The relationship between maximum tolerance and motor activation during transcutaneous spinal stimulation is unaffected by the carrier frequency or vibration

Effects of non-invasive cervical spinal cord neuromodulation by trans-spinal electrical stimulation on cortico-muscular descending patterns in upper extremity of chronic stroke

Background: Trans-spinal electrical stimulation (tsES) to the intact spinal cord poststroke may modulate the cortico-muscular control in stroke survivors with diverse lesions in the brain. This work aimed to investigate the immediate effects of tsES on the cortico-muscular descending patterns during voluntary upper extremity (UE) muscle contractions by analyzing cortico-muscular coherence (CMCoh) and electromyography (EMG) in people with chronic stroke. Methods: Twelve chronic stroke participants were recruited to perform wrist-hand extension and flexion tasks at submaximal levels of voluntary contraction for the corresponding agonist flexors and extensors. During the tasks, the tsES was delivered to the cervical spinal cord with rectangular biphasic pulses. Electroencephalography (EEG) data were collected from the sensorimotor cortex, and the EMG data were recorded from both distal and proximal UE muscles. The CMCoh, laterality index (LI) of the peak CMCoh, and EMG activation level parameters under both non-tsES and tsES conditions were compared to evaluate the immediate effects of tsES on the cortico-muscular descending pathway. Results: The CMCoh and LI of peak CMCoh in the agonist distal muscles showed significant increases (p < 0.05) during the wrist-hand extension and flexion tasks with the application of tsES. The EMG activation levels of the antagonist distal muscle during wrist-hand extension were significantly decreased (p < 0.05) with tsES. Additionally, the proximal UE muscles exhibited significant decreases (p < 0.05) in peak CMCoh and EMG activation levels by applying tsES. There was a significant increase (p < 0.05) in LI of peak CMCoh of proximal UE muscles during tsES. Conclusion: The cervical spinal cord neuromodulation via tsES enhanced the residual descending excitatory control, activated the local inhibitory circuits within the spinal cord, and reduced the cortical and proximal muscular compensatory effects. These results suggested the potential of tsES as a supplementary input for improving UE motor functions in stroke rehabilitation.

The effects of transcutaneous spinal cord stimulation delivered with and without high-frequency modulation on spinal and corticospinal excitability

Transcutaneous spinal cord stimulation (TSCS) has been shown to improve motor recovery in people with spinal cord injury (SCI). Some groups deliver TSCS modulated with a kHz-frequency (TSCS-kHz); the intensity used for TSCS-kHz is usually set based on the motor threshold for TSCS, even though TSCS-kHz threshold is considerably higher than TSCS. As a result, TSCS-kHz interventions tend to be delivered at low intensities with respect to the motor threshold (~40%). In this study, we compared the effects of sub-threshold TSCS and TSCS-kHz, when delivered at similar intensity relative to their own motor threshold. Experiment I compared the after-effects of 20 min of sub-threshold (40% threshold) TSCS and TSCS-kHz on spinal and corticospinal excitability in able-bodied participants. Experiment II assessed the dose-response relationship of delivering short (10-pulse) trains of TSCS and TSCS-kHz at three different current intensities relative to the threshold (40%, 60%, and 80%). Experiment I found that 20 min of TSCS-kHz at a 40% threshold decreased posterior root reflex amplitude (p < 0.05), whereas TSCS did not. In experiment II, motor-evoked potential (MEP) amplitude increased following short trains of TSCS and TSCS-kHz of increasing intensity. MEP amplitude was significantly greater for TSCS-kHz compared with TSCS when delivered at 80% of the threshold (p < 0.05). These results suggest that TSCS and TSCS-kHz have different effects when delivered at similar intensity relative to their own threshold; both for immediate effects on corticospinal excitability and following prolonged stimulation on spinal excitability. These different effects may be utilized for optimal rehabilitation in people with SCI.

