Some of you might have heard of repetitive transcranial magnetic stimulation (rTMS) and its use in chronic pain. Basically rTMS uses magnetic fields to generate electrical currents within the brain. This is a direct way of altering neuronal firing or excitability in the brain and a number of research groups have been investigating whether it might be used to treat chronic pain by altering pain processing in the brain. To get an idea of what it's all about check out this section of the BBC's Horizon programme "The Secret World of Pain".
While that video is emotionally compelling and at face value looks really promising, I lost count of the number of ways in which the placebo effect might be being ramped up in that interaction. In fact I personally felt that the programme did not really offer the viewer enough balance or caveats there. So what does the best evidence tell us about the efficacy of this technique? For a change we get to talk about our own research because after carrying out a Cochrane review on this very topic we have just written a commentary in the journal PAIN® that discusses the current state of the evidence for rTMS in chronic pain management.
Our review found a bunch (19) of small studies of rTMS. Overall the data was quite varied but when pooled it suggested a small effect. When we broke the data into pre-planned subgroups we found a small short-term effect on pain of single doses of high frequency rTMS applied to the motor cortex. Great news - it seemed to work better than sham (placebo) stimulation. But as usual there were reasons to be a tad less cheerful. The effect was small and while it tickled the feet of clinical importance it didn't clearly hit that target. There were also problems with risk of bias, particularly the tricky challenge of effectively blinding the studies, and we know that these issues tend to exaggerate effect sizes.
In fairness to rTMS these were single-dose one-off treatment studies. Maybe more doses would be more effective. Also they almost all recruited patients with severe neuropathic pain that didn't respond to anything else either, so not the easiest pain to influence. When we looked the few studies that delivered multiple doses the results were conflicting and inconsistent and there really aren't enough of them to make a confident judgement.
The point that we make in our commentary is that the evidence does suggest that rTMS might modulate pain, but this data has its problems, the studies are small and the evidence is a mixed bag of quality. There are more small, exploratory studies being published regularly, all justified by and based upon the initial promise of the earlier work but what is really needed is for researchers to take a nice big sample, deliver a good robust course of rTMS treatment using the parameters that look most promising right now (high frequency stimulation to the motor cortex), measure their outcomes over a decent time span, and all of this with good tight methods and better blinding. Sounds easy (but of course it never is)! But without this effort the next time we update our Cochrane review we might still only be able to say "maybe, maybe not".