headache/facial pain made up respectively 18.7% and 12% of their patients’ chief complaints (1).

Chiropractors regularly use spinal manipulative treatment (SMT) in the management of neck pain and headache, even though an association with vertebral artery dissection has been reported. SMT is used so regularly by chiropractors that visits to a chiropractor have been used as a proxy for SMT in case–control studies done by Rothwell et al. and Cassidy et al. (2, 3). Both of these studies reported an increased risk of vertebrobasilar (VBA) stroke associated with chiropractic visits for persons under 45 years of age. However, Cassidy et al. found that the association was similar to visits to a primary care physician and suggested that the association between chiropractic care and stroke was non-causal.

The main purpose of this study was to replicate in a U.S. population the Cassidy et al. case–control study that was performed in a Canadian population. The Cassidy study has been hailed as one of the best designed investigations of the association between chiropractic manipulative treatment and VBA stroke. The current study also intended to compare the association with recent primary care physician (PCP) care and VBA stroke in samples of the U.S. commercial and Medicare Advantage (MA) populations.

The secondary aim of this study was to gauge the practicality of using chiropractic visits as a proxy measure for exposure to spinal manipulation.

Pertinent Results:

There were 1,159 VBA stroke cases and 4,633 controls in the commercial study sample with an average age of 65.1 years and a proportion of males at 64.8%. There were 670 stroke cases and 2,680 matched controls included in the MA study with an average age of 76.1 years and 58.6% were male. The prevalence rate of VBA stroke in the commercial population was 0.0032% and 0.021% in the MA population.

A high percentage of cases had reported at least one comorbid condition (71.5% of cases in the commercial study and 88.5% of the cases in the MA study). The most common comorbid conditions included hypertensive disease, ischemic heart disease, disease of pulmonary circulation, other forms of heart disease, hypercholesterolemia, and diseases of the endocrine glands.

Visits to chiropractors within 30 days prior to hospital admission occurred in 1.6% of commercially insured stroke cases, as compared to 1.3% of controls visiting chiropractors within 30 days prior to their index date.

Visits to a PCP within 30 days prior to the index date occurred in 18.9% of the stroke cases, whereas only 6.8% of controls had visited a PCP.

In the MA sample, the percentage of exposure for chiropractic visits was lower within the 30-day hazard period for cases (0.3%) than for controls (0.9%). On the other hand, the percentage of exposures for PCP visits was higher for cases (21.3%) than for controls (12.9%).

No association between chiropractic visits and VBA stroke was found for the overall sample, or when stratified by age. However, there was an association between PCP visits and VBA stroke that was present in all age groups and lengths of hazard periods.

Clinical Application & Conclusions:

In spite of what several other case-control studies have reported, this study found no significant association between exposure to chiropractic care and the risk of VBA stroke. This finding strengthens the viewpoint that chiropractic care is not likely a cause of VBA strokes; then again, it does not totally exclude cervical manipulation as a possible cause or contributory factor in the occurrence of VBA stroke.

The secondary analysis showed that manipulation may or may not have been reported at every chiropractic visit. Thus, using chiropractic visits as a proxy for manipulation is not reliable.

Study Methods:

This was a case–control study based on the experiences of commercially insured and MA health plan members located across all but one state in the U.S.

Cases were identified using administrative claims data, which were also used to determine patient characteristics and health service utilization. The stroke cases included all patients admitted to an acute care hospital with VBA occlusion and stenosis strokes during the study period. Patients were excluded from the study if they had more than one admission for a VBA stroke.

Each stroke case was matched to four age and gender matched controls who were randomly selected from the sample of qualified members. Both cases and controls were randomly sorted prior to the matching.

Exposures were defined as any encounters with a chiropractor or a primary care physician (PCP) prior to the index date (i.e., the date of admission for the VBA stroke).

Separate analyses were performed for the commercially insured and the MA populations via conditional logistic regression models to examine the association between the exposures and VBA strokes. The odds ratio of having the VBA stroke and the effect of total number of chiropractic visits and PCP visits within the hazard period were calculated. The hazard periods were one day, 3 days, 7 days, 14 days and 30 days. Separate analyses were performed on patients in the commercial population stratified by age groups under 45 years and 45 years and up.

A secondary analysis was performed to ascertain whether using chiropractic visits as a proxy for spinal manipulation was appropriate. The analysis involved querying the commercial and MA databases to identify the proportions of VBA stroke cases and matched controls for which a chiropractic spinal manipulative treatment or a manual therapy procedure (CPT code 97140) was or was not recorded.

Study Strengths / Weaknesses

This was a well-done case-control study that adds to the understanding of the proposed relationship between chiropractic manipulation and VBA-stroke. The main study limitation was that information was gathered using claims data instead of more reliable methods, such as patient interviews and medical records.

The use of administrative claims data does not provide contextual information surrounding the clinical encounters between physicians and their patients. Since comprehensive clinical chart audits were not performed in this study, it is not possible to know what actually occurred in the clinical encounters.

Information about recent trauma and activities that have been reported to be potential risk factors for VBA stroke in case studies were not available in the claims data. Furthermore, the claims data did not provide accurate and complete reporting of other health disorders (e.g., migraine and recent infection) that have been reported as being associated with VBA stroke (4).

Current procedural terminology (CPT) codes were used to report clinical procedures. However, CPT codes for chiropractic manipulative procedures only provide information as to the number of spinal regions manipulated; they do not specify which particular spinal regions were manipulated.

Further, claims data did not discriminate between the various types of manipulative techniques that are utilized by chiropractors (e.g., thrust, non-thrust, rotational, soft tissue mobilization, etc.).

Only 2 VBA stoke cases were included in the 0–30 days hazard period for the younger (< 45 yrs) group that received chiropractic care in the commercial cohort and no stroke cases were included in the other hazard periods. This could have been the reason the results of this study conflicted with previous similar case–control studies regarding the association of chiropractic care with VBA stroke. Another possible reason for the difference in results between studies is that the hospital claims data in the U.S. was not likely as accurate as it was in Ontario, Canada.

Additional References:

  1. Christensen M, Kollasch M, Hyland J, Rosner A. Chapter 8 – Patient Conditions. In Practice Analysis of Chiropractic: A Project Report, Survey Analysis, and Summary of the Practice of Chiropractic Within the United States. Greeley, CO: The National Board of Chiropractic Examiners. 2010:95–120.
  2. Cassidy J, Boyle E, Cote P et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population-based case–control and case-crossover study. Spine 2008; 33(Suppl 4): S176–83.
  3. Rothwell D, Bondy S, Williams J. Chiropractic manipulation and stroke: a population-based case–control study. Stroke 2001; 32(5): 1054–60.
  4. Haneline M, Lewkovich G. A narrative review of pathophysiological mechanisms associated with cervical artery dissection. Journal of the Canadian Chiropractic Association 2007; (51)3: 140-51.

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