Trans-spinal Direct Current Stimulation Research Articles

Direct Current Stimulation over the Primary Motor Cortex, Cerebellum, and Spinal Cord to Modulate Balance Performance: A Randomized Placebo-Controlled Trial.

Existing applications of non-invasive brain stimulation in the modulation of balance ability are focused on the primary motor cortex (M1). It is conceivable that other brain and spinal cord areas may be comparable or more promising targets in this regard. This study compares transcranial direct current stimulation (tDCS) over (i) the M1, (ii) the cerebellum, and (iii) trans-spinal direct current stimulation (tsDCS) in the modulation of balance ability. Forty-two sports students were randomized in this placebo-controlled study. Twenty minutes of anodal 1.5 mA t/tsDCS over (i) the M1, (ii) the cerebellum, and (iii) the spinal cord, as well as (iv) sham tDCS were applied to each subject. The Y Balance Test, Single Leg Landing Test, and Single Leg Squat Test were performed prior to and after each intervention. The Y Balance Test showed significant improvement after real stimulation of each region compared to sham stimulation. While tsDCS supported the balance ability of both legs, M1 and cerebellar tDCS supported right leg stand only. No significant differences were found in the Single Leg Landing Test and the Single Leg Squat Test. Our data encourage the application of DCS over the cerebellum and spinal cord (in addition to the M1 region) in supporting balance control. Future research should investigate and compare the effects of different stimulation protocols (anodal or cathodal direct current stimulation (DCS), alternating current stimulation (ACS), high-definition DCS/ACS, closed-loop ACS) over these regions in healthy people and examine the potential of these approaches in the neurorehabilitation.

Transspinal Direct Current Electrical Stimulation Selectively Affects the Excitability of the Corticospinal System, Depending on the Intensity but Not Motor Skills.

Transspinal direct current stimulation (tsDCS) is a non-invasive technique used to modulate spinal cord activity. However, the effects and mechanisms of this stimulation are currently not comprehensively known. This study aimed to estimate the effect of different intensities of tsDCS applied at the level of cervical enlargement of the spinal cord (C7-Th1 segments) on the excitability of the corticospinal system (CSS) and the correction of motor skills in healthy subjects. The effect of tsDCS was estimated by the motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS) in the primary motor cortex (M1). The study involved 54 healthy adults aged 22 ± 4 years. The application of 11 min anodal tsDCS at the level of the cervical spine C7-Th1 with a current intensity of 2.5 mA did not change the MEP amplitude of the upper limb muscles, in contrast to the data that we previously obtained with a current intensity of 1.5 mA. We also found no difference in the effect of 2.5 mA stimulation on motor skill correction in healthy subjects in the nine-hole peg test (9-HPT) and the serial reaction time task (SRT) as with 1.5 mA stimulation. Our data show that an increase in the intensity of stimulation does not lead to an increase in the effects but rather reduces the effects of stimulation. These results provide information about the optimally appropriate stimulation current intensities to induce CSS excitability and the ability of tsDCS to influence motor skills in healthy adults.

The intensity of transspinal direct current stimulation affects the excitability of the corticospinal system

BACKGROUND: Transspinal direct current stimulation (tsDCS) affects the corticospinal system, one of the central human systems associated with controlling precise voluntary movements. Stimulation effects are very sensitive to montage and protocols of applied stimulation because they can involve different neuronal mechanisms. AIM: This study aimed to estimate the effects of parameters of anodal tsDCS applied at the level of the spinal cord (C7–Th1 segments) with cervical enlargement to determine the excitability of the corticospinal system and the correction of motor skills in healthy people. METHODS: The study involved 81 healthy adults aged 21.19±3.20 years. The effect of tsDCS was assessed using motor-evoked potentials from the first dorsal interosseous (FDI) muscle by transcranial magnetic stimulation in the primary motor cortex before stimulation, immediately after stimulation, and after 15 min. RESULTS: The application of 11-min anodal tsDCS at the C7–Th1 level with a current of 1.5 mA affects the FDI muscle, initially reducing the amplitude of transcranial magnetic stimulation induced motor-evoked potentials immediately after stimulation. The amplitude of the motor-evoked potentials increases after 15 min of stimulation. tsDCS with an intensity of 2.5 mA does not affect the change in the amplitude of motor-evoked potentials. Similarly, no difference was found in the effect of 1.5 mA stimulation on the correction of motor skills in healthy adults at the nine-hole peg test and the serial reaction time task as with 2.5 mA. CONCLUSION: This study adds information about the optimally appropriate current intensities of stimulation to induce corticospinal system excitability and the ability of tsDCS to influence motor skills in healthy adults.

