The Immediate and Short-Term Effects of Transcutaneous Spinal Cord Stimulation and Peripheral Nerve Stimulation on Corticospinal Excitability

Grid-based transcutaneous spinal cord stimulation: probing neuromodulatory effect in spinal flexion reflex circuits

Objective.Non-invasive spinal stimulation has the potential to modulate spinal excitability. This study explored the modulatory capacity of sub-motor grid-based transcutaneous spinal cord stimulation (tSCS) applied to the lumbar spinal cord in neurologically intact participants. Our objective was to examine the effect of grid spinal stimulation on polysynaptic reflex pathways involving motoneurons and interneurons likely activated by Aβ/δfiber-mediated cutaneous afferents.Approach.Stimulation was delivered using two grid electrode montages, generating a net electric field in transverse or diagonal directions. We administered tSCS with the center of the grid aligned with the T10-T11 spinous process. Participants were seated for the 20 min stimulation duration. At 30 min after the cessation of spinal stimulation, we examined neuromodulatory effects on spinal circuit excitability in the tibialis anterior muscle by employing the classical flexion reflex paradigms. Additionally, we evaluated spinal motoneuron excitability using theH-reflex paradigm in the soleus muscle to explore the differential effects of tSCS on the polysynaptic versus monosynaptic reflex pathway and to test the spatial extent of the grid stimulation.Main results.Our findings indicated significant neuromodulatory effects on the flexion reflex, resulting in a net inhibitory effect, regardless of the grid electrode montages. Our data further indicated that the flexion reflex duration was significantly shortened only by the diagonal montage.Significance.Our results suggest that grid-based tSCS may specifically modulate spinal activities associated with polysynaptic flexion reflex pathways, with the potential for grid-specific targeted neuromodulation.

Differential effects of stimulation waveform and intensity on the neural structures activated by lumbar transcutaneous spinal cord stimulation

Lumbar transcutaneous spinal cord stimulation (TSS) evokes synchronized muscle responses, termed spinally evoked motor response (sEMR). Whether the structures TSS activates to evoke sEMRs differ when TSS intensity and waveform are varied is unknown. In 15 participants (9 F, 6 M), sEMRs were evoked by TSS over L1-L3 (at sEMR threshold and suprathreshold intensities) with conventional (one 400-µs biphasic pulse) or high-frequency burst (ten 40-µs biphasic pulses at 10 kHz) stimulus waveforms in vastus medialis (VM), tibialis anterior (TA), and medial gastrocnemius (MG) muscles. TSS was paired with transcranial magnetic stimulation (TMS) over the contralateral motor cortex at relative interstimulus intervals (ISIs) (-10 ms to 11 ms), centered on the ISI when TSS and TMS inputs simultaneously activated VM motoneurons. Doublet TSS was delivered at 80-ms ISI. For VM, the area of the combined response evoked by paired TMS and TSS was not facilitated at any ISI. For TA and MG, combined responses were facilitated by ∼40-100% when TMS activated the motoneurons before or at a similar time as TSS, particularly with suprathreshold TSS. Additionally, for TA, there was greater suppression of the second sEMR evoked by TSS doublets using suprathreshold conventional TSS compared to high-frequency burst TSS (P < 0.001). The results suggest that for VM TSS activated predominantly motor axons, but for TA and MG facilitation of the sEMR by TMS suggests that TSS activated sensory axons. Stimulation waveforms had similar outcomes in most conditions.NEW & NOTEWORTHY Transcutaneous spinal cord stimulation (TSS) can evoke muscle responses by activation of sensory and/or motor axons. The relative contribution of these varies across the muscles tested. We found evidence for activation of sensory axons with TSS for tibialis anterior and medial gastrocnemius but not for vastus medialis. In cases where sensory axons were activated, conventional TSS activated relatively more sensory axons than high-frequency burst TSS.

