Brain Modulation by Electric Currents in Fibromyalgia: A Structured Review on Non-invasive Approach With Transcranial Electrical Stimulation

Abstract

Fibromyalgia syndrome (FMS) is a complex disorder where widespread musculoskeletal pain is associated with many heterogenous symptoms ranging from affective disturbances to cognitive dysfunction and central fatigue. FMS is currently underdiagnosed and often very poorly responsive to pharmacological treatment. Pathophysiology of the disease remains still obscure even if in the last years fine structural and functional cerebral abnormalities have been identified, principally by neurophysiological and imaging studies delineating disfunctions in pain perception, processing and control systems. On such basis, recently, neurostimulation of brain areas involved in mechanism of pain processing and control (primary motor cortex: M1 and dorsolateral prefrontal cortex: DLPFC) has been explored by means of different approaches and particularly through non-invasive brain stimulation techniques (transcranial magnetic and electric stimulation: TMS and tES). Here we summarize studies on tES application in FMS. The great majority of reports, based on direct currents (transcranial direct currents stimulation: tDCS) and targeting M1, showed efficacy on pain measures and less on cognitive and affective symptoms, even if several aspects as maintenance of therapeutical effects and optimal stimulation parameters remain to be established. Differently, stimulation of DLPFC, explored in a few studies, was ineffective on pain and showed limited effects on cognitive and affective symptoms. Very recently new tES techniques as high-density tDCS (HD-tDCS), transcranial random noise stimulation (tRNS) and tDCS devices for home-based treatment have been explored in FMS with interesting even if very preliminary results opening interesting perspectives for more effective, well tolerated, cheap and easy therapeutic approaches.

Keywords: fibromyalgia (FM); non-invasive brain stimulation (NIBS); tDCS — transcranial direct current stimulation; tRNS (transcranial random noise stimulation); transcranial electrical stimulation (tES).

FIGURE 1

Different forms of transcranial electrical stimulation. mA (milliampere). Stimulating current can be: direct, continuous (transcranial direct current stimulation: tDCS) (1) that can be anodal or cathodal; alternate (2) with polarity changing a different frequency between anode and cathode at fixed (2A transcranial alternate current stimulation: tACS) or randomly changing frequency (2B transcranial random noise stimulation: tRNS). Different montages (3) can also be applied (anodal stimulation of M1 is exemplified in the picture). In classical tDCS (3A) anode is on the target and cathode in the reference area (contralateral supraorbital region); in the case of High definition-tDCS (HD-tDCS) (3B) anode (more smaller in size to increase focality) is positioned over the target area and is surrounded by four equally-spaced cathodes.

FIGURE 2

The stochastic Resonance, a phenomenon invoked to explain the effects of tACS and more in particular of tRNS. According to this principle a signal containing a high level of noise has more chance to increase excitability of neurons that are near to activation threshold making them to discharge.

FIGURE 3

Scores for risk bias analysis for all items evaluated in each study examined. “+”: low risk; “?”: unclear risk; “-”: high risk.