Efficacy of Transcutaneous Spinal Stimulation versus Whole Body Vibration for Spasticity Reduction in Persons with Spinal Cord Injury

Optimizing Transcutaneous Spinal Stimulation: Excitability of Evoked Spinal Reflexes is Dependent on Electrode Montage

Background

There is growing interest in use of transcutaneous spinal stimulation (TSS) for people with neurologic conditions both to augment volitional control (by facilitating motoneuron excitability), and to decrease spasticity (by activating inhibitory networks). Various electrode montages are used during TSS, with little understanding of how electrode position influences spinal circuit activation. We sought to identify the thoracolumbar electrode montage associated with the most robust activation of spinal circuits by comparing posterior root-muscle reflexes (PRM reflexes) elicited by 6 montages. Additionally, we assessed tolerability of the stimulation during PRM reflex testing.

Methods

Fifteen adults with intact neurological systems participated in this randomized crossover study. PRM reflexes were evoked transcutaneously using electrode montages with dorsal-ventral (DV) or dorsal-midline (DM) current flow. DV montages included: [1] cathode over T11/T12, anodes over iliac crests (DV-I), [2] cathode over T11/T12, anodes over umbilicus (DV-U), [3] dual paraspinal cathodes at T11/12, anodes over iliac crests (DV-PI), and [4] dual paraspinal cathodes at T11/12, anodes over umbilicus (DV-PU). DM montages included: [5] cathode over T11/12, anode 5cm caudal (DM-C), and [6] cathode over T11/12, anode 5cm rostral (DM-R). PRM reflex recruitment curves were obtained in the soleus muscle of both lower extremities.

Results

DV-U and DV-I montages elicited bilateral reflexes with lower reflex thresholds and larger recruitment curve area than other montages. There were no differences in response amplitude at 120% of RT(1.2xRT) or tolerability among montages.

Conclusions

Differences in spinal circuit recruitment are reflected in the response amplitude of the PRM reflexes. DV-I and DV-U montages were associated with lower reflex thresholds, indicating that motor responses can be evoked with lower stimulation intensity. DV-I and DV-U montages therefore have the potential for lower and more tolerable interventional stimulation intensities. Our findings optimize electrode placement for interventional TSS and PRM reflex assessments.

FUNDAMENTAL LIMITATIONS OF KILOHERTZ-FREQUENCY CARRIERS IN AFFERENT FIBER RECRUITMENT WITH TRANSCUTANEOUS SPINAL CORD STIMULATION

The use of kilohertz-frequency (KHF) waveforms has rapidly gained momentum in transcutaneous spinal cord stimulation (tSCS) to restore motor function after paralysis. However, the mechanisms by which these fast-alternating currents depolarize efferent and afferent fibers remain unknown. Our study fills this research gap by providing a hypothesis-and evidence-based investigation using peripheral nerve stimulation, lumbar tSCS, and cervical tSCS in 25 unimpaired participants together with computational modeling. Peripheral nerve stimulation experiments and computational modeling showed that KHF waveforms negatively impact the processes required to elicit action potentials, thereby increasing response thresholds and biasing the recruitment towards efferent fibers. While these results translate to tSCS, we also demonstrate that lumbar tSCS results in the preferential recruitment of afferent fibers, while cervical tSCS favors recruitment of efferent fibers. Given the assumed importance of proprioceptive afferents in motor recovery, our work suggests that the use of KHF waveforms should be reconsidered to maximize neurorehabilitation outcomes, particularly for cervical tSCS. We posit that careful analysis of the mechanisms that mediate responses elicited by novel approaches in tSCS is crucial to understanding their potential to restore motor function after paralysis.

Multi-session transcutaneous spinal cord stimulation prevents chloride homeostasis imbalance and the development of hyperreflexia after spinal cord injury in rat

Spasticity is a complex and multidimensional disorder that impacts nearly 75% of individuals with spinal cord injury (SCI) and currently lacks adequate treatment options. This sensorimotor condition is burdensome as hyperexcitability of reflex pathways result in exacerbated reflex responses, co-contractions of antagonistic muscles, and involuntary movements. Transcutaneous spinal cord stimulation (tSCS) has become a popular tool in the human SCI research field. The likeliness for this intervention to be successful as a noninvasive anti-spastic therapy after SCI is suggested by a mild and transitory improvement in spastic symptoms following a single stimulation session, but it remains to be determined if repeated tSCS over the course of weeks can produce more profound effects. Despite its popularity, the neuroplasticity induced by tSCS also remains widely unexplored, particularly due to the lack of suitable animal models to investigate this intervention. Thus, the basis of this work was to use tSCS over multiple sessions (multi-session tSCS) in a rat model to target spasticity after SCI and identify the long-term physiological improvements and anatomical neuroplasticity occurring in the spinal cord. Here, we show that multi-session tSCS in rats with an incomplete (severe T9 contusion) SCI (1) decreases hyperreflexia, (2) increases the low frequency-dependent modulation of the H-reflex, (3) prevents potassium-chloride cotransporter isoform 2 (KCC2) membrane downregulation in lumbar motoneurons, and (4) generally augments motor output, i.e., EMG amplitude in response to single pulses of tSCS, particularly in extensor muscles. Together, this work displays that multi-session tSCS can target and diminish spasticity after SCI as an alternative to pharmacological interventions and begins to highlight the underlying neuroplasticity contributing to its success in improving functional recovery.