Transcutaneous Spinal Cord Stimulation to Stabilize Seated Systolic Blood Pressure in Persons With Chronic Spinal Cord Injury: Protocol Development

Transcutaneous spinal cord stimulation (tSCS) is an emerging therapeutic strategy to target spinal autonomic circuitry to normalize and stabilize blood pressure (BP) in hypotensive persons living with chronic spinal cord injury (SCI). Our aim is to describe our current methodological approach to identify individual tSCS parameters that result in the maintenance of seated systolic blood pressure (SBP) within a pre-defined target range. The parent study is a prospective, randomized clinical trial in which eligible participants will undergo multiple mapping sessions to optimize tSCS parameter settings to promote stable SBP within a target range of 110-120 mm Hg for males and 100-120 mm Hg for females. Parameter mapping includes cathode electrode placement site (T7/8, T9/10, T11/12, and L1/2), stimulation frequency (30, 60 Hz), current amplitudes (0-120 mA), waveform (mono- and biphasic), pulse width (1000 μs), and use of carrier frequency (0, 10 kHz). Each participant will undergo up to 10 mapping sessions involving different electrode placement sites and parameter settings. BP will be continuously monitored throughout each mapping session. Stimulation amplitude (mA) will be increased at intervals of between 2 and 10 mA until one of the following occurs: 1) seated SBP reaches the target range; 2) tSCS intensity reaches 120 mA; or 3) the participant requests to stop. Secondary outcomes recorded include 1) symptoms related to autonomic dysreflexia and orthostatic hypotension, 2) Likert pain scale, and 3) skin appearance after removal of the tSCS electrode. Clinical Trials Registration: NCT05180227.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

Objective.Transcutaneous spinal cord stimulation (tSCS) has been gaining momentum as a non-invasive rehabilitation approach to restore movement to paralyzed muscles after spinal cord injury (SCI). However, its low selectivity limits the types of movements that can be enabled and, thus, its potential applications in rehabilitation.Approach.In this cross-over study design, we investigated whether muscle recruitment selectivity of individual muscles could be enhanced by multielectrode configurations of tSCS in 16 neurologically intact individuals. We hypothesized that due to the segmental innervation of lower limb muscles, we could identify muscle-specific optimal stimulation locations that would enable improved recruitment selectivity over conventional tSCS. We elicited leg muscle responses by delivering biphasic pulses of electrical stimulation to the lumbosacral enlargement using conventional and multielectrode tSCS.Results.Analysis of recruitment curve responses confirmed that multielectrode configurations could improve the rostrocaudal and lateral selectivity of tSCS. To investigate whether motor responses elicited by spatially selective tSCS were mediated by posterior root-muscle reflexes, each stimulation event was a paired pulse with a conditioning-test interval of 33.3 ms. Muscle responses to the second stimulation pulse were significantly suppressed, a characteristic of post-activation depression suggesting that spatially selective tSCS recruits proprioceptive fibers that reflexively activate muscle-specific motor neurons in the spinal cord. Moreover, the combination of leg muscle recruitment probability and segmental innervation maps revealed a stereotypical spinal activation map in congruence with each electrode's position.Significance. Improvements in muscle recruitment selectivity could be essential for the effective translation into stimulation protocols that selectively enhance single-joint movements in neurorehabilitation.

Transcutaneous spinal stimulation in people with and without spinal cord injury: Effect of electrode placement and trains of stimulation on threshold intensity

Transcutaneous spinal cord stimulation (TSS) is purported to improve motor function in people after spinal cord injury (SCI). However, several methodology aspects are yet to be explored. We investigated whether stimulation configuration affected the intensity needed to elicit spinally evoked motor responses (sEMR) in four lower limb muscles bilaterally. Also, since stimulation intensity for therapeutic TSS (i.e., trains of stimulation, typically delivered at 15-50 Hz) is sometimes based on the single-pulse threshold intensity, we compared these two stimulation types. In non-SCI participants (n = 9) and participants with a SCI (n = 9), three different electrode configurations (cathode-anode); L1-midline (below the umbilicus), T11-midline and L1-ASIS (anterior superior iliac spine; non-SCI only) were compared for the sEMR threshold intensity using single pulses or trains of stimulation which were recorded in the vastus medialis, medial hamstring, tibialis anterior, medial gastrocnemius muscles. In non-SCI participants, the L1-midline configuration showed lower sEMR thresholds compared to T11-midline (p = 0.002) and L1-ASIS (p < 0.001). There was no difference between T11-midline and L1-midline for participants with SCI (p = 0.245). Spinally evoked motor response thresholds were ~13% lower during trains of stimulation compared to single pulses in non-SCI participants (p < 0.001), but not in participants with SCI (p = 0.101). With trains of stimulation, threshold intensities were slightly lower and the incidence of sEMR was considerably lower. Overall, stimulation threshold intensities were generally lower with the L1-midline electrode configuration and is therefore preferred. While single-pulse threshold intensities may overestimate threshold intensities for therapeutic TSS, tolerance to trains of stimulation will be the limiting factor in most cases.

