Cervical trans-spinal direct current stimulation: a modelling-experimental approach

Sofia Rita Fernandes,Pedro Cavaleiro Miranda,Mamede De Carvalho,Ricardo Salvador,Mariana Pereira

doi:10.1186/s12984-019-0589-6

Abstract

BackgroundTrans-spinal direct current stimulation (tsDCS) is a non-invasive technique with promising neuromodulatory effects on spinal cord (SC) circuitry. Computational studies are essential to guide effective tsDCS protocols for specific clinical applications. This study aims to combine modelling and experimental studies to determine the electrode montage that maximizes electric field (E-field) delivery during cervical tsDCS.MethodsCurrent and E-field distributions in the cervical SC were predicted for four electrode montages in a human realistic model using computational methods. A double-blind crossover and randomized exploratory study was conducted using the montage that maximized E-field delivery. tsDCS was applied for 15 min in 10 healthy subjects (anodal, cathodal, sham, with polarity assigned to the cervical electrode), with a current intensity of 2.5 mA, resulting in a total current charge density delivery of 90 mC/cm2. Upper limb motor (transcranial magnetic stimulation) and sensory evoked potentials (MEP, SEP), M-waves, H-reflex and F-wave responses were analysed. Central and peripheral conduction times were determined using MEP. Repeated measures ANOVA and Friedman test were used for statistical analysis (significance level α = 0.05).ResultsAll montages presented higher current density and E-field magnitudes in the cervical SC region between the electrodes. However, electrodes at C3 and T3 spinous processes (C3-T3) originated the highest E-field magnitude (0.50 V/m). Using C3-T3 montage we observed significant changes in N9 SEP latency (p = 0.006), but significance did not persist in pairwise comparisons (sham-anodal: p = 0.022; sham-cathodal: p = 0.619; anodal-cathodal: p = 0.018; α = 0.017, Bonferroni corrected). MEP latency and central motor conduction time (CMCT) modified significantly on stimulation (p = 0.007 and p = 0.015, respectively). In addition, pairwise comparisons confirmed significant differences between sham and cathodal conditions after Bonferroni correction for MEP latency (sham-anodal: p = 0.868; sham-cathodal: p = 0.011; anodal-cathodal: p = 0.023) and CMCT (sham-anodal: p = 0.929; sham-cathodal: p = 0.010; anodal-cathodal: p = 0.034).ConclusionsComputational models predicted higher E-field delivery in the cervical SC for the C3-T3 montage. Polarity-dependent effects in motor responses were reported using this montage consistent with spinal motor modulation. tsDCS experimental protocol designs should be guided by modelling studies to improve effectiveness.

Highlights

Trans-spinal direct current stimulation is a non-invasive technique with promising neuromodulatory effects on spinal cord (SC) circuitry
Trans-spinal direct current stimulation has recently emerged as a non-invasive technique with promising neuromodulatory effects on spinal circuitry related to motor and sensory responses of the upper and lower limbs [7, 8]. tsDCS has a similar approach as transcranial direct current stimulation, a noninvasive brain stimulation method for modulating cortical excitability [28]
Whereas anodal transcranial direct current stimulation (tDCS) produces facilitation in cortical motor responses [28], exploratory tsDCS studies in humans report a variety of polarity-dependent effects in spinal motor responses when stimulating the cervical SC: facilitation of motor responses of abductor digiti minimi (ADM) and abductor policis brevis (APB) muscles was observed during cathodal tsDCS applied over C7 spinous process (s.p.), with the anode over the right deltoid muscle (C7-rD montage, [4]); increased amplitude of motor evoked potentials (MEP) of the flexor carpi radialis (FCR) was observed independently of the polarity of the electrodes, placed at C7 s.p. and cervicomental angle (C7-Cervicomental angle (CMA) montage; [22])

Summary

Introduction

Trans-spinal direct current stimulation (tsDCS) is a non-invasive technique with promising neuromodulatory effects on spinal cord (SC) circuitry. Trans-spinal direct current stimulation (tsDCS) has recently emerged as a non-invasive technique with promising neuromodulatory effects on spinal circuitry related to motor and sensory responses of the upper and lower limbs [7, 8]. TsDCS has a similar approach as transcranial direct current stimulation (tDCS), a noninvasive brain stimulation method for modulating cortical excitability [28] It applies a constant low intensity electric current through surface electrodes. Cervical tsDCS was observed to increase corticophrenic pathway excitability, considering a C4-CMA montage: increased diaphragmatic MEP amplitude was observed independently of C4 polarity and tidal volume was increased with the cathode placed at C4 [25]

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