Repetitive Transcranial Magnetic Stimulation for the Treatment of Spinal Cord Injury: Current Status and Perspective

Spinal cord injury (SCI) can lead to devastating dysfunctions and complications, significantly impacting patients' quality of life and aggravating the burden of disease. Since the main pathological mechanism of SCI is the disruption of neuronal circuits, the primary therapeutic strategy for SCI involves reconstructing and activating circuits to restore neural signal transmission. Repetitive transcranial magnetic stimulation (rTMS), a noninvasive brain stimulation technique, can modulate the function or state of the nervous system by pulsed magnetic fields. Here, we discuss the basic principles and potential mechanisms of rTMS for treating SCI, including promoting the reconstruction of damaged circuits in the spinal cord, activating reorganization of the cerebral cortex and circuits, modulating the balance of inputs to motoneurons, improving the microenvironment and intrinsic regeneration ability in SCI. Based on these mechanisms, we provide an overview of the therapeutic effects of rTMS in SCI patients with motor dysfunction, spasticity and neuropathic pain. We also discuss the challenges and prospectives of rTMS, especially the potential of combination therapy of rTMS and neural progenitor cell transplantation, and the synergistic effects on promoting regeneration, relay formation and functional connectivity. This review is expected to offer a relatively comprehensive understanding and new perspectives of rTMS for SCI treatment.

Spinally targeted paired associated stimulation with high-frequency peripheral component induces spinal level plasticity in healthy subjects

A novel variant of paired-associative stimulation (PAS) consisting of high-frequency peripheral nerve stimulation (PNS) and high-intensity transcranial magnetic stimulation (TMS) above the motor cortex, called high-PAS, can lead to improved motor function in patients with incomplete spinal cord injury. In PAS, the interstimulus interval (ISI) between the PNS and TMS pulses plays a significant role in the location of the intended effect of the induced plastic changes. While conventional PAS protocols (single TMS pulse often applied with intensity close to resting motor threshold, and single PNS pulse) usually require precisely defined ISIs, high-PAS can induce plasticity at a wide range of ISIs and also in spite of small ISI errors, which is helpful in clinical settings where precise ISI determination can be challenging. However, this also makes the determination of high-PAS level of plasticity induction more challenging and calls for more research on the mechanism of action of high-PAS. We sought to determine if the TMS-induced orthodromic activation in upper motor neurons and PNS-induced antidromic activation in lower motor neurons arriving simultaneously to the intervening synapses at the spinal cord level can be shown to induce acute changes at the targeted location, unlike an otherwise identical but cortically targeted equivalent. Ten healthy subjects participated in two separate sessions, where high-PAS induced activation was set to target spinal (SPINAL) or cortical (CORTICAL) levels with ISI manipulation between otherwise identically applied TMS and PNS pulses. The outcomes were assessed with motor-evoked potentials (MEPs) and Hoffmann (H)-reflex before (PRE), immediately after, and 30 and 60 min after (POST, POST30, POST60) the intervention. MEPs were significantly enhanced in both interventions. In the SPINAL but not in the CORTICAL session, maximal H-reflex amplitudes significantly increased at two timepoints, indicating an increase in spinal excitability. The H/M ratio (maximal H-reflex normalized to maximal M-wave) also showed a significant increase from PRE to POST30 timepoint in the SPINAL session when compared with the CORTICAL equivalent. These results confirm that spinally targeted high-PAS with individualized ISIs indeed has an effect at the spinal level in the sensorimotor system. High-PAS is a novel PAS variant that has shown promising results in motor rehabilitation of individuals with SCI and these new findings contribute to the understanding of its mechanism of action. This provides further evidence for high-PAS as an option for clinical settings to target plasticity at different levels of the corticospinal tract.

Noninvasive Electrical Modalities to Alleviate Respiratory Deficits Following Spinal Cord Injury

(1) Background: Respiratory dysfunction is a debilitating consequence of cervical and thoracic spinal cord injury (SCI), resulting from the loss of cortico-spinal drive to respiratory motor networks. This impairment affects both central and peripheral nervous systems, disrupting motor control and muscle innervation, which is essential for effective breathing. These deficits significantly impact the health and quality of life of individuals with SCI. Noninvasive stimulation techniques targeting these networks have emerged as a promising strategy to restore respiratory function. This study systematically reviewed the evidence on noninvasive electrical stimulation modalities targeting respiratory motor networks, complemented by previously unpublished data from our research. (2) Methods: A systematic search of five databases (PubMed, Ovid, Embase, Science Direct, and Web of Science) identified studies published through 31 August 2024. A total of 19 studies involving 194 participants with SCI were included. Unpublished data from our research were also analyzed to provide supplementary insights. (3) Results: Among the stimulation modalities reviewed, spinal cord transcutaneous stimulation (scTS) emerged as a particularly promising therapeutic approach for respiratory rehabilitation in individuals with SCI. An exploratory clinical trial conducted by the authors confirmed the effectiveness of scTS in enhancing respiratory motor performance using a bipolar, 5 kHz-modulated, and 1 ms pulse width modality. However, the heterogeneity in SCI populations and stimulation protocols across studies underscores the need for further standardization and individualized optimization to enhance clinical outcomes. (4) Conclusions: Developing standardized and individualized neuromodulatory protocols, addressing both central and peripheral nervous system impairments, is critical to optimizing respiratory recovery and advancing clinical implementation.

