Pain relief through expectation supersedes descending inhibitory deficits in fibromyalgia patients

Abstract 

In healthy adults, expectations can modulate the activity of inhibitory bulbo-spinal projections, and can even block the analgesic properties of counter-irritation – a phenomenon that triggers descending inhibition. Since descending inhibition is known to be deficient in fibromyalgia (FM) patients, we tested the possibility that expectancy-mediated analgesia would improve, or even kick-start, the deficient inhibitory responses of FM patients. By measuring subjective pain ratings, spinal withdrawal reflexes, and somatosensory evoked potentials (SEP), it was possible to test whether or not expectancy-mediated analgesia involved descending inhibition in FM patients. Here, we show that expectations of analgesia radically change the subjective experience of pain, but do not eliminate evidence of spinal hyperexcitability in FM patients. We found that expectations of analgesia reduce subjective pain ratings and decrease SEP amplitudes, confirming that expectations influence thalamocortical processes. However, even when analgesia was experienced, the spinal activity of FM patients was abnormal, showing heightened reflex responses. This demonstrates that, unlike healthy subjects, the modulation of pain by expectations in FM fails to influence spinal activity. These results indicate that FMs are capable of expectancy-induced analgesia but that, for them, this form of analgesia does not depend on the recruitment of descending inhibitory projections.

Keywords: Fibromyalgia, Expectations, Withdrawal reflex, Spine, Brain, Descending inhibition