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Blame game: why protopathic bias keeps chiropractors in the cross hairs

Updated: Jul 11, 2023

Cervical spinal manipulative therapy (cSMT) is practised by chiropractors and other practitioners of manual medicine around the world. With neck pain being the fourth greatest cause of years lived with disability, it is a frequent presentation to chiropractors. As experts in non-surgical, non-pharmaceutical spine care, chiropractors are highly trained in manual methods of care, including hands-on and instrument-assisted spinal manipulation to manage neck pain, headaches and a range of other musculoskeletal disorders.

Over the course of the last several decades, attention has focused on the association between cSMT and vascular damage in the neck leading to stroke. Often involving a temporal association due to the proximity in time between the application of treatment and the onset of stroke symptoms, chiropractors have found themselves in civil litigation facing allegations of clinical negligence.

Despite the current body of scientific evidence not supporting the premise of causation, critics of chiropractic (and plaintiff lawyers in negligence cases) continue to blame cervical dissection and related stroke on the application of high-velocity, low amplitude manual thrust manipulation. So why is it that the debate over the safety of cSMT rumbles on? The answer may lie in what is known as protopathic bias.

Protopathic bias occurs when the treatment for the symptoms of an undiagnosed condition appear to cause the outcome. Most commonly seen in conditions for which drugs are prescribed, it is also known as reverse causality. In medicine, one oft-cited example of protopathic bias is the prescription of non-steroidal anti-inflammatory (NSAID) medication for symptoms of heart failure before it is diagnosed, leading to accusations that NSAIDs cause heart failure. Protopathic bias gives rise to overestimation of risk of treatment interventions.

Cervical artery dissection is a cause of stroke in young adults. It affects males slightly more than females (53%-57%) and it has been found to occur more frequently during the winter months. The risk of vertebral artery dissection is estimated to be one per 100,000 person years (one per 5 million person weeks).

Cervical artery dissection affects either the carotid or vertebral arteries and can be intracranial or extracranial, with extracranial carotid artery dissections being the most common. Dissection results from a tear in the intima of the artery, giving rise to bleeding within the media and causing the layers to separate, with a thrombus, or thrombus fragments, forming in the resulting lumen. Strokes most commonly occur in the two weeks following the dissection, with the risk diminishing significantly after this.

Risk factors include hypertension (high blood pressure) but sufferers are less likely to have high cholesterol. Other risk factors are migraine with aura, a history of recent infection, and the presence of connective tissue disorders, such as Ehlers-Danlos syndrome, osteogenesis imperfecta, Marfan's syndrome and fibromuscular dysplasia. Perhaps most significant is a recent history of minor cervical trauma, giving rise to the non-specific symptoms that cause millions to seek chiropractic treatment each year around the world.

While classic cervical artery dissections may involve significant focal neurological signs such as Horner's syndrome (more often seen in carotid artery dissections), vertigo, dysarthria, visual field deficit, ataxia and diplopia (more characteristic of vertebral artery dissection), symptoms are commonly benign in the early stages and may present as neck pain and vague occipital headaches - symptoms that are indistinguishable from non-specific, mechanical neck pain. This clinical similarity has given rise to the view that rather than being causative, cSMT is incidental to a dissection in progress, later manifesting as a stroke.

With protopathic bias at play, cSMT is often targeted as the causative agent in cases of dissection and resulting stroke despite causation not having been established in any robust trial.

This has been demonstrated in a recent paper by Whedon et al [1], which specifically evaluated the association between cervical artery dissection and spinal manipulation in over 5000 U.S. adults receiving either cSMT, or medical evaluation and management, or neither. The investigators used a range of different analyses of the data, none of which demonstrated any consistent increased association of cervical artery dissection with cSMT compared to standard evaluation and management; in fact cSMT showed a decreased degree of association compared to evaluation and management in population controls. The authors concluded that there is strong evidence that any association between cSMT and cervical artery dissection is not causal in nature. It is more likely, they say, that patients with neck pain and related symptoms seek out care from a cSMT provider, a medical provider, or both rather than having a specific risk from cSMT.

These findings mirror the findings of previous studies [2], [3], [4], [5], [6], [7] that have failed to find evidence of causation where the application of cSMT has been associated with stroke.

Despite the lack of evidence supporting causation and the presence of protopathic bias, it seems unlikely that chiropractors will succeed anytime soon in shaking off criticism from the profession's detractors. For now, at least, they remain in the cross hairs. Such continued scrutiny calls for the highest levels of professional conduct and a robust commitment to evidence-based, people-centred, interprofessional and collaborative approaches at all levels of the chiropractic profession.


[1] Whedon JM, Peterson C et al. The association between cervical artery dissection and spinal manipulation among US adults. Eur Spine J, 2023 Jun 8; epub ahead of print.

[2] Cassidy JD, Boyle E et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population based case control and case crossover study. Eur Spine J, 2008, Apr; 17 (Supp 1): 176-83

[3] Chaibi A, Russell MB. A risk-benefit assessment strategy to exclude cervical artery dissection in spinal manual therapy: a comprehensive review. Ann Med 2019 Mar; 51(2): 118-127.

[4] Church EW, Sieg EP et al. Systematic review and meta-analysis of chiropractic care and cervical artery dissection: no evidence for causation. Cureus 2016 Feb; 16(8): e498

[5] Kosloff TM, Elton D et al. Chiropractic care and the risk of vertebrobasilar stroke: results of a case-control study in US commercial and Medicare Advantage populations. Chirop Man Ther 2015 Jun 16; 23: 19

[6] Rubinstein SM, Peerdman SM et al. A systematic review of the risk factors for cervical artery dissection. Stroke 2005 Jul; 36(7): 1575-80.

[7] Whedon JM, McKenzie TA et al. Risk of traumatic injury associated with chiropractic spinal manipulative therapy in Medicare part B beneficiaries aged 66 to 99 years. Spine 2015 Feb 15; 40(4): 264-70.

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