Fibromyalgia (FM) is a chronic disease that affects more than five million people annually in the US. The population prevalence ranges from 2% to 5.4% according to the 2010 American Society of Rheumatology diagnostic criteria. The female: male ratio has changed according to the diagnostic criteria. When applying the 1990 diagnostic criteria (ACR 1990) it is 7:1 and according to the 2010 criteria (ACR 2010) and revised in 2016, this ratio is 2.3:1. Regardless of the variation in prevalence between the sexes, it is a syndrome characterized by generalized musculoskeletal pain, fatigue, non-restorative sleep, cognitive changes, depressive symptoms and other correlates of autonomic dysfunction, such as colon syndrome. irritability and bladder tenesmus. In addition, FM has a level of psychological distress associated with catastrophism and depressive symptoms, which can worsen the prognosis and reinforce disability more markedly than is observed in other chronic pain conditions. FM is the prototype of dysfunction syndromes that course with pain by processes mediated by the central and peripheral nervous system. Patients with FM have been associated with an increased risk of stroke. In fact, it is a syndrome with great social impact, which demands advances in the understanding of diagnostic and therapeutic processes. In this direction, an attempt has been made to identify factors associated with the potential for response to treatment: potential for response to placebo, clinical, psychological, neurophysiological factors, levels of endorphins and neurotrophins associated with clinical response. The quest to understand the effects of these markers aims to personalize therapy and identify factors that can modify the clinical effect of treatments and the organization of neurobiological systems.

Although the results with the use of tDCS have been promising in the treatment of several chronic pain conditions, as demonstrated in a recent meta-analysis conducted by the Pain and Neuromodulation research group at Hospital de Clínicas de Porto Alegre (Zortea et al., 2019). ) it is necessary to consider that expectations about pain can directly affect nociceptive processing. Such expectations contribute to the placebo effect that has been shown to be mediated by opioids. The study by Eippert et al. (2009) showed that naloxone administration reduces the neural and behavioral effects of placebo, as well as placebo-induced responses in several cortical and subcortical areas that constitute the descending pain modulatory system (e.g., rACC, PAG, RVM, and hypothalamus). ). Furthermore, it abolished the rACC-PAG coupling induced by the placebo intervention. The expectation of placebo-induced analgesia is positively correlated with the availability of MOR. On the contrary, negative suggestion reduces the analgesic effects of synthetic opiates. All these findings support the close association between opioids and expectations that drive placebo-mediated analgesia.

Therefore, this clinical trial aims to compare the effectiveness of anodic tDCS applied to the left dorsolateral prefrontal cortex (DLPFC) compared to sham tDCS in FM, according to susceptibility to the placebo effect and serum endorphin levels, in the following results ( 1) treatment efficacy, which includes daily measurements recorded in a Brief Pain Inventory (BPI) application, which allows assessing pain from a multidimensional perspective (pain intensity and interference with general activities, mood, mobility, work, personal relationships, sleep and enjoyment of life, etc.) (primary outcome), the primary outcome will be the Brief Pain Inventory (BPI) score, which assesses pain levels, sleep quality, mood, fatigue, disability, use of analgesics, etc. Secondary outcomes are the impact of pain on quality of life, depressive symptoms. Predictors of the placebo effect will also be evaluated, through the Verbal Numerical Pain Scale (NPS, 0-10) scores in the last week and through the BPI scores, using a multiple hierarchical model. Among potential predictors, there is disability due to pain, psychological profile, drugs in use. In addition to these, the index of inflammatory markers (serum interleukins IL1-, IL-6, IL-10; TNF-alpha and C-reactive protein); serum levels of endorphins, brain-derived neurotrophic factor (BDNF), S-100B protein and the polymorphisms Val66Met of the BDNF gene and Val158Met of the COMT enzyme gene. Will be included 100 women with FM, randomized to receive active (n=50) or sham (n=50) tDCS, aged between 18 and 65 years. Randomization will be stratified according to the placebo effect response potential, determined after a simulated tDCS session. A variation in NPS (0-10) equal to or greater than 30% after a simulated tDCS session will classify them as high responders to the placebo effect. The follow-up time will be four weeks after a single treatment session. In this way, it is intended to produce consistent evidence for the use of this low-cost technique in the treatment of patients of the Unified Health System, which will be directed to patients with chronic pain at the same time, which will provide data to subsidize the feasibility of studying the effect of this complementary therapy in other conditions, such as refractory depression and neurorehabilitation. In addition, to provide data to better understand tDCS response predictors in outcomes that are related to the impact of pain on the quality of life of conditions that generate a lot of suffering and high costs to the health system.