Safety and Feasibility of Cervical and Thoracic Transcutaneous Spinal Cord Stimulation to Improve Hand Motor Function in Children With Chronic Spinal Cord Injury

OBJECTIVE

In adults with cervical spinal cord injury (SCI), transcutaneous spinal stimulation (scTS) has improved upper extremity strength and control. This novel noninvasive neurotherapeutic approach combined with training may modulate the inherent developmental plasticity of children with SCI, providing even greater improvements than training or stimulation alone. Because children with SCI represent a vulnerable population, we first must establish the safety and feasibility of any potential novel therapeutic approach. The objectives of this pilot study were to determine the safety, feasibility, and proof of principle of cervical and thoracic scTS for short-term effect on upper extremity strength in children with SCI.

MATERIALS AND METHODS

In this nonrandomized, within-subject repeated measure design, seven participants with chronic cervical SCI performed upper extremity motor tasks without and with cervical (C3-C4 and C6-C7) and thoracic (T10-T11) site scTS. Safety and feasibility of using cervical and thoracic sites scTS were determined by the frequency count of anticipated and unanticipated risks (eg, pain, numbness). Proof-of-principle concept was tested via change in force production during hand motor tasks.

RESULTS

All seven participants tolerated cervical and thoracic scTS across the three days, with a wide range of stimulation intensities (cervical sites = 20-70 mA and thoracic site = 25-190 mA). Skin redness at the stimulation sites was observed in four of 21 assessments (19%) and dissipated in a few hours. No episode of autonomic dysreflexia was observed or reported. Hemodynamic parameters (systolic blood pressure and heart rate) remained within stable limits (p > 0.05) throughout the assessment time points at baseline, with scTS, and after the experiment. Hand-grip and wrist-extension strength increased (p < 0.05) with scTS.

CONCLUSIONS

We indicated that short-term application of scTS via two cervical and one thoracic site is safe and feasible in children with SCI and resulted in immediate improvements in hand-grip and wrist-extension strength in the presence of scTS.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT04032990.

First Use of Non-Invasive Spinal Cord Stimulation in Motor Rehabilitation of Children with Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is characterized by the degeneration of spinal alpha motorneurons. Nusinersen demonstrated good efficacy in the early disease phases. The feasibility of transcutaneous spinal cord stimulation (tSCS) in motor rehabilitation of patients with spinal cord injury has been demonstrated. We hypothesize that tSCS may activate intact and restored by nusinersen motorneurons and slow down the decline in motor activity, and may contribute to the development of motor skills in children with SMA. A case series is presented. Five children (6-13 years old) with SMA type II or III participated in the study. They were treated with nusinersen for ~2 years. Application of tSCS was carried out during physical therapy for 30-40 min per day in the course of 10-14 days. Outcome measures were goniometry of joints with contracture, forced vital capacity (FVC), RULM and HFMSE scales. The participants tolerated the stimulation well. The reduction of the contracture was ≥5 deg. RULM and HFMSE increased by ~1-2 points. Predicted FVC increased by 1-7% in three participants. Each participant expanded their range of active movements and/or learned new motor skills. Spinal cord stimulation may be an effective rehabilitation method in patients treated with nusinersen. More research is needed.

