People with fibromyalgia are less responsive to gentle touches usually considered pleasant, which is driven by differences in how their brains process and understand touch, a recent study suggests.
The study, “Anhedonia to Gentle Touch in Fibromyalgia: Normal Sensory Processing but Abnormal Evaluation,” was published in the journal Brain Sciences.
Soft touching or brushing can normally induce feelings of pleasure by activating C-tactile (CT) fibers, a type of touch-sensitive nerve in the skin. Pleasant activation of CT fibers leads to activity in a part of the brain called the posterior insula.
These fibers have also been implicated in processing sensations of pain. As such, they could have a role in fibromyalgia.
In the study, researchers in Sweden assessed how CT fibers responded to soft brushing in 31 people with fibromyalgia (mean age 39) and 29 healthy controls matched for age. All participants were women. On clinical evaluation, the fibromyalgia group reported significantly greater pain catastrophizing — the inability to stop thinking about pain and its amplification — insomnia, body mass index, blood pressure, and anxiety/depression, as well as poorer overall health compared to the controls. Also, only patients reported experiencing pain.
Participants underwent MRI scans, and a researcher would apply one of two types of brushing to their left forearms: a slow brush, which is known to effectively stimulate CT fibers, and a faster brush that is typically not as effective at inducing pleasure. Patients and controls were asked to rate the pleasure they derived from touch, as well as their current level of overall pain.
Unlike in prior studies, results showed that the controls rated both fast and slow brushing as similarly pleasant. The researchers said that this could be due to a ceiling effect resulting from the long and uncomfortable environment of the MRI scanner, making any positive stimulus especially pleasant.
In contrast, the fibromyalgia group rated slow brushing as pleasant, but fast brushing as unpleasant. Compared to controls, people with fibromyalgia rated lower pleasure with slow brushing.
Although pain scores were overall higher in patients, both groups reported less pain after slow brushing than after fast brushing.
Activity in the posterior insula was similar in patients and controls, and stronger with slow brushing than with fast brushing. However, results showed differences in the association between posterior insula activation and self-reported sensation: in controls, higher pleasure was associated with more posterior insula activation, while higher pain ratings were associated with less activation. The FM group had the opposite pattern: more activation associated with pain, and less with pleasure.
“Our results suggest that FM [fibromyalgia] patients do not exhibit hypersensitivity to pleasant touch at an early processing stage, as we found no difference in neural activation in the posterior insula,” the researchers wrote. “However, we found altered activation patterns during the evaluation of positive stimuli and reporting of current pain levels.”
In other words, in people with fibromyalgia, the initial neurological response to touch is the same as in healthy individuals, but later processing in the brain leads to significant differences.
“Taken together, our results suggest intact early-stage sensory processing of positive tactile stimuli but dysfunctional evaluative processing,” the researchers wrote. “These findings contribute to our understanding of the mechanisms underlying anhedonia [an inability to feel pleasure] in FM.”
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