Fibromyalgia patients perceive touch differently compared to healthy individuals, suggesting that there is abnormal processing of signals in C-type skin nerve fibers in fibromyalgia.
A research team from the National Center for Complementary and Integrative Health in Bethesda, Maryland, investigated whether patients with fibromyalgia rated the perception of touch differently than healthy individuals. They also wanted to explore if opioid signaling was involved in any potential findings.
Earlier studies have suggested that patients with fibromyalgia have less available opioid receptors in their brain. In addition to their involvement in pain signaling, opioid receptors mediate feelings of reward to natural behaviors.
Enrolling 24 fibromyalgia patients and 28 healthy controls, the study, “Touch Perception Altered by Chronic Pain and by Opioid Blockade,“ explored the effects of touch by slow or fast brushing of the participant’s forearm. The participants were asked to rate both pleasure and intensity of the strokes.
Healthy individuals rated the slow brushing as more pleasant but less intense – a non-surprising finding given that the C-type nerves involved in the perception of touch are more strongly activated by slow touch.
In contrast, people with fibromyalgia rated the slow and fast movements as equally pleasurable and intense. The level of pleasure and intensity did, however, not differ from the ratings of healthy individuals.
Study participants then received either the opioid receptor-blocking drug naloxone or a placebo. The results, published in the journal eNeuro, showed that the healthy participants tended to rate both slow and fast touch as slightly more pleasurable when receiving naloxone. On the other hand, fibromyalgia patients did not report any change in pleasure after receiving naloxone but rated both fast and slow touch as less intense than patients receiving placebo.
The authors present the seemingly contra-intuitive argument that, in healthy individuals, blocking opioid receptors makes touch more pleasurable. To support this idea, they cite studies showing that monkeys receiving opioid-blocking drugs seek and receive more grooming. This could, however, be attributed to a wish to make up for the lost pleasure, a scenario that can be compared to drug addicts increasing the dose when they become tolerant to a drug’s effects.
A more likely explanation is that the higher pleasure ratings in healthy individuals were associated with another test: while the participants received naloxone, they were also exposed to a painful stimuli inside an MRI scanner. According to earlier research, the initial emotional state of a person might determine the response to opioid blockade, increasing comfort-seeking behavior. Given the circumstances, healthy participants may have received more comfort by the gentle strokes.
The interpretation of these findings is, therefore, not an easy task. Still, the study indicates that patients with fibromyalgia have abnormal touch processing, as well as altered opioid signaling. More research is needed to determine whether these findings might mirror underlying fibromyalgia pathology.
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