Background and aims: Previous literature reports that tDCS over the primary motor cortex (M1) may improve pain symptoms in patients with fibromyalgia (FM). We aim to provide evidence on this treatment and compare the effects of stimulation over M1 vs. operculo-insular cortex (OIC), a more specific pain processing area. 

Methods: 78 FM female patients were randomly assigned to 3 treatment groups (left M1, OIC or sham). We delivered 15 consecutive (3-week) 20-minute sessions of 2-mA anodal tDCS. Visual Analogue Scales (VAS) for pain and other core symptoms of fibromyalgia, and quantitative sensory testing (mean algometry at the 18 tender points), were applied before and after treatment. During the intervention, adverse effects were controlled using a standardized questionnaire.

Results: The three groups were comparable in the severity of FM symptoms prior to the intervention. The treatment effect was significant for pain assessed by VAS, regardless of the group: all patients, including those under sham stimulation, showed improvement in perceived pain. In contrast, tDCS had no effect on pain thresholds obtained by algometry. No major adverse effects were reported.

Conclusions: Stimulation over M1 and OIC did not show greater pain relief than sham stimulation, and thus treatment effects may be considered non-specific. Given our results and previous literature, we suggest that the outcome variables choice is critical to assess the effectiveness of tDCS, a safe intervention. Due to the complexity of FM symptomatology, further analyses of the effectiveness of tDCS in other comorbid symptoms (as depression or cognitive dysfunction) are needed.

EFFECTS OF TRANSCRANIAL DIRECT CURRENT STIMULATION (TDCS) ON CLINICAL AND EXPERIMENTAL PAIN IN PATIENTS WITH FIBROMYALGIA