Functional and Affective Effects of a Brief tDCS Intervention in Fibromyalgia and Chronic Fatigue Syndrome (FibFatDCS)

Abstract Background: Fibromyalgia (FM) and chronic fatigue syndrome (CFS) are characterized by persistent, multidimensional symptoms arising from complex brain-body interactions that impair functioning and quality of life. Noninvasive neuromodulation techniques such as transcranial direct current stimulation (tDCS) have been proposed as adjunctive approaches to support short-term functional changes within distributed neural systems, although evidence remains limited and heterogeneous. Objective: To examine immediate pre-post changes in multidimensional clinical outcomes following a standardized tDCS protocol in patients with FM and CFS, and to explore potential diagnosis-related response patterns. Methods: Fifteen patients (FM: n = 5; CFS: n = 10) received 10 sessions of tDCS (20 minutes per session) administered over two consecutive weeks (five sessions per week). Patient-reported outcomes were assessed at baseline and immediately after treatment completion. Measures included the Short Form-36 Health Survey (SF-36), Fibromyalgia Impact Questionnaire (FIQ), Hospital Anxiety and Depression Scale (HADS), Composite Autonomic Symptom Score (COMPASS), pain thermometer, Epworth Sleepiness Scale, Fatigue Severity Scale (FSS), and Fatigue Assessment Scale (FAS). Pre-post comparisons were conducted at the whole-sample level, followed by exploratory analyses stratified by diagnosis. Results: At baseline, patients with FM showed lower COMPASS, FSS, FAS, and HADS scores, and higher SF-36 scores, compared to patients with CFS, indicating a relatively less severe clinical profile. Across the sample, pre-post comparisons indicated improvements in FIQ and HADS scores. Exploratory group-specific analyses suggested that FM patients exhibited reductions in depressive symptoms, whereas CFS patients showed improvements in quality of life (SF-36) and functional impact (FIQ). No significant changes were observed in subjective pain intensity or fatigue severity (FSS). Conclusions: These findings suggest partially distinct clinical profiles and differential responsiveness to tDCS across FM and CFS, within the context of baseline differences between groups. Improvements appeared domain-specific, with affective symptoms responding in FM and functional outcomes in CFS, while core symptoms such as pain and fatigue remained unchanged over the intervention period. This pattern provides preliminary support for the idea that short-term neuromodulation may preferentially influence higher-order regulatory processes rather than core symptom generators, underscoring the need for tailored, multidimensional treatment approaches. Nevertheless, given the small sample size, absence of a control condition, and exploratory nature of subgroup analyses, these results should be interpreted with caution. Further controlled

studies are needed to clarify the reliability, specificity, and clinical relevance of these effects.