Research advancements on nerve guide conduits for nerve injury repair

Peripheral nerve injury (PNI) is one of the most serious causes of disability and loss of work capacity of younger individuals. Although PNS has a certain degree of regeneration, there are still challenges like disordered growth, neuroma formation, and incomplete regeneration. Regarding the management of PNI, conventional methods such as surgery, pharmacotherapy, and rehabilitative therapy. Treatment strategies vary depending on the severity of the injury. While for the long nerve defect, autologous nerve grafting is commonly recognized as the preferred surgical approach. Nevertheless, due to lack of donor sources, neurological deficits and the low regeneration efficiency of grafted nerves, nerve guide conduits (NGCs) are recognized as a future promising technology in recent years. This review provides a comprehensive overview of current treatments for PNI, and discusses NGCs from different perspectives, such as material, design, fabrication process, and composite function.

Timing-dependent synergies between motor cortex and posterior spinal stimulation in humans

Volitional movement requires descending input from the motor cortex and sensory feedback through the spinal cord. We previously developed a paired brain and spinal electrical stimulation approach in rats that relies on convergence of the descending motor and spinal sensory stimuli in the cervical cord. This approach strengthened sensorimotor circuits and improved volitional movement through associative plasticity. In humans, it is not known whether posterior epidural spinal cord stimulation targeted at the sensorimotor interface or anterior epidural spinal cord stimulation targeted within the motor system is effective at facilitating brain evoked responses. In 59 individuals undergoing elective cervical spine decompression surgery, the motor cortex was stimulated with scalp electrodes and the spinal cord was stimulated with epidural electrodes, with muscle responses being recorded in arm and leg muscles. Spinal electrodes were placed either posteriorly or anteriorly, and the interval between cortex and spinal cord stimulation was varied. Pairing stimulation between the motor cortex and spinal sensory (posterior) but not spinal motor (anterior) stimulation produced motor evoked potentials that were over five times larger than brain stimulation alone. This strong augmentation occurred only when descending motor and spinal afferent stimuli were timed to converge in the spinal cord. Paired stimulation also increased the selectivity of muscle responses relative to unpaired brain or spinal cord stimulation. Finally, clinical signs suggest that facilitation was observed in both injured and uninjured segments of the spinal cord. The large effect size of this paired stimulation makes it a promising candidate for therapeutic neuromodulation. KEY POINTS: Pairs of stimuli designed to alter nervous system function typically target the motor system, or one targets the sensory system and the other targets the motor system for convergence in cortex. In humans undergoing clinically indicated surgery, we tested paired brain and spinal cord stimulation that we developed in rats aiming to target sensorimotor convergence in the cervical cord. Arm and hand muscle responses to paired sensorimotor stimulation were more than five times larger than brain or spinal cord stimulation alone when applied to the posterior but not anterior spinal cord. Arm and hand muscle responses to paired stimulation were more selective for targeted muscles than the brain- or spinal-only conditions, especially at latencies that produced the strongest effects of paired stimulation. Measures of clinical evidence of compression were only weakly related to the paired stimulation effect, suggesting that it could be applied as therapy in people affected by disorders of the central nervous system.

Modulations in neural pathways excitability post transcutaneous spinal cord stimulation among individuals with spinal cord injury: a systematic review

Introduction

Transcutaneous spinal cord stimulation (TSCS), a non-invasive form of spinal cord stimulation, has been shown to improve motor function in individuals living with spinal cord injury (SCI). However, the effects of different types of TSCS currents including direct current (DC-TSCS), alternating current (AC-TSCS), and spinal paired stimulation on the excitability of neural pathways have not been systematically investigated. The objective of this systematic review was to determine the effects of TSCS on the excitability of neural pathways in adults with non-progressive SCI at any level.