Optimizing assessment of low frequency H-reflex depression in persons with spinal cord injury

Considering the growing interest in clinical applications of neuromodulation, assessing effects of various modulatory approaches is increasingly important. Monosynaptic spinal reflexes undergo depression following repeated activation, offering a means to quantify neuromodulatory influences. Following spinal cord injury (SCI), changes in reflex modulation are associated with spasticity and impaired motor control. To assess disrupted reflex modulation, low-frequency depression (LFD) of Hoffman (H)-reflex excitability is examined, wherein the amplitudes of conditioned reflexes are compared to an unconditioned control reflex. Alternatively, some studies utilize paired-pulse depression (PPD) in place of the extended LFD train. While both protocols induce similar amounts of H-reflex depression in neurologically intact individuals, this may not be the case for persons with neuropathology. We compared the H-reflex depression elicited by PPD and by trains of 3-10 pulses to an 11-pulse LFD protocol in persons with incomplete SCI. The amount of depression produced by PPD was less than an 11-pulse train (mean difference = 0.137). When compared to the 11-pulse train, the 5-pulse train had a Pearson's correlation coefficient (R) of 0.905 and a coefficient of determination (R2) of 0.818. Therefore, a 5-pulse train for assessing LFD elicits modulation similar to the 11-pulse train and thus we recommend its use in lieu of longer trains.

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.

Multi-session transcutaneous spinal cord stimulation prevents chloridehomeostasis imbalance and the development of spasticity after spinal cordinjury in rat

Spasticity is a complex and multidimensional disorder that impacts nearly 75% of individuals with spinal cord injury (SCI) and currently lacks adequate treatment options. This sensorimotor condition is burdensome as hyperexcitability of reflex pathways result in exacerbated reflex responses, co-contractions of antagonistic muscles, and involuntary movements. Transcutaneous spinal cord stimulation (tSCS) has become a popular tool in the human SCI research field. The likeliness for this intervention to be successful as a noninvasive anti-spastic therapy after SCI is suggested by a mild and transitory improvement in spastic symptoms following a single stimulation session, but it remains to be determined if repeated tSCS over the course of weeks can produce more profound effects. Despite its popularity, the neuroplasticity induced by tSCS also remains widely unexplored, particularly due to the lack of suitable animal models to investigate this intervention. Thus, the basis of this work was to use tSCS over multiple sessions (multi-session tSCS) in a rat model to target spasticity after SCI and identify the long-term physiological improvements and anatomical neuroplasticity occurring in the spinal cord. Here, we show that multi-session tSCS in rats with an incomplete (severe T9 contusion) SCI (1) decreases hyperreflexia, (2) increases the low frequency-dependent modulation of the H-reflex, (3) prevents potassium-chloride cotransporter isoform 2 (KCC2) membrane downregulation in lumbar motoneurons, and (4) generally augments motor output, i.e., EMG amplitude in response to single pulses of tSCS, particularly in extensor muscles. Together, this work displays that multi-session tSCS can target and diminish spasticity after SCI as an alternative to pharmacological interventions and begins to highlight the underlying neuroplasticity contributing to its success in improving functional recovery.

Whole-Body Vibration Impacts on the Degree of Toe Angle and Its Correlation to the Knee Osteoarthritis Index during Level Walking among Female University Students: A Randomized Controlled Trial

Whole-body vibration (WBV) training is used for ankle rehabilitation as it stimulates muscle spindles to excite tonic vibration reflexes, and improves muscle strength, power, joint proprioception, balance, and flexibility. Thus, this study aims to determine the impact of whole-body vibration on the degree of the toe angle and the correlation between the toe angle and knee osteoarthritis index during level walking among female university students. A randomized controlled trial was conducted with 42 participants divided into two groups. The control group performed only home-based exercise (HBE) after education, and the study group received WBV with HBE. The functional status of participants to predict osteoarthritis was evaluated using the Western Ontario and McMaster osteoarthritis index (WOMAC), where the toe angle degree and WOMAC index were assessed before and six weeks after training. The results showed a significant improvement in the toe-in angle of HBE + WBV compared to the HBE group (p = 0.02), and in HBE + WBV, an improvement of the toe angle showed a 43% decrease in the WOMAC index (p = 0.001). In conclusion, WBV for the ankle and foot training program positively affected the degree of the toe angle, that directly affected the subtalar and ankle joint mechanics. Trial registration in the Pan African Clinical Trial Registry PACTR202304816093190 (registered retrospectively, date of registration: 18 April 2023).

