DISCUSSION

The case reports (Tables ​(Tables11 and ​and2)2) confirm previous reports6 associating upper spinal manipulation with a range of complications. The most serious problems, which some experts now describe as ‘well-recognized’,22 are vertebral artery dissections due to intimal tearing as a result of over-stretching the artery during rotational manipulation. This seems to occur most commonly at the level of the atlantoaxial joint.20 Intimal injury can be followed by intramural bleeding or pseudoaneurysm formation, which can result in thrombosis, embolism20 or arterial spasm.22

The retrospective case series (Table 3) confirm that spinal manipulation is associated with risks such as vascular accidents and non-vascular complications. Such adverse effects are being reported from several countries and often have serious consequences. The therapists involved are mostly chiropractors; this predominance is probably due to the fact that these therapists use spinal manipulation more frequently than other practitioners. Most of the incidents reported in case series or surveys had not been previously reported, indicating that under-reporting may frequently be high.

The two prospective case series42,43 corroborate the results from several earlier investigations50 showing that mild to moderate adverse effects occur in a large proportion of patients receiving spinal manipulation. These adverse effects are transient and non-serious but nevertheless seriously affect many patients.42,50 Risk-benefit evaluations of spinal manipulation must therefore account not just for serious complications but also for such adverse events.

Case-control and other studies confirm that upper spinal manipulation is associated with risks44-47 and that spinal manipulation is an independent risk factor for vertebral artery dissection.46 Many chiropractors insist that a causal link is questionable or unlikely, as the early signs of arterial dissections include neck pain, which could be the reason for a patient to consult a chiropractor, therefore these possible associations could be false.23,51 Smith et al. tried to account for this particular confounder and still found spinal manipulation to be a risk factor.46

The three surveys disclose more complications. They suggest that many therapists are now becoming aware of the risks of spinal manipulation.48,49 Two of the surveys49,52 also confirm that under-reporting is frequently close to 100%.

It seems unfair to assess the risk of spinal manipulation as practised by well-trained chiropractors alongside that associated with untrained therapists (Tables ​(Tables11 and ​and2).2). Chiropractors may argue that it takes years of experience to learn the fine psychomotor control required for skilled manipulations. Certainly skill and experience are important, and it is relevant to differentiate between different professions, as done in Tables ​Tables11 and ​and2.2. On the other hand, skill is a quality not easily controlled for in such research; even some chiropractors may be more skilled than others. Moreover, this review is aimed at evaluating the risk of an intervention (spinal manipulation) and not that of a profession (chiropractic). In fact, this review shows that the implicated practitioners are not only chiropractors but also surgeons, shiatsu practitioners, ‘bonesetters’ and general practitioners (Table 2).

Collectively, these data suggest that spinal manipulation is associated with frequent, mild and transient adverse effects as well as with serious complications which can lead to permanent disability or death. Yet causal inferences are, of course, problematic. Vascular accidents may happen spontaneously or could have causes other than spinal manipulation. A temporal relationship is insufficient to establish causality, and recall bias can further obscure the truth. Moreover, denominators are rarely available. Consequently the frequency of serious adverse effects is currently unknown. Estimates by chiropractors vary (e.g. 6.4 per 10 million manipulations of the upper spine and 1 per 100 million manipulations of the lower spine).53 These figures, however, may be over-optimistic. Retrospective investigations have repeatedly shown that under-reporting is close to 100%.13,52 This level of under-reporting would render such estimates nonsensical. At present, there is no sufficiently large and rigorous prospective study to generate reliable incidence figures; previous studies have failed to investigate those patients which were lost at follow-up. This could be the subgroup which has been harmed. It is therefore essential that future studies follow up close to 100% of the initial patient sample.

The effectiveness of spinal manipulation for most indications is less than convincing.5 A risk-benefit evaluation is therefore unlikely to generate positive results: with uncertain effectiveness and finite risks, the balance cannot be positive. Cautious attitudes towards upper spinal manipulation are therefore becoming more widespread: ‘special caution should be exercised when performing first-line cervical manipulation and simple, honest and easily understandable information about there risks should be included when informed consent is obtained.’54

Some therapists have started advocating screening patients for risk factors before treatment.55-57 Based on cadaver studies of human vertebral arteries, Cagnie et al.58 have suggested that, in the presence of arteriosclerotic changes, the stretching and compression effects of rotational manipulation may constitute a risk factor for vascular accidents. These authors concluded that ‘therapists should avoid manipulative techniques at all levels of the cervical spine in the presence of any indirect sign of arteriosclerotic disease or in the presence of calcified arterial walls or tortuosities of the vessel.’58 Others have suggested that high homocystein levels constitute a risk factor for arterial dissection.59 Spinal manipulation might therefore be contraindicated in such individuals. The effectiveness of screening has, however, not been convincingly demonstrated. The chiropractic profession tends to downplay the risks: ‘chiropractic services are safe’;60 ‘the healthy vertebral artery is not at risk from properly performed chiropractic manipulative procedures.’61 Others argue that ‘the occurrence of cerebrovascular accidents in the chiropractic population is 0.000008%’,62 that causality is not proven or even unlikely,61,63-66 that other interventions are more risky (see below),67 that the mechanical forces employed for spinal manipulation are too low to cause injury,68 or that there is a ploy from the medical establishment to sideline chiropractors.69-71 In the light of the evidence summarized above, such attitudes do not seem to be in the best interest of patients.

It is, of course, important to present any risk-benefit assessment fairly and in the context of similar evaluations of alternative therapeutic options. One such option is drug therapy. The drugs in question—non-steroidal anti-inflammatory drugs (NSAIDs)—cause considerable problems, for example gastrointestinal and cardiovascular complications.72,73 Thus spinal manipulation could be preferable to drug therapy. But there are problems with this line of argument: the efficacy of NSAIDs is undoubted but that of spinal manipulation is not, and moreover, the adverse effects of NSAIDs are subject to post-marketing surveillance while those of spinal manipulation are not. Thus we are certain about the risks and benefits of the former and uncertain about those of the latter. Finally, it should be mentioned that other therapeutic options (e.g. exercise therapy or massage) have not been associated with significant risks at all.

This systematic review has several limitations. Even though the search strategy was thorough, some relevant published articles might have been missed. High levels of under-reporting or recall bias might distort the overall picture generated. Publication bias could have exerted a similar effect. For instance, it is possible that journals of complementary medicine are unlikely to publish findings which might be considered ‘negative’.74

In conclusion, spinal manipulation, particularly when performed on the upper spine, has repeatedly been associated with serious adverse events. Currently the incidence of such events is unknown. Adherence to informed consent, which currently seems less than rigorous,75 should therefore be mandatory to all therapists using this treatment. Considering that spinal manipulation is used mostly for self-limiting conditions and that its effectiveness is not well established,5 we should adopt a cautious attitude towards using it in routine health care.