For several decades, high-velocity low-amplitude cervical spine manipulation has been associated with rare instances of vertebrobasilar dissection and stroke. A simplified summary posits one view pointing to case reports and media stories describing instances of stroke after cervical manipulation. The suggested mechanism hypothesizes neck extension and rotation during cervical manipulation can damage vertebral arteries, leading to ischemic stroke. Another view points out that case-based evidence is circumstantial, and therefore inadequate to establish a causal relationship. But circumstantial evidence can’t just be dismissed as if it doesn’t exist. So, where does that leave us?
A Way Forward
Scientific evidence is needed to move past beliefs and opinions. Fortunately, several published studies offer essential evidence. A recent article authored by Gorrell and colleagues and published in the Journal of Manual & Manipulative Therapy is an example.(1) The article describes a study designed to help answer the following questions:
1. How does the length of vertebral arteries change during manual cervical high velocity low amplitude manipulation?
2. How much slack is normally present in the vertebral arteries?
3. How much do vertebral arteries stretch before mechanical damage is observed?
To answer these questions, investigators inserted piezoelectric ultrasound crystals into vertebral arteries of 7 un-embalmed cadavers. Three chiropractors with between 7 and 20 years of experience applied supine cervical manipulation (2 chiropractors per cadaver) toward each cervical segment and on each side in a randomly generated sequence. Diversified-style thrusts began with head and neck flexion, lateral flexion and rotation. One thrust was directed anteriorly, imparting rotation, while the other was directed medially and slightly inferior. Vertebral artery length changes were measured during 518 manipulative thrusts. The investigators also measured: a) the amount of slack normally present; and b) the amount of stretch needed to induce mechanical injury.
Key Findings
- Mean vertebral artery length changes were greatest on the non-thrust side, ranging from 5%, when manipulation was applied toward C5, to 1% when applied toward C1.
- Mean elongation needed to take up slack in the vertebral arteries was 33.5%.
- Mechanical injury occurred when the vertebral arteries reached a mean length change of 51%.
Interpretation
Vertebral artery length changes during cervical manipulation were less than needed to account for the normal amount of slack. Thus, vertebral arteries were not stretched with cervical manipulation in this study. This evidence combined with findings that mechanical injury requires substantially more elongation past the first stretching point, strongly suggests vertebral arteries are not damaged by stretching during cervical manipulation.
There are, of course, several limitations to this study. Several questions remain, such as whether these findings translate to studies in living people. However, these findings are consistent with other studies investigating biomechanical characteristics of major cervical arteries during cervical manipulation.
Causality Criteria
A natural question arising from this research could be: Does it conclusively settle the question? The short answer is no. But it does provide meaningful evidence on the topic. To render decisions on causality, we must consider different types of evidence. The following are criteria commonly considered in questions of causality.(2)
1. Temporality: This criterion states that exposure (e.g., cervical manipulation) must precede a problem (e.g., stroke). Numerous case studies describe stroke immediately after cervical manipulation. This evidence suggests a possible causal relationship. As described earlier, temporal evidence must be supported by other evidence to establish a causal relationship.
2. Strength of association: Statistically strong relationships between an intervention (e.g., cervical manipulation) and a problem (e.g., stroke) are usually needed to support conclusions of causality. Several studies report small relationships between cervical manipulation and stroke, similar to those observed in control groups. However, for relatively rare conditions such as stroke, small associations may still indicate a causal relationship. The key factor here is that the statistical relationships are similar to those observed in control groups, suggesting the association between cervical manipulation and stroke is most likely circumstantial.(3–5)
3. Consistency: To demonstrate causal relationships, population-based studies must show consistent findings among varied populations. In the case of cervical manipulation and stroke, studies among different populations consistently suggest the relationship is most likely circumstantial.(3–5)
4. Specificity: Causal relationships are more likely when a single type of exposure (e.g., cervical manipulation) matches a condition (e.g., stroke). Population-based studies report that strokes happen similarly after exposure to either chiropractors or primary care practitioners. This evidence also suggests a relationship between cervical manipulation and stroke is most likely circumstantial. (3–5)
5. Biological plausibility: This criterion states plausible mechanisms are needed. A common proposed mechanism suggests cervical arteries are damaged by stretching during cervical manipulation. Findings from the study conducted by Gorrell et al., are consistent with other studies of cervical arteries, suggesting this mechanism is not likely biologically plausible.(6–8)
6. Experiment: This criterion suggests casual relationships should be supported by experimental studies. Experimental research using animal models to date does not support a causal relationship between cervical manipulation and stroke.(9,10)
7. Biological gradient: Data supporting higher incidence of stroke with more exposure to cervical manipulation would support the possibility of a causal relationship. However, dose-response data are not available. Therefore, questions about biological gradient are largely unanswered.