Pendulum test parameters are useful for detecting knee muscle hypertonia and quantifying response to an intrathecal baclofen bolus injection

Our objective was to determine which pendulum test parameters are useful for detecting hypertonia in the knee muscles and assessing the group and individual responses to intrathecal baclofen (ITB) bolus injection among prospective pump recipients. We included 15 neurological patients with lower limb hypertonia (mainly spinal cord injury, n  = 7) and collected data the day before (baseline), and 2.5 and 5.0 h after the 50-µg ITB bolus injection. For comparison, data were collected in 15 healthy controls. The average over six test repetitions was obtained for the number of oscillations, swing time (SwingT), amplitudes of the first flexion and extension, maximum angular velocities of the first flexion (F1V) and extension (E1V), relaxation index, and damping coefficient (DampC). Across the patient group, all pendulum parameters indicated a significant decrease in hypertonia from baseline to postinjection (analysis of variance P  ≤ 0.004), except DampC. On the basis of the cutoffs from the receiver operating characteristic curve, all parameters were good or excellent discriminators of hypertonia in patients from normotonia in controls (area under the curve ≥0.85), with the highest sensitivity for SwingT and E1V (≥93%). Furthermore, all parameters except F1V revealed a significant shift from preinjection hypertonia to postinjection normotonia among patients (McNamar test P  ≤ 0.002, DampC excluded due to missing data), with the greatest responsiveness for E1V and relaxation index (≥73%). The results confirm the overall usefulness of pendulum test parameters in this patient population and indicate that some parameters are better at detecting hypertonia (SwingT, E1V) whereas others (E1V, relaxation index) are more responsive to the ITB injection.

Neurophysiological and clinical outcome measures of the impact of electrical stimulation on spasticity in spinal cord injury: Systematic review and meta-analysis

This systematic review and meta-analysis aims to determine whether non-invasive electrical stimulation (ES) is effective at reducing spasticity in people living with spinal cord injury (SCI). PubMed, Web of Science, Scopus and Cochrane Central Register of Controlled Trials databases were searched in April 2022. Primary outcome measures were the Ashworth scale (AS), Modified Ashworth scale (MAS), Pendulum test and the Penn spasm frequency scale (PSFS). Secondary outcomes were the Hoffman (H)- reflex, motor-evoked potentials (MEPs) and posterior-root reflexes (PRRs). A random-effects model, using two correlation coefficients, ( C o r r = 0.1 , C o r r = 0.2 ) determined the difference between baseline and post-intervention measures for RCTs. A quantitative synthesis amalgamated data from studies with no control group (non-RCTs). Twenty-nine studies were included: five in the meta-analysis and 17 in the amalgamation of non-RCT studies. Twenty studies measured MAS or AS scores, 14 used the Pendulum test and one used the PSFS. Four measured the H-reflex and no studies used MEPs or PRRs. Types of ES used were: transcutaneous electrical nerve stimulation (TENS), transcutaneous spinal cord stimulation (TSCS), functional electrical stimulation (FES) cycling and FES gait. Meta-analyses of 3 studies using the MAS and 2 using the Pendulum test were carried out. For MAS scores, non-invasive ES was effective at reducing spasticity compared to a control group (p = 0.01, C o r r = 0.1 ; p = 0.002, C o r r = 0.2 ). For Pendulum test outcomes, there was no statistically significant difference between intervention and control groups. Quantitative synthesis of non-RCT studies revealed that 22 of the 29 studies reported improvement in at least one measure of spasticity following non-invasive ES, 13 of which were statistically significant (p < 0.05). Activation of the muscle was not necessary to reduce spasticity. Non-invasive ES can reduce spasticity in people with SCI, according to MAS scores, for both RCT and non-RCT studies, and Pendulum test values in non-RCT studies. This review could not correlate between clinical and neurophysiological outcomes; we recommend the additional use of neurophysiological outcomes for future studies. The use of TSCS and TENS, which did not induce a muscle contraction, indicate that activation of afferent fibres is at least required for non-invasive ES to reduce spasticity.

Influence of Spine Curvature on the Efficacy of Transcutaneous Lumbar Spinal Cord Stimulation

Transcutaneous spinal cord stimulation is a non-invasive method for neuromodulation of sensorimotor function. Its main mechanism of action results from the activation of afferent fibers in the posterior roots-the same structures as targeted by epidural stimulation. Here, we investigated the influence of sagittal spine alignment on the capacity of the surface-electrode-based stimulation to activate these neural structures. We evaluated electromyographic responses evoked in the lower limbs of ten healthy individuals during extension, flexion, and neutral alignment of the thoracolumbar spine. To control for position-specific effects, stimulation in these spine alignment conditions was performed in four different body positions. In comparison to neutral and extended spine alignment, flexion of the spine resulted in a strong reduction of the response amplitudes. There was no such effect on tibial-nerve evoked H reflexes. Further, there was a reduction of post-activation depression of the responses to transcutaneous spinal cord stimulation evoked in spinal flexion. Thus, afferent fibers were reliably activated with neutral and extended spine alignment. Spinal flexion, however, reduced the capacity of the stimulation to activate afferent fibers and led to the co-activation of motor fibers in the anterior roots. This change of action was due to biophysical rather than neurophysiological influences. We recommend applying transcutaneous spinal cord stimulation in body positions that allow individuals to maintain a neutral or extended spine.