Transspinal direct current stimulation with an intensity of 2.5 mA does not affect the corticospinal system excitability and motor skills

BACKGROUND: Noninvasive brain stimulation effectively affects movements, including the spinal cord level. Stimulation effects are very sensitive to montage and protocols of applied stimulation because they can involve different neuronal mechanisms. AIM: This study aimed to estimate the effect of anodal transspinal direct current stimulation (tsDCS) with an intensity of 2.5 mA applied at the spinal cord level (C7–Th1 segments) with cervical enlargement on the corticospinal system excitability and motor skills. METHODS: The study involved 54 healthy adults aged 21.19±3.20 years. The effect of tsDCS was assessed using motorevoked potentials from the first dorsal interosseous (FDI) muscle by transcranial magnetic stimulation in the primary motor cortex before stimulation, immediately after stimulation, and after 15 min. RESULTS: The application of an 11-min anodal tsDCS with a current value of 2.5 mA at the C7–Th1 level did not affect the motorevoked potentials of FDI. Statistically, changes in motorevoked potentials amplitudes did not differ between groups receiving anodal tsDCS and sham stimulation. In addition, anodal tsDCS did not affect motor skills. An individual’s ability to coordinate fingers and manipulate objects effectively (a measure of dexterity) in the nine-hole peg test and pressing a key in response to a visual stimulus in the serial reaction time task did not differ from that with sham stimulation. CONCLUSION: 2.5 mA anodal tsDCS on cervical enlargement does not affect the corticospinal system excitability or change motor skills associated with precise hand movements.

Towards chronic non-invasive stimulation: what can you learn from pain research?

This scientific commentary refers to 'Long-term analgesic effect of trans-spinal direct current stimulation compared to non-invasive motor cortex stimulation in complex regional pain syndrome, by Hodaj et al. (https://doi.org/10.1093/braincomms/fcad191).

Long-term analgesic effect of trans-spinal direct current stimulation compared to non-invasive motor cortex stimulation in complex regional pain syndrome.

The aim of the present study was to compare the analgesic effect of motor cortex stimulation using high-frequency repetitive transcranial magnetic stimulation or transcranial direct current stimulation and transcutaneous spinal direct current stimulation in patients with complex regional pain syndrome. Thirty-three patients with complex regional pain syndrome were randomized to one of the three treatment groups (repetitive transcranial magnetic stimulation, n = 11; transcranial direct current stimulation, n = 10; transcutaneous spinal direct current stimulation, n = 12) and received a series of 12 sessions of stimulation for 3 weeks (induction phase) and 11 sessions for 4 months (maintenance therapy). The primary end-point was the mean pain intensity assessed weekly with a visual numerical scale during the month prior to treatment (baseline), the 5-month stimulation period and 1 month after the treatment. The weekly visual numerical scale pain score was significantly reduced at all time points compared to baseline in the transcutaneous spinal direct current stimulation group, at the last two time points in the repetitive transcranial magnetic stimulation group (end of the 5-month stimulation period and 1 month later), but at no time point in the transcranial direct current stimulation group. A significant pain relief was observed at the end of induction phase using transcutaneous spinal direct current stimulation compared to repetitive transcranial magnetic stimulation (P = 0.008) and to transcranial direct current stimulation (P = 0.003). In this trial, transcutaneous spinal direct current stimulation was more efficient to relieve pain in patients with complex regional pain syndrome compared to motor cortex stimulation techniques (repetitive transcranial magnetic stimulation, transcranial direct current stimulation). This efficacy was found during the induction phase and was maintained thereafter. This study warrants further investigation to confirm the potentiality of transcutaneous spinal direct current stimulation as a therapeutic option in complex regional pain syndrome.