Optimization of Transspinal Stimulation Applications for Motor Recovery after Spinal Cord Injury: Scoping Review

Spinal cord injury (SCI) is a debilitating condition that can significantly affect an individual's life, causing paralysis, autonomic dysreflexia, and chronic pain. Transspinal stimulation (TSS) is a non-invasive form of neuromodulation that activates the underlying neural circuitries of the spinal cord. Application of TSS can be performed through multiple stimulation protocols, which may vary in the electrodes' size or position as well as stimulation parameters, and which may influence the response of motor functions to the stimulation. Due to the novelty of TSS, it is beneficial to summarize the available evidence to identify the range of parameters that may provide the best outcomes for motor response. The PubMed and Google Scholar databases were searched for studies examining the effects of TSS on limb motor function. A literature search yielded 34 studies for analysis, in which electrode placement and stimulation parameters varied considerably. The stimulation protocols from each study and their impact on limb motor function were summarized. Electrode placement was variable based on the targeted limb. Studies for the upper limbs targeted the cervical enlargement with anatomical placement of the cathode over the cervical vertebral region. In lower-limb studies, the cathode(s) were placed over the thoracic and lumbar vertebral regions, to target the lumbar enlargement. The effects of carrier frequency were inconclusive across the studies. Multisite cathodal placements yielded favorable motor response results compared to single-site placement. This review briefly summarized the current mechanistic evidence of the effect of TSS on motor response after SCI. Our findings indicate that optimization of stimulation parameters will require future randomized controlled studies to independently assess the effects of different stimulation parameters under controlled circumstances.

Using a high-frequency carrier does not improve comfort of transcutaneous spinal cord stimulation

Objective.Spinal cord neuromodulation has gained much attention for demonstrating improved motor recovery in people with spinal cord injury, motivating the development of clinically applicable technologies. Among them, transcutaneous spinal cord stimulation (tSCS) is attractive because of its non-invasive profile. Many tSCS studies employ a high-frequency (10 kHz) carrier, which has been reported to reduce stimulation discomfort. However, these claims have come under scrutiny in recent years. The purpose of this study was to determine whether using a high-frequency carrier for tSCS is more comfortable at therapeutic amplitudes, which evoke posterior root-muscle (PRM) reflexes.Approach.In 16 neurologically intact participants, tSCS was delivered using a 1 ms long monophasic pulse with and without a high-frequency carrier. Stimulation amplitude and pulse duration were varied and PRM reflexes were recorded from the soleus, gastrocnemius, and tibialis anterior muscles. Participants rated their discomfort during stimulation from 0 to 10 at PRM reflex threshold.Main Results.At PRM reflex threshold, the addition of a high-frequency carrier (0.87 ± 0.2) was equally comfortable as conventional stimulation (1.03 ± 0.18) but required approximately double the charge to evoke the PRM reflex (conventional: 32.4 ± 9.2µC; high-frequency carrier: 62.5 ± 11.1µC). Strength-duration curves for tSCS with a high-frequency carrier had a rheobase that was 4.8× greater and a chronaxie that was 5.7× narrower than the conventional monophasic pulse, indicating that the addition of a high-frequency carrier makes stimulation less efficient in recruiting neural activity in spinal roots.Significance.Using a high-frequency carrier for tSCS is equally as comfortable and less efficient as conventional stimulation at amplitudes required to stimulate spinal dorsal roots.

Evoked sensations with transcutaneous electrical stimulation with different frequencies, waveforms, and electrode configurations

BACKGROUND

Current Perception Threshold (CPT) is a technique used for diagnostic purposes that applies sinusoidal currents transcutaneously at 5 Hz, 250 Hz, and 2KHz to preferentially excite C, Aδ, and Aβ afferent nerve fibers correspondingly. This fact may be interesting for evoking different electrotactile sensations for a wide variety of applications.

METHODS

Sensations evoked by 5 Hz, 250 Hz, and 2KHz frequencies; sinusoidal, square, and 250 μs-pulsed waveforms; and conventional and concentric electrode configurations were analyzed in 19 healthy volunteers. Stimuli were applied in the dorsum of the hand in a double-blind manner and CPTs were defined based on participants' verbal feedback. After each stimulus participants filled in a form with sensation modality, irradiation, intensity, and emotion descriptors.

RESULTS

The frequency showed a significant effect on the four domains of evoked sensations and the waveform showed a significant effect on the modality domain. For most waveform and electrode configuration combinations, 5 Hz evoked mostly a low-intensity prickling sensation; 250 Hz mostly evoked an uncomfortable medium-intensity tingling sensation; and 2KHz mostly evoked a low-intensity tingling sensation. No thermal or noxious sensations were evoked. A significant interaction effect was only found between the frequency and the waveform factors. The electrode configuration did not show either a significant effect on the evoked sensations or an interaction effect with the frequency or waveform type.

