Is spinal manipulative therapy (SMT) dangerous? This question has kept us on this blog busy for quite some time now. To me, there is little doubt that SMT can cause adverse effects some of which are serious. But many chiropractors seem totally unconvinced. Perhaps this new overview of reviews might help to clarify the issue. Its aim was to elucidate and quantify the risk of serious adverse events (SAEs) associated with SMT.
The authors searched five electronic databases from inception to December 8, 2015 and included reviews on any type of studies, patients, and SMT technique. The primary outcome was SAEs. The quality of the included reviews was assessed using a measurement tool to assess systematic reviews (AMSTAR). Since there were insufficient data for calculating incidence rates of SAEs, they used an alternative approach; the conclusions regarding safety of SMT were extracted for each review, and the communicated opinion were judged by two reviewers independently as safe, harmful, or neutral/unclear. Risk ratios (RRs) of a review communicating that SMT is safe and meeting the requirements for each AMSTAR item, were calculated.
A total of 283 eligible reviews were identified, but only 118 provided data for synthesis. The most frequently described adverse events (AEs) were stroke, headache, and vertebral artery dissection. Fifty-four reviews (46%) expressed that SMT is safe, 15 (13%) expressed that SMT is harmful, and 49 reviews (42%) were neutral or unclear. Thirteen reviews reported incidence estimates for SAEs, roughly ranging from 1 in 20,000 to 1 in 250,000,000 manipulations. Low methodological quality was present, with a median of 4 of 11 AMSTAR items met (interquartile range, 3 to 6). Reviews meeting the requirements for each of the AMSTAR items (i.e. good internal validity) had a higher chance of expressing that SMT is safe.
The authors concluded that it is currently not possible to provide an overall conclusion about the safety of SMT; however, the types of SAEs reported can indeed be significant, sustaining that some risk is present. High quality research and consistent reporting of AEs and SAEs are needed.
This article is valuable, if only for the wealth of information one can extract from it. There are, however, numerous problems. One is that the overview included mostly reviews of the effectiveness of SMT for various conditions. We know that studies of SMT often do not even mention AEs. If such studies are then pooled in a review, they inevitably generate an impression of safety. But this would, of course, be a false-positive result!
The authors of the overview are aware of this problem and address it in the following paragraph: “When only considering the subset of reviews, where the objective was to investigate AEs (37 reviews), then 8 reviews (22%) expressed that SMT is safe, 13 reviews (35%) expressed that SMT is harmful and 16 reviews (43%) were neutral or unclear regarding the safety of SMT. Hence, there is a tendency that a bigger proportion of these reviews are expressing that SMT is harmful compared to the full sample of reviews…”
To my surprise, I found several of my own reviews in the ‘neutral or unclear’ category. Here are the verbatim conclusions of three of them:
- It is concluded that serious cerebrovascular complications of spinal manipulation continue to be reported.
- The most common serious adverse events are vertebrobasilar accidents, disk herniation, and cauda equina syndrome.
- These data indicate that mild and transient adverse events seem to be frequent. Serious adverse events are probably rare but their incidence can only be estimated at present.
I find it puzzling how this could be classified as neutral or unclear. The solution of the puzzle might lie in the methodology used: “we appraised the communicated opinions of each review concerning the safety of SMT based on their conclusions regarding the AEs and SAEs. This was done by two reviewers independently (SMN, LK), who judged the communicated opinions as either ‘safe’, ‘neutral/unclear’ or ‘harmful’, based on the qualitative impression the reviewers had when reading the conclusions. The reviewers had no opinion about the safety/harmfulness of SMT before commencing the judgements. Cohen’s weighted Kappa was calculated for the agreement between the reviewers, with a value of 0.40–0.59 indicating ‘fair agreement’, 0.60–0.74 indicating ‘good agreement’ and ≥0.75 indicating ‘excellent agreement’. Disagreements were resolved by a third reviewer (MH).”
In other words, the categorisation was done on the basis of subjective judgements of two researchers. It seems obvious that, if their attitude was favourable towards SMT, their judgements would be influenced. The three examples from my own work cited above indicates to me that their verdicts were indeed far from objective.
So what is the main message here? In my view, it can be summarized in the following quote from the overview: “a bigger proportion of these reviews are expressing that SMT is harmful …”
Yes, yes, yes – I know that, if you are a chiropractor (or other practitioner using mostly SMT), you are unlikely to agree with this!
Perhaps you can agree with this statement then:
As long as there is reasonable doubt about the safety of SMT, and as long as we cannot be sure that SMT generates more good than harm, we should be very cautious using it for routine healthcare and do rigorous research to determine the truth (it’s called the precautionary principle and applies to all types of healthcare).