8. Analogy: Studying analogous circumstances can provide corroborating evidence for causal relationships. Though some suggest cervical artery injury from motor vehicle accidents are analogous to cervical manipulation, physical trauma from motor vehicle accidents is very different from care applied by a licensed professional. Therefore, no analogous circumstances can help support or refute a causal relationship.
9. Coherence: This criterion describes the need to consider all available evidence. When most evidence points toward the same conclusion, we can be more confident in it. Currently, the only evidence pointing toward a causal relationship is temporality. Scientific evidence for 2 criteria (biological gradient & analogy) isn’t available. Other evidence consistently points toward a circumstantial relationship.
Key Take-Away Points
- The questions about a causal relationship between cervical manipulation and stroke are not completely settled. However, scientific evidence from both population-based and biomechanical studies consistently suggests the relationship is most likely circumstantial and that vertebral artery damage from cervical manipulation is probably not biologically plausible in most circumstances.
- Additional research is needed to answer some remaining questions such as whether biomechanical research findings observed in cadavers are similar in living people. Dose-response research is also needed.
- Perhaps the greatest need is research to help practitioners reliably identify patients in early stages of cervical artery dissection because symptoms can mimic neck and head pain from musculoskeletal sources. Until those diagnostic methods are developed, consider symptoms of neurological origin that can indicate a stroke in progress (11):
- muscle weakness
- hemiplegia
- reduced vibratory and/or other sensations
- abnormal speech
- aphasia
- cranial nerve dysfunction
- visual field deficit
- imbalance
- difficulty walking
- coordination loss
- nystagmus
- confusion
- memory deficit
- vomiting
- loss of consciousness
Dr. Vining is associate dean of clinical research, as well as a professor, at the Palmer Center for Chiropractic Research, Palmer College of Chiropractic, in Davenport, Iowa.
References
- Gorrell LM, Sawatsky A, Edwards WB, Herzog W. Vertebral arteries do not experience tensile force during manual cervical spine manipulation applied to human cadavers. J Man Manip Ther. 2022 Nov 15;1–9.
- Fedak KM, Bernal A, Capshaw ZA, Gross S. Applying the Bradford Hill criteria in the 21st century: How data integration has changed causal inference in molecular epidemiology. Emerg Themes Epidemiol. 2015;12(1):1–10.
- 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;33(Supplement):S176–83.
- Cassidy JD, Boyle E, Côté P, Hogg-Johnson S, Bondy SJ, Haldeman S. Risk of Carotid Stroke after Chiropractic Care: A Population-Based Case-Crossover Study. J Stroke Cerebrovasc Dis. 2017 Apr;26(4):842–50.
- Whedon JM, Song Y, Mackenzie TA, Phillips RB, Lukovits TG, Lurie JD. Risk of stroke after chiropractic spinal manipulation in medicare B beneficiaries aged 66 to 99 years with neck pain. J Manipulative Physiol Ther. 2015 Feb;38(2):93–101.
- Herzog W, Tang C, Leonard T. Internal Carotid Artery Strains During High-Speed, Low-Amplitude Spinal Manipulations of the Neck. J Manipulative Physiol Ther. 2015 Nov;38(9):664–71.
- Herzog W, Leonard TR, Symons B, Tang C, Wuest S. Vertebral artery strains during high-speed, low amplitude cervical spinal manipulation. J Electromyogr Kinesiol. 2012 Oct;22(5):740–6.
- Moser N, Mior S, Noseworthy M, Côté P, Wells G, Behr M, et al. Effect of cervical manipulation on vertebral artery and cerebral haemodynamics in patients with chronic neck pain: a crossover randomised controlled trial. BMJ Open. 2019 May 28;9(5):e025219.
- Guan T, Zeng Y, Qi J, Qin B, Fu S, Wang G, et al. Effects of Cervical Rotatory Manipulation on Internal Carotid Artery in Hemodynamics Using an Animal Model of Carotid Atherosclerosis: A Safety Study. Med Sci Monit Int Med J Exp Clin Res. 2019 Mar 31;25:2344–51.
- Wynd S, Anderson T, Kawchuk G. Effect of cervical spine manipulation on a pre-existing vascular lesion within the canine vertebral artery. Cerebrovasc Dis Basel Switz. 2008;26(3):304–9.
- Leach RA. Patients with symptoms and signs of stroke presenting to a rural chiropractic practice. J Manipulative Physiol Ther. 2010 Jan;33(1):62–9.