Transcranial direct current stimulation alleviates nociceptive behavior in male rats with neuropathic pain by regulating oxidative stress and reducing neuroinflammation.

Transcranial direct curent stimulation (tDCS) and trans-spinal direct current stimulation (tsDCS) are promising therapies for pain that can alter the excitability of neuronal activity in cerebral cortex. The aim of the study is to investigate the therapeutic effects of direct current stimulation (DCS) over the spinal cord and cerebral cortex on oxidative stress and neuroinflammation in rats with chronic constriction injury (CCI).Male Wistar rats were randomly divided into four experimental groups: Sham, CCI, CCI + tDCS and CCI + tsDCS. The neuropathic pain model was induced by using the CCI model. Rats with neuropathy were treated with cathodal tDCS and tsDCS stimulations consisting of 0.5 mA for 30 min a day for 7 days from day 8 onwards. Locomotor activity was measured by open-field test and nociceptive behavior was assessed by hot-plate, tail-flick and Randall-Selitto tests. Following the behavioral experiments, total oxidant capacity (TOC), total antioxidant capacity (TAC) and proinflammatory cytokine levels were evaluated in spinal cord and cerebral cortex tissues. The CCI model induced significant mechanical and thermal hyperalgesia. Nociceptive behaviors in rats with CCI were reversed by DCS treatment. Higher TOC and lower TAC levels were detected in the spinal cord and cerebral cortex tissues of the CCI rats compared to the control. tsDCS treatment amended oxidant/antioxidant status. Moreover, tsDCS modulated the central levels of Tumor necrosis factor-α (TNF-α), interleukin 1-beta (IL-1β), IL-6 and IL-18. tsDCS stimulation showed better therapeutic effect on neuropathic pain by regulating oxidant/antioxidant levels and reducing neuroinflammation. DCS, especially at spinal level, may be a promising therapeutic strategy that can be used alone or in combination with other effective treatments for alleviating neuropathic pain.

Effectiveness of Repetitive Transcranial Magnetic Stimulation Combined With Transspinal Electrical Stimulation on Corticospinal Excitability for Individuals With Incomplete Spinal Cord Injury: A Pilot Study.

Repetitive Transcranial Magnetic Stimulation (rTMS) and transspinal electrical stimulation (tsES) have been proposed as a novel neurostimulation modality for individuals with incomplete spinal cord injury (iSCI). In this study, we integrated magnetic and electrical stimulators to provide neuromodulation therapy to individuals with incomplete spinal cord injury (iSCI). We designed a clinical trial comprising an 8-week treatment period and a 4-week treatment-free observation period. Cortical excitability, clinical features, inertial measurement unit and surface electromyography were assessed every 4 weeks. Twelve individuals with iSCI were recruited and randomly divided into a combined therapy group, a magnetic stimulation group, an electrical stimulation group, or a sham stimulation group. The magnetic and electric stimulations provided in this study were intermittent theta-burst stimulation (iTBS) and 2.5-mA direct current (DC) stimulation, respectively. Combined therapy, which involves iTBS and transspinal DC stimulation (tsDCS), was more effective than was iTBS alone or tsDCS alone in terms of increasing corticospinal excitability. In conclusion, the effectiveness of 8-week combined therapy in increasing corticospinal excitability faded 4 weeks after the cessation of treatment. According to the results, combination of iTBS rTMS and tsDCS treatment was more effective than was iTBS rTMS alone or tsDCS alone in enhancing corticospinal excitability. Although promising, the results of this study must be validated by studies with longer interventions and larger sample sizes.

Clinical transspinal direct current stimulation for pain treatment. Evaluation including factors predicting treatment outcome.