CONCLUSIONS

Transcutaneous electrical stimulation may evoke different sensations at different frequencies due to the preferential activation of different fiber types. The results of these analysis could be used to enhance human-machine/computer-interaction systems based on electrotactile feedback.

Combined Transcutaneous Electrical Spinal Cord Stimulation and Task-Specific Rehabilitation Improves Trunk and Sitting Functions in People with Chronic Tetraplegia

The aim of this study was to examine the effects of transcutaneous electrical spinal cord stimulation (TSCS) and conventional task-specific rehabilitation (TSR) on trunk control and sitting stability in people with chronic tetraplegia secondary to a spinal cord injury (SCI). Five individuals with complete cervical (C4-C7) cord injury participated in 24-week therapy that combined TSCS and TSR in the first 12 weeks, followed by TSR alone for another 12 weeks. The TSCS was delivered simultaneously at T11 and L1 spinal levels, at a frequency ranging from 20-30 Hz with 0.1-1.0 ms. pulse width biphasically. Although the neurological prognosis did not manifest after either treatment, the results show that there were significant increases in forward reach distance (10.3 ± 4.5 cm), right lateral reach distance (3.7 ± 1.8 cm), and left lateral reach distance (3.0 ± 0.9 cm) after the combinational treatment (TSCS+TSR). The stimulation also significantly improved the participants' trunk control and function in sitting. Additionally, the trunk range of motion and the electromyographic response of the trunk muscles were significantly elevated after TSCS+TSR. The TSCS+TSR intervention improved independent trunk control with significantly increased static and dynamic sitting balance, which were maintained throughout the TSR period and the follow-up period, indicating long-term sustainable recovery.

A Review of Functional Restoration From Spinal Cord Stimulation in Patients With Spinal Cord Injury

Traumatic spinal cord injury often leads to loss of sensory, motor, and autonomic function below the level of injury. Recent advancements in spinal cord electrical stimulation (SCS) for spinal cord injury have provided potential avenues for restoration of neurologic function in affected patients. This review aims to assess the efficacy of spinal cord stimulation, both epidural (eSCS) and transcutaneous (tSCS), on the return of function in individuals with chronic spinal cord injury. The current literature on human clinical eSCS and tSCS for spinal cord injury was reviewed. Seventy-one relevant studies were included for review, specifically examining changes in volitional movement, changes in muscle activity or spasticity, or return of cardiovascular pulmonary, or genitourinary autonomic function. The total participant sample comprised of 327 patients with spinal cord injury, each evaluated using different stimulation protocols, some for sensorimotor function and others for various autonomic functions. One hundred eight of 127 patients saw improvement in sensorimotor function, 51 of 70 patients saw improvement in autonomic genitourinary function, 32 of 32 patients saw improvement in autonomic pulmonary function, and 32 of 36 patients saw improvement in autonomic cardiovascular function. Although this review highlights SCS as a promising therapeutic neuromodulatory technique to improve rehabilitation in patients with SCI, further mechanistic studies and stimulus parameter optimization are necessary before clinical translation.

Transcutaneous spinal cord stimulation combined with locomotor training to improve walking ability in people with chronic spinal cord injury: study protocol for an international multi-centred double-blinded randomised sham-controlled trial (eWALK)

STUDY DESIGN

An international multi-centred, double-blinded, randomised sham-controlled trial (eWALK).

OBJECTIVE

To determine the effect of 12 weeks of transcutaneous spinal stimulation (TSS) combined with locomotor training on walking ability in people with spinal cord injury (SCI).

SETTING

Dedicated SCI research centres in Australia, Spain, USA and Scotland.

METHODS

Fifty community-dwelling individuals with chronic SCI will be recruited. Participants will be eligible if they have bilateral motor levels between T1 and T11, a reproducible lower limb muscle contraction in at least one muscle group, and a Walking Index for SCI II (WISCI II) between 1 and 6. Eligible participants will be randomised to one of two groups, either the active stimulation group or the sham stimulation group. Participants allocated to the stimulation group will receive TSS combined with locomotor training for three 30-min sessions a week for 12 weeks. The locomotor sessions will include walking on a treadmill and overground. Participants allocated to the sham stimulation group will receive the same locomotor training combined with sham stimulation. The primary outcome will be walking ability with stimulation using the WISCI II. Secondary outcomes will record sensation, strength, spasticity, bowel function and quality of life.