Methods

The following databases were searched from their inception until June 2022: MEDLINE ALL, Embase, Web of Science, Cochrane Library, and clinical trials. A total of 4,431 abstracts were screened, and 23 articles were included.

Results

Nineteen studies used TSCS at the thoracolumbar enlargement for lower limb rehabilitation (gait & balance) and four studies used cervical TSCS for upper limb rehabilitation. Sixteen studies measured spinal excitability by reporting different outcomes including Hoffmann reflex (H-reflex), flexion reflex excitability, spinal motor evoked potentials (SMEPs), cervicomedullay evoked potentials (CMEPs), and cutaneous-input-evoked muscle response. Seven studies measured corticospinal excitability using motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS), and one study measured somatosensory evoked potentials (SSEPs) following TSCS. Our findings indicated a decrease in the amplitude of H-reflex and long latency flexion reflex following AC-TSCS, alongside an increase in the amplitudes of SMEPs and CMEPs. Moreover, the application of the TSCS-TMS paired associative technique resulted in spinal reflex inhibition, manifested by reduced amplitudes in both the H-reflex and flexion reflex arc. In terms of corticospinal excitability, findings from 5 studies demonstrated an increase in the amplitude of MEPs linked to lower limb muscles following DC-TSCS, in addition to paired associative stimulation involving repetitive TMS on the brain and DC-TSCS on the spine. There was an observed improvement in the latency of SSEPs in a single study. Notably, the overall quality of evidence, assessed by the modified Downs and Black Quality assessment, was deemed poor.

Discussion

This review unveils the systematic evidence supporting the potential of TSCS in reshaping both spinal and supraspinal neuronal circuitries post-SCI. Yet, it underscores the critical necessity for more rigorous, high-quality investigations.

Initial feasibility evaluation of the RISES system: An innovative and activity-based closed-loop framework for spinal cord injury rehabilitation and recovery

Background

Electrical stimulation of the spinal cord may improve rewiring of the affected pathways. Immediate modulation of stimulation parameters, and its effects of it on kinematics and electromyographic variables is unclear.

Methods

This study piloted the safety and feasibility of the Reynolds Innovative Spinal Electrical Stimulation (RISES) technology with a focus on its novel closed-loop setting. This personalized, task-specific non-invasive stimulation system enables real-time stimulation parameter modulation and supports multi-data acquisition and storage. Four SCI participants underwent a clinical trial coupled with activity-based training. Primary safety outcome measures included adverse events (AEs) and skin integrity; secondary measures were vital signs, pain, and fatigue assessed at the pre, mid, and post-stimulation sessions. The trial included open-loop and closed-loop blocks of transcutaneous spinal cord stimulation (tSCS).

Results

Results showed no serious adverse events, with skin integrity unaffected. Vital signs and pain showed no significant differences across session timepoints. Fatigue levels differed significantly with post-session > mid-session > pre-session. Comparisons between open-loop and closed-loop blocks showed no significant differences in setup time, vital signs, pain, or fatigue. Average stimulation duration per task was significantly longer for open-loop (467.6 sec) than Closed-loop (410.8 sec).

Conclusions

RISES, demonstrated safety and feasibility. Further work will focus on clinical efficacy.

A motor learning-based postural intervention with a robotic trunk support trainer to improve functional sitting in spinal cord injury: case report

STUDY DESIGN

Single-subject-research-design.

OBJECTIVES

To improve seated postural control in a participant with spinal cord injury (SCI) with a robotic Trunk-Support-Trainer (TruST).

SETTING

Laboratory.

METHODS

TruST delivered "assist-as-needed" forces on the participant's torso during a motor learning-and-control-based intervention (TruST-intervention). TruST-assistive forces were progressed and matched to the participant's postural trunk control gains across six intervention sessions. The T-shirt test was used to capture functional improvements while dressing the upper body. Kinematics were used to compute upper body excursions (cm) and velocity (cm2), and sitting workspace area (cm2). Functional trunk dynamometry was used to examine muscle force (Kg). Surface electromyography (sEMG) was applied to measure trunk muscle activity. The Borg Rating of Perceived Exertion (RPE) was used to monitor physical exertion during TruST-intervention. A two-standard-deviation bandwidth method was adopted for data interpretation.