Clinical transspinal direct current stimulation for pain treatment. Evaluation including factors predicting treatment outcome.

TH-196. Clinical transspinal direct current stimulation for pain treatment. Evaluation including factors predicting treatment outcome

TH-196. Clinical transspinal direct current stimulation for pain treatment. Evaluation including factors predicting treatment outcome

Neural data-driven model of spinal excitability changes induced by transcutaneous electrical stimulation in spinal cord injury subjects.

The efficacy of trans-spinal direct current stimulation (tsDCS) as neurorehabilitation technology remains sub-optimal, partly due to the variability introduced by subject-specific neurophysiological features and stimulation conditions (e.g. electrode placement, stimulating amplitude, polarity, etc.) Hence, current therapies apply tsDCS in an open-loop fashion, resulting in a lack of standardized protocols for controlling elicited neuronal adaptations in closed-loop. Through the combination of high-density electromyogram (HD-EMG) decomposition, biophysical neuronal modelling and metaheuristic optimization, this work presents a novel neural data-driven framework for estimating subject-specific features and quantifying acute neuronal adaptations elicited by tsDCS on incomplete spinal cord injury subjects. This approach consists of calibrating the anatomical parameters (e.g. soma diameter) of in silico $\alpha-$motoneuron (MN) models for firing similarly to in vivo MNs decoded from HD-EMG. Assuming that cathodal-tsDCS elicits excitability changes in the MN pool, while preserving their anatomical parameters, optimization of an excitability gain common to the entire pool was performed to minimize discrepancies in firing rate and recruitment time between in vivo and in silico MNs after cathodal-tsDCS. This quantification of excitability changes on MN models calibrated in a person specific way enables closing the loop with neuro-modulation devices to tailor neurorehabilitation therapies. Clinical Relevance - This framework addresses a key limitation in non-invasive neuro-modulative technologies via a novel model-assisted framework that enables quantifying acute excitability changes induced on a person-specific in silico MN pool calibrated using in vivo neural data. This will enable the development of advanced controllers for modulating targeted neuronal adaptations in closed-loop.

Differential Effects of Invasive Anodal Trans-spinal Direct Current Stimulation on Monosynaptic Excitatory Postsynaptic Potentials, Ia Afferents Excitability, and Motoneuron Intrinsic Properties Between Superoxide Dismutase Type-1 Glycine to Alanine Substitution at Position 93 and Wildtype Mice

In presymptomatic amyotrophic lateral sclerosis (ALS), spinal motoneurons (MNs) have reduced firing patterns and synaptic excitation levels. Preliminary data indicated that in the SOD1 G93A mouse model of ALS, monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in spinal MN by Ia proprioceptive afferent stimulation could be facilitated by trans-spinal direct current stimulation (tsDCS). However, which element of the Ia afferent-MN circuit is affected by tsDCS, and whether tsDCS-induced EPSP facilitation is a general phenomenon or specific to the superoxide dismutase type-1 (SOD1) Glycine to Alanine substitution at position 93 (G93A) mutation, remain to be determined. In this study, we have applied 15-minute tsDCS to the lumbar segments of presymptomatic SOD1 and wildtype (WT) mice and explored its impact on MN passive membrane properties, EPSP amplitude, and Ia afferent activity. While anodal tsDCS induced short-lasting EPSP facilitation in both SOD1 and WT mice, Ia afferent activity and passive membrane properties were altered only in SOD1 mice. Interestingly, EPSP amplitudes of SOD1 mice remained facilitated for at least 1 h after current application, but no long-lasting effect was observed in WT mice. Moreover, anodal tsDCS failed to induce any long-lasting changes in MN passive membrane properties in both SOD1 and WT mice. Conversely, cathodal tsDCS decreased Ia afferent induced EPSP amplitudes only during current application in SOD1 MNs, and no significant effects on Ia afferents excitability were observed. Our findings indicate the high susceptibility of SOD1 MNs to tsDCS and highlight the potential of this neuromodulation technique for the treatment of ALS.