TRIAL REGISTRATION

ANZCTR.org.au identifier ACTRN12620001241921.

Feasibility of transcutaneous spinal direct current stimulation combined with locomotor training after spinal cord injury

Study Design:

Feasibility study, consisting of random-order, cross-over study of a single intervention session, followed by a parallel-arm study of 16 sessions

Objectives:

To investigate the feasibility of a novel combinatorial approach with simultaneous delivery of transcutaneous spinal direct current stimulation (tsDCS) and locomotor training (tsDCS+LT) after spinal cord injury, compared to sham stimulation and locomotor training (sham+LT), and examine preliminary effects on walking function.

Setting:

Clinical research center in the southeastern United States

Methods:

Eight individuals with chronic incomplete spinal cord injury (ISCI) completed the two-part protocol. Feasibility was assessed based on safety (adverse responses), tolerability (pain, spasticity, skin integrity), and protocol achievement (session duration, intensity). Walking function was assessed with the 10-meter and 6-minute walk tests.

Results:

There were no major adverse responses. Minimal reports of skin irritation and musculoskeletal pain were consistent between groups. Average training peak heart rate as percent of maximum (mean(SD); tsDCS+LT: 66(4)%, sham+LT: 69(10)%) and Borg ratings of perceived exertion (tsDCS+LT: 17.5(1.2), sham+LT: 14.4(1.8)) indicate both groups trained at high intensities. Walking speed gains exceeded the minimal clinically important difference (MCID) in three of four who received tsDCS+LT (0.18(0.29) m/s) and one of four in sham+LT (−0.05(0.23) m/s). Gains in walking endurance exceeded the MCID in one of four in each group (tsDCS+LT: 36.4(69.0) m, sham+LT: 4.9(56.9) m).

Conclusions:

Combinatorial tsDCS and locomotor training is safe and feasible for individuals with chronic ISCI, even those with considerable walking impairment. Study outcomes support the need to investigate the efficacy of this approach.

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.

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.

Augmented Transcutaneous Stimulation Using an Injectable Electrode: A Computational Study

Minimally invasive neuromodulation technologies seek to marry the neural selectivity of implantable devices with the low-cost and non-invasive nature of transcutaneous electrical stimulation (TES). The Injectrode^® is a needle-delivered electrode that is injected onto neural structures under image guidance. Power is then transcutaneously delivered to the Injectrode using surface electrodes. The Injectrode serves as a low-impedance conduit to guide current to the deep on-target nerve, reducing activation thresholds by an order of magnitude compared to using only surface stimulation electrodes. To minimize off-target recruitment of cutaneous fibers, the energy transfer efficiency from the surface electrodes to the Injectrode must be optimized. TES energy is transferred to the Injectrode through both capacitive and resistive mechanisms. Electrostatic finite element models generally used in TES research consider only the resistive means of energy transfer by defining tissue conductivities. Here, we present an electroquasistatic model, taking into consideration both the conductivity and permittivity of tissue, to understand transcutaneous power delivery to the Injectrode. The model was validated with measurements taken from (n = 4) swine cadavers. We used the validated model to investigate system and anatomic parameters that influence the coupling efficiency of the Injectrode energy delivery system. Our work suggests the relevance of electroquasistatic models to account for capacitive charge transfer mechanisms when studying TES, particularly when high-frequency voltage components are present, such as those used for voltage-controlled pulses and sinusoidal nerve blocks.