RESULTS

After TruST-intervention, the participant halved the time needed to don and doff a T-shirt, increased muscle force of trunk muscles (mean = 3 kg), acquired a steadier postural sitting control without vision (mean excursion baseline: 76.0 ± 2 SD = 5.25 cm and post-intervention: 44.1 cm; and mean velocity baseline: 3.0 ± 2 SD = 0.2 cm/s and post-intervention: 1.8 cm/s), and expanded his sitting workspace area (mean baseline: 36.7 ± 2 SD = 36.6 cm2 and post-intervention: 419.2 cm2). The participant increased his tolerance to counteract greater TruST-force perturbations in lateral and posterior directions. Furthermore, abdominal muscle activity substantially augmented after completion of TruST-intervention across all perturbation directions.

CONCLUSIONS

Our data indicate a potential effectiveness of TruST-intervention to promote functional sitting in SCI.

Effect of Cervical Transcutaneous Spinal Cord Stimulation on Sensorimotor Cortical Activity during Upper-Limb Movements in Healthy Individuals

Transcutaneous spinal cord stimulation (tSCS) can improve upper-limb motor function after spinal cord injury. A number of studies have attempted to deduce the corticospinal mechanisms which are modulated following tSCS, with many relying on transcranial magnetic stimulation to provide measures of corticospinal excitability. Other metrics, such as cortical oscillations, may provide an alternative and complementary perspective on the physiological effect of tSCS. Hence, the present study recorded EEG from 30 healthy volunteers to investigate if and how cortical oscillatory dynamics are altered by 10 min of continuous cervical tSCS. Participants performed repetitive upper-limb movements and resting-state tasks while tSCS was delivered to the posterior side of the neck as EEG was recorded simultaneously. The intensity of tSCS was tailored to each participant based on their maximum tolerance (mean: 50 ± 20 mA). A control session was conducted without tSCS. Changes to sensorimotor cortical activity during movement were quantified in terms of event-related (de)synchronisation (ERD/ERS). Our analysis revealed that, on a group level, there was no consistency in terms of the direction of ERD modulation during tSCS, nor was there a dose-effect between tSCS and ERD/ERS. Resting-state oscillatory power was compared before and after tSCS but no statistically significant difference was found in terms of alpha peak frequency or alpha power. However, participants who received the highest stimulation intensities had significantly weakened ERD/ERS (10% ERS) compared to when tSCS was not applied (25% ERD; p = 0.016), suggestive of cortical inhibition. Overall, our results demonstrated that a single 10 min session of tSCS delivered to the cervical region of the spine was not sufficient to induce consistent changes in sensorimotor cortical activity among the entire cohort. However, under high intensities there may be an inhibitory effect at the cortical level. Future work should investigate, with a larger sample size, the effect of session duration and tSCS intensity on cortical oscillations.

Neural Substrates of Transcutaneous Spinal Cord Stimulation: Neuromodulation across Multiple Segments of the Spinal Cord

Transcutaneous spinal cord stimulation (tSCS) has the potential to promote improved sensorimotor rehabilitation by modulating the circuitry of the spinal cord non-invasively. Little is currently known about how cervical or lumbar tSCS influences the excitability of spinal and corticospinal networks, or whether the synergistic effects of multi-segmental tSCS occur between remote segments of the spinal cord. The aim of this review is to describe the emergence and development of tSCS as a novel method to modulate the spinal cord, while highlighting the effectiveness of tSCS in improving sensorimotor recovery after spinal cord injury. This review underscores the ability of single-site tSCS to alter excitability across multiple segments of the spinal cord, while multiple sites of tSCS converge to facilitate spinal reflex and corticospinal networks. Finally, the potential and current limitations for engaging cervical and lumbar spinal cord networks through tSCS to enhance the effectiveness of rehabilitation interventions are discussed. Further mechanistic work is needed in order to optimize targeted rehabilitation strategies and improve clinical outcomes.