Trans-Spinal Direct Current Stimulation in Spasticity: A Literature Mini-Review

Spasticity is common after a stroke and has a negative impact on functional and quality-of-life measures. There is an unmet medical need to provide safe and effective treatment using non-pharmacological approaches. Trans-spinal direct current stimulation (tsDCS) is an emerging modality for non-invasive neuromodulation that induces reduction of spinal excitability leading to a decrease in spasticity. We describe current treatment options for spasticity, including a literature review about the use of tsDCS in patients with spasticity. We found four clinical studies that used tsDCS to treat spasticity for different neurological conditions including hereditary spastic paraplegia, upper extremity spasticity following stroke, multiple sclerosis, and incomplete chronic spinal cord injury. Spasticity was the primary outcome in three of the studies and a secondary outcome in the final study. The three studies that addressed spasticity as the primary outcome found that active tsDCS decreased spasticity compared to sham. These studies suggest that tsDCS can modulate spinal motor and sensory spinal pathways through the use of specific electrode montages and stimulation parameters. This therapy can improve motor functions and may represent a viable treatment option for spasticity.

Trans-Spinal Direct Current Stimulation Targets Ca2+ Channels to Induce Persistent Motor Unit Responses.

Trans-spinal direct current stimulation (tsDCS) is a neuromodulatory approach to augment spinal cord activity to improve function after neurological disease and injury. Little is known about the mechanisms underlying tsDCS actions on the motor system. The purpose of this study is to determine the role for a persistent inward current (PIC)-like response in motoneurons in mediating tsDCS actions. We recorded single motor units from the extensor and flexor carpi radialis muscles in healthy sedated rats and measured unit activity changes produced by cervical enlargement cathodal and anodal tsDCS (c-tsDCS; a-tsDCS). Both c-tsDCS and a-tsDCS immediately increased spontaneous motor unit firing during stimulation. After c-tsDCS was stopped, spontaneous firing persisted for a substantial period (165 ± 5s), yet after a-tsDCS activity shortly returned to baseline (27 ± 7s). Administration of the L-type calcium channel blocker Nimodipine reduced spontaneous motor unit firing during c-tsDCS and blocked the persistent response. By contrast, Nimodipine did not change unit firing during a-tsDCS but the short persistent response was blocked. Computer simulation using a two-compartment neuronal model replicated the main experimental observations: larger and more persistent responses during and after c-tsDCS than a-tsDCS. Using reduced Ca2+ conductance to model Nimodipine action, a reduced response during c-tsDCS and elimination of the persistent response was observed. Our experimental findings, supported by computer simulation, show that c-tsDCS can target Ca2+ conductances to augment motoneuron activity. As tsDCS is well-tolerated in humans, this knowledge informs therapeutic treatment strategies to achieve rehabilitation goals after injury; in particular, to increase muscle force.

Selective augmentation of corticospinal motor drive with trans-spinal direct current stimulation in the cat

BackgroundA key outcome for spinal cord stimulation for neurorehabilitation after injury is to strengthen corticospinal system control of the arm and hand. Non-invasive, compared with invasive, spinal stimulation minimizes risk but depends on muscle-specific actions for restorative functions. ObjectiveWe developed a large-animal (cat) model, combining computational and experimental techniques, to characterize neuromodulation with transcutaneous spinal direct current stimulation (tsDCS) for facilitation of corticospinal motor drive to specific forelimb muscles. MethodsAcute modulation of corticospinal function by tsDCS was measured using motor cortex-evoked muscle potentials (MEPs). The effects of current intensity, polarity (cathodal, anodal), and electrode position on specific forelimb muscle (biceps vs extensor carpi radialis, ECR) MEP modulation were examined. Locations of a key target, the motoneuron pools, were determined using neuronal tracing. A high-resolution anatomical (MRI and CT) model was developed for computational simulation of spinal current flow during tsDCS. ResultsEffects of tsDCS on corticospinal excitability were robust and immediate, therefore supporting MEPs as a sensitive marker of tsDCS targeting. Varying cathodal/anodal current intensity modulated MEP enhancement/suppression, with higher cathodal sensitivity. Muscle-specificity depended on cathode position; the rostral position preferentially augmented biceps responses and the caudal position, ECR responses. Precise anatomical current-flow modeling, supplemented with target motor pool distributions, can explain tsDCS focality on muscle groups. ConclusionAnatomical current-flow modeling with physiological validation based on MEPs provides a framework to optimize muscle-specific tsDCS interventions. tsDCS targeting of representative motor pools enables muscle- and response-specific neuromodulation of corticospinal motor drive.