When Spinal Neuromodulation Meets Sensorimotor Rehabilitation: Lessons Learned From Animal Models to Regain Manual Dexterity After a Spinal Cord Injury

Electrical neuromodulation has strongly hit the foundations of spinal cord injury and repair. Clinical and experimental studies have demonstrated the ability to neuromodulate and engage spinal cord circuits to recover volitional motor functions lost after the injury. Although the science and technology behind electrical neuromodulation has attracted much of the attention, it cannot be obviated that electrical stimulation must be applied concomitantly to sensorimotor rehabilitation, and one would be very difficult to understand without the other, as both need to be finely tuned to efficiently execute movements. The present review explores the difficulties faced by experimental and clinical neuroscientists when attempting to neuromodulate and rehabilitate manual dexterity in spinal cord injured subjects. From a translational point of view, we will describe the major rehabilitation interventions employed in animal research to promote recovery of forelimb motor function. On the other hand, we will outline some of the state-of-the-art findings when applying electrical neuromodulation to the spinal cord in animal models and human patients, highlighting how evidences from lumbar stimulation are paving the path to cervical neuromodulation.

Transcutaneous spinal cord stimulation and motor responses in individuals with spinal cord injury: A methodological review

Background

Transcutaneous spinal cord stimulation (tSCS) is a non-invasive modality in which electrodes can stimulate spinal circuitries and facilitate a motor response. This review aimed to evaluate the methodology of studies using tSCS to generate motor activity in persons with spinal cord injury (SCI) and to appraise the quality of included trials.

Methods

A systematic search for studies published until May 2021 was made of the following databases: EMBASE, Medline (Ovid) and Web of Science. Two reviewers independently screened the studies, extracted the data, and evaluated the quality of included trials. The electrical characteristics of stimulation were summarised to allow for comparison across studies. In addition, the surface electromyography (EMG) recording methods were evaluated.

Results

A total of 3753 articles were initially screened, of which 25 met the criteria for inclusion. Studies were divided into those using tSCS for neurophysiological investigations of reflex responses (n = 9) and therapeutic investigations of motor recovery (n = 16). The overall quality of evidence was deemed to be poor-to-fair (10.5 ± 4.9) based on the Downs and Black Quality Checklist criteria. The electrical characteristics were collated to establish the dosage range across stimulation trials. The methods employed by included studies relating to stimulation parameters and outcome measurement varied extensively, although some trends are beginning to appear in relation to electrode configuration and EMG outcomes.

Conclusion

This review outlines the parameters currently employed for tSCS of the cervicothoracic and thoracolumbar regions to produce motor responses. However, to establish standardised procedures for neurophysiological assessments and therapeutic investigations of tSCS, further high-quality investigations are required, ideally utilizing consistent electrophysiological recording methods, and reporting common characteristics of the electrical stimulation administered.

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

Rehabilitative interventions involving electrical stimulation show promise for neuroplastic recovery in people living with Spinal Cord Injury (SCI). However, the understanding of how stimulation interacts with descending and spinal excitability remain unclear. In this study we compared the immediate and short-term (within a few minutes) effects of pairing Transcranial Magnetic Stimulation (TMS) with transcutaneous Spinal Cord stimulation (tSCS) and Peripheral Nerve Stimulation (PNS) on Corticospinal excitability in healthy subjects. Three separate experimental conditions were assessed. In Experiment I, paired associative stimulation (PAS) was applied, involving repeated pairing of single pulses of TMS and tSCS, either arriving simultaneously at the spinal motoneurones (PAS_0ms) or slightly delayed (PAS_5ms). Corticospinal and spinal excitability, and motor performance, were assessed before and after the PAS interventions in 24 subjects. Experiment II compared the immediate effects of tSCS and PNS on corticospinal excitability in 20 subjects. Experiment III compared the immediate effects of tSCS with tSCS delivered at the same stimulation amplitude but modulated with a carrier frequency (in the kHz range) on corticospinal excitability in 10 subjects. Electromyography (EMG) electrodes were placed over the Tibialis Anterior (TA) soleus (SOL) and vastus medialis (VM) muscles and stimulation electrodes (cathodes) were placed on the lumbar spine (tSCS) and lateral to the popliteal fossa (PNS). TMS over the primary motor cortex (M1) was paired with tSCS or PNS to produce Motor Evoked Potentials (MEPs) in the TA and SOL muscles. Simultaneous delivery of repetitive PAS (PAS_0ms) increased corticospinal excitability and H-reflex amplitude at least 5 min after the intervention, and dorsiflexion force was increased in a force-matching task. When comparing effects on descending excitability between tSCS and PNS, a subsequent facilitation in MEPs was observed following tSCS at 30-50 ms which was not present following PNS. To a lesser extent this facilitatory effect was also observed with HF- tSCS at subthreshold currents. Here we have shown that repeated pairing of TMS and tSCS can increase corticospinal excitability when timed to arrive simultaneously at the alpha-motoneurone and can influence functional motor output. These results may be useful in optimizing stimulation parameters for neuroplasticity in people living with SCI.

