The question whether spinal manipulative therapy (SMT) has any specific therapeutic effects is still open. This fact must irritate ardent chiropractors, and they therefore try everything to dispel our doubts. One way would be to demonstrate a dose-effect relationship between SMT and the clinical outcome. But, for several reasons, this is not an easy task.
This RCT was aimed at identifying the dose-response relationship between visits for SMT and chronic cervicogenic headache (CGH) outcomes; to evaluate the efficacy of SMT by comparison with a light massage control.
The study included 256 adults with chronic CGH. The primary outcome was days with CGH in the prior 4 weeks evaluated at the 12- and 24-week primary endpoints. Secondary outcomes included CGH days at remaining endpoints, pain intensity, disability, perceived improvement, medication use, and patient satisfaction. Participants were randomized to 4 different dose levels of chiropractic SMT: 0, 6, 12, or 18 sessions. They were treated 3 times per week for 6 weeks and received a focused light-massage control at sessions when SMT was not assigned. Linear dose effects and comparisons to the no-manipulation control group were evaluated at 6, 12, 24, 39, and 52 weeks.
A linear dose-response was observed for all follow-ups, a reduction of approximately 1 CGH day/4 weeks per additional 6 SMT visits (p<.05); a maximal effective dose could not be determined. CGH days/4 weeks were reduced from about 16 to 8 for the highest and most effective dose of 18 SMT visits. Mean differences in CGH days/4 weeks between 18 SMT visits and control were -3.3 (p=.004) and -2.9 (p=.017) at the primary endpoints, and similar in magnitude at the remaining endpoints (p<.05). Differences between other SMT doses and control were smaller in magnitude (p > .05). CGH intensity showed no important improvement nor differed by dose. Other secondary outcomes were generally supportive of the primary.
The authors concluded that there was a linear dose-response relationship between SMT visits and days with CGH. For the highest and most effective dose of 18 SMT visits, CGH days were reduced by half, and about 3 more days per month than for the light-massage control.
This trial would make sense, if the effectiveness of SMT for CGH had been a well-documented fact, and if the study had rigorously controlled for placebo-effects.
But guess what?
Neither of these conditions were met.
A recent review concluded that there are few published randomized controlled trials analyzing the effectiveness of spinal manipulation and/or mobilization for TTH, CeH, and M in the last decade. In addition, the methodological quality of these papers is typically low. Clearly, there is a need for high-quality randomized controlled trials assessing the effectiveness of these interventions in these headache disorders. And this is by no means the only article making such statements; similar reviews arrive at similar conclusions. In turn, this means that the effects observed after SMT are not necessarily specific effects due to SMT but could easily be due to placebo or other non-specific effects. In order to avoid confusion, one would need a credible placebo – one that closely mimics SMT – and make sure that patients were ‘blinded’. But ‘light massage’ clearly does not mimic SMT, and patients obviously were aware of which interventions they received.
So, an alternative – and I think at least as plausible – conclusion of the data provided by this new RCT is this:
Chiropractic SMT is associated with a powerful placebo response which, of course, obeys a dose-effect relationship. Thus these findings are in keeping with the notion that SMT is a placebo.
And why would the researchers – who stress that they have no conflicts of interest – mislead us by making this alternative interpretation of their findings not abundantly clear?
I fear, the reason might be simple: they also seem to mislead us about their conflicts of interest: they are mostly chiropractors with a long track record of publishing promotional papers masquerading as research. What, I ask myself, could be a stronger conflict of interest?
(Pity that a high-impact journal like SPINE did not spot these [not so little] flaws)