Trans-Spinal Electrical Stimulation Therapy for Functional Rehabilitation after Spinal Cord Injury: Review.

Spinal cord injury (SCI) is one of the most debilitating injuries in the world. Complications after SCI, such as respiratory issues, bowel/bladder incontinency, pressure ulcers, autonomic dysreflexia, spasticity, pain, etc., lead to immense suffering, a remarkable reduction in life expectancy, and even premature death. Traditional rehabilitations for people with SCI are often insignificant or ineffective due to the severity and complexity of the injury. However, the recent development of noninvasive electrical neuromodulation treatments to the spinal cord have shed a ray of hope for these individuals to regain some of their lost functions, a reduction in secondary complications, and an improvement in their life quality. For this review, 250 articles were screened and about 150 were included to summarize the two most promising noninvasive spinal cord electrical stimulation methods of SCI rehabilitation treatment, namely, trans-spinal direct current stimulation (tsDCS) and trans-spinal pulsed current stimulation (tsPCS). Both treatments have demonstrated good success in not only improving the sensorimotor function, but also autonomic functions. Due to the noninvasive nature and lower costs of these treatments, in the coming years, we expect these treatments to be integrated into regular rehabilitation therapies worldwide.

Effects of paired stimulation with specific waveforms on cortical and spinal plasticity in subjects with a chronic spinal cord injury

Paired stimulation can cause neuroplasticity in corticospinal and spinal pathways in subjects with a chronic spinal cord injury (SCI). We aimed to know the effects of different waveforms using paired stimulations with bicycling in subjects with a chronic SCI. Recruited subjects with an SCI underwent three treatment interventions in random order for 4-20 min followed by 30 min of bicycling (control, repetitive transcranial magnetic stimulation (TMS; rTMS) at 20 Hz with transspinal direct current stimulation (tsDCS), and intermittent theta burst stimulation (iTBS) with tsDCS with a 1-week gap period. A TMS method was employed to record the resting motor threshold (RMT), the 90% values of which was used as the stimulation intensity, and the Hoffman (H)-reflex was measured by stimulating the tibial nerve in the popliteal fossa. The RMT, motor evoked potential (MEP) latency, MEP peak-to-peak amplitude, and H-reflex latency as primary variables and lower extremity motor scale (LEMS) and modified Ashworth spasticity scale (MAS) as secondary variables were analyzed before and after the interventions. The MEP latency, MEP amplitude, and LEMS significantly improved with the rTMS-iTBS/tsDCS or the rTMS-20 Hz/tsDCS (p < 0.050) protocols compared to the control intervention. All other outcome measures, including RMT, H-reflex latency, and MAS score showed some changes but did not fully attain a level of significance. The paired stimulation with rTMS-iTBS/tsDCS was equally effective to produce neuroplastic effect in subjects with chronic SCI compared to the conventional TMS-20 Hz/tsDCS intervention.