Noninvasive spinal stimulation safely enables upright posture in children with spinal cord injury

In children with spinal cord injury (SCI), scoliosis due to trunk muscle paralysis frequently requires surgical treatment. Transcutaneous spinal stimulation enables trunk stability in adults with SCI and may pose a non-invasive preventative therapeutic alternative. This non-randomized, non-blinded pilot clinical trial (NCT03975634) determined the safety and efficacy of transcutaneous spinal stimulation to enable upright sitting posture in 8 children with trunk control impairment due to acquired SCI using within-subject repeated measures study design. Primary safety and efficacy outcomes (pain, hemodynamics stability, skin irritation, trunk kinematics) and secondary outcomes (center of pressure displacement, compliance rate) were assessed within the pre-specified endpoints. One participant did not complete the study due to pain with stimulation on the first day. One episode of autonomic dysreflexia during stimulation was recorded. Following hemodynamic normalization, the participant completed the study. Overall, spinal stimulation was well-tolerated and enabled upright sitting posture in 7 out of the 8 participants.

Scoliosis due to trunk muscle paralysis frequently requires surgical treatment in children with spinal cord injury. The authors demonstrate the safety and efficacy of transcutaneous spinal stimulation to enable upright sitting posture in 7/8 children with trunk control impairment in a within-subjects, repeated measures pilot clinical trial.

Utility and Feasibility of Transcutaneous Spinal Cord Stimulation for Patients With Incomplete SCI in Therapeutic Settings: A Review of Topic

Transcutaneous Spinal Cord Stimulation (TSCS) has been shown to enhance the excitability of spinal neural circuits. This excitation is associated with enhanced voluntary performance in patients with incomplete SCI (iSCI). Though there is much we do not know, combining this altered state of exciability with therapy has the potential to enhance the outcomes associated with activity-based interventions. It is a promising tool to augment the work being done in therapeutic settings with the potential to expedite recovery. There is, however, a lag in assimilating the science for clinical practice. This article will examine current literature related to the application of TSCS in combination with therapeutic interventions for motor recovery and aims to elucidate trends in waveform selection, duration and frequency, and combinatorial therapies that may inform clinical practice. With specific consideration for therapeutic settings, potential benefits, applications, and pitfalls for clinical use are considered. Finally, the next steps in research to move toward wider clinical utility are discussed.

The relationship between maximum tolerance and motor activation during transcutaneous spinal stimulation is unaffected by the carrier frequency or vibration

Transcutaneous spinal stimulation (TSS) is a useful tool to modulate spinal sensorimotor circuits and has emerged as a potential treatment for motor disorders in neurologically impaired populations. One major limitation of TSS is the discomfort associated with high levels of stimulation during the experimental procedure. The objective of this study was to examine if the discomfort caused by TSS can be alleviated using different stimulation paradigms in a neurologically intact population. Tolerance to TSS delivered using conventional biphasic balanced rectangular pulses was compared to two alternative stimulation paradigms: a 5 kHz carrier frequency and biphasic balanced rectangular pulses combined with vibrotactile stimulation. In ten healthy participants, tolerance to TSS was examined using both single-pulse (0.2 Hz) and continuous (30 Hz) stimulation protocols. In both the single-pulse and continuous stimulation protocols, participants tolerated significantly higher levels of stimulation with the carrier frequency paradigm compared to the other stimulation paradigms. However, when the maximum tolerable stimulation intensity of each stimulation paradigm was normalized to the intensity required to evoke a lower limb muscle response, there were no statistical differences between the stimulation paradigms. Our results suggest that, when considering the intensity of stimulation required to obtain spinally evoked motor potentials, neither alternative stimulation paradigm is more effective at reducing discomfort than the conventional, unmodulated pulse configuration.