OC18: Transspinal direct current stimulation for pain treatment. Objective proof of concept, initial clinical findings and BDNF polymorphism

Introduction Transspinal Direct Current Stimulation (tsDCS) has been reported to block pain signaling. To approach clinical usage, two studies were performed. In both, the effect of BDNF polymorphism (Val/Met) was investigated. Materials and methods Study I Nineteen healthy controls, the anode over the tenth thoracic vertebra, the cathode over the shoulder. tsDCS: 2.5 A for 20 minutes, double blinded cross over design. Laser Evoked Potentials (LEPs): Stimulation of the dorsum of the hand and foot respectively, recordings at baseline, immediately after- and 30 minutes after the tsDCS stimulation. Amplitude and latency of the N2 peak was assessed and the subjects rated pain intensity. BDNF polymorphism was investigated in DNA from buccal mucosa cells. Study II Patients with neuropathic pain due to polyneuropathy are recruited. tsDCS is given at three days with one day between, BDNF polymorphism investigated as above. Results Study I Group level: LEP amplitudes successively reduced over time. Placebo; latencies unaffected from upper-, slightly increased from lower extremities. Verum; transiently increased from upper-, accumulatively increased from lower extremities in proportion to experienced pain. Individual level: Large variations. BDNF polymorphism influences the effect of stimulation with less pain reduction in those with less active BDNF (Val/Val). Study II Fourteen patients have been investigated. The results will be presented at the conference. Conclusions Short term, anodal thoracic tsDCS affects transspinal pain signaling with large inter-individual differences. Individualized tsDCS may be a treatment option for neuropathic pain. The type of BDNF one carries, seem to influence the effect of stimulation.

Influence of BDNF Genotype on the Effect of Neuromodulation on Pain Signalling and Pain Perception in Humans

Trans-spinal Direct Current Stimulation (tsDCS) can alleviate pain perception in humans through plastic processes. Brain Derived Neurotrophic Factor (BDNF) has been shown to influence a large number of physiological processes including those underpinning neural plasticity. A common polymorphism of BDNF (val66met), reduces the efficiency of plastic processes. We studied the effect of this BDNF polymorphism on the signalling and perception of thermal pain and to what degree these can be influenced by tsDCS in young healthy adults.

Neuromodulation of lower limb motor pathways with trans-spinal direct current stimulation: an overview of current findings

Introduction The spinal cord (SC) is a complex structure containing several neuronal circuits related with motor function of the upper and lower limbs, operating under the influence of higher centres. Central nervous diseases can change the responses of the spinal motor circuits leading to its dysfunction. Over the last decade, there has been a growing interest in the study of trans-spinal direct current stimulation (tsDCS) as a potential therapeutic tool to modulate spinal circuits through the application of electric currents delivered non-invasively [1]. Computational modelling studies are potentially useful to optimise electrodes montage in order to target current delivery to specific spinal region and pathways [2]. The aim of this study is to describe the more effective tsDCS electrode montages to maximise the modulation effects in lower limbs, as derived from a narrative review of the literature. Materials and methods A literature narrative review was carried out through Pubmed database and manual search, considering the following selection criteria: research papers published in journals with impact factor in the areas of neuroscience, neurophysiology and biomedical engineering from 2008 to 2018; use of keywords related with the topic (tsDCS, trans- spinal, lumbar, thoracic, spinal cord, motor pathways, computational modelling); written in English. A qualitative analysis was performed over the literature selected considering the following items: electrode montage; tsDCS protocol; methods for assessing motor responses; observed changes; computational results; induced electric field (EF) distribution. Results Published studies showed different neuromodulation effects on the motor responses of the lower limb. Whereas some studies reported a reduction of spinal motoneurons excitability using anodal tsDCS, others described higher motor unit recruitment with cathodal stimulation (T10–T12), suggesting modulation of Ia-motoneuron (MN) synapse. Other authors failed to replicate a modulatory effect applying tsDCS [3]. Modelling studies predicted EF magnitudes sufficient for neuromodulation in the montages previously applied in the spinal segments to be modulated when these are located between the electrodes [2]. Discussion and conclusions: Literature review indicates that experimental findings mainly depend on electrode polarity and position over the SC. tsDCS is a tool amenable to modulate motor circuits excitability in the spinal cord. This intervention can have clinical implications in the treatment of conditions like spasticity. However, future studies should consider EF magnitude and its orientation relative to spinal neurons. We propose that future clinical protocols should be guided by computational modelling to increase the chances of consistent positive results [4].