Algorithms for Automated Calibration of Transcutaneous Spinal Cord Stimulation to Facilitate Clinical Applications

Abstract

Transcutaneous spinal cord stimulation (tSCS) is a promising intervention that can benefit spasticity control and augment voluntary movement in spinal cord injury (SCI) and multiple sclerosis. Current applications require expert knowledge and rely on the thorough visual analysis of electromyographic (EMG) responses from lower-limb muscles to optimize attainable treatment effects. Here, we devised an automated tSCS setup by combining an electrode array placed over low-thoracic to mid-lumbar vertebrae, synchronized EMG recordings, and a self-operating stimulation protocol to systematically test various stimulation sites and amplitudes. A built-in calibration procedure classifies the evoked responses as reflexes or direct motor responses and identifies stimulation thresholds as recommendations for tSCS therapy. We tested our setup in 15 individuals (five neurologically intact, five SCI, and five Parkinson’s disease) and validated the results against blinded ratings from two clinical experts. Congruent results were obtained in 13 cases for electrode positions and in eight for tSCS amplitudes, with deviations of a maximum of one position and 5 to 10 mA in amplitude in the remaining cases. Despite these minor deviations, the calibration found clinically suitable tSCS settings in 13 individuals. In the remaining two cases, the automatic setup and both experts agreed that no reflex responses could be detected. The presented technological developments may facilitate the dissemination of tSCS into non-academic environments and broaden its use for diagnostic and therapeutic purposes.

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

“…With the L1-midline configuration, the MG muscle was generally recruited first. These results are consistent with those of previous studies (Krenn et al, 2013;Militskova et al, 2020;Roy et al, 2012;Salchow-Hömmen et al, 2019;Sayenko et al, 2015) and are likely due to the proximity of the relevant spinal roots to the cathode. Overall, while the difference between the T11 and L1 cathode electrode placements was small in our study, the L1-midline electrode configuration elicited sEMRs in more lower limb muscles for a given stimulation intensity, especially in people with a SCI where threshold intensities were not as divergent between these two electrode configurations as non-SCI individuals.…”

“…However, despite the relatively diffuse current induced by TSS, dorsal roots closer to the cathode electrode tend to experience higher currents, which leads to uneven dispersion of current across the motoneurone pools (Rattay et al, 2000 ). For example, proximal leg muscles appear to be recruited preferentially when the cathode is positioned over T11–T12, while distal muscles appear to be recruited preferentially when the cathode is positioned over L1–L2 (Krenn et al, 2013 ; Militskova et al, 2020 ; Roy et al, 2012 ; Salchow‐Hömmen et al, 2019 ; Sayenko et al, 2015 ). Much less is known about the choice of anode location choice in determining how stimulation interacts with the target motoneurone pools.…”

Targeting Transcutaneous Spinal Cord Stimulation Using a Supervised Machine Learning Approach Based on Mechanomyography

“…To classify the EMG responses according to the response amplitude characteristics, the recorded EMG signals during double-pulse stimulation pass through a data processing pipeline implemented in Python 3.9 and adapted from our previously published algorithm for EMG-based analysis [ 11 ]. Firstly, the stimulation artifacts are detected in the EMG to synchronize the sensor acquisition precisely with the stimulation.…”

“…The EMG responses evoked by the first stimulus and their suppression after the second stimulus indicate the recruitment of afferents [ 10 ]. The placement of the back electrode can then be adjusted in a manual trial-and-error manner [ 8 ] or in an automated process [ 11 ]. In the latter, the EMG responses are classified according to the amplitude of the response to the first and suppression characteristics of the response to the second stimulation pulse.…”

Transcutaneous Spinal Cord Stimulation: Advances in an Emerging Non-Invasive Strategy for Neuromodulation

“…Technological developments to further ease the use of transcutaneous spinal cord stimulation in clinical environments are a necessary prerequisite for its wide acceptance and lasting integration into rehabilitation practice. Salchow-Hömmen and colleagues tackled this question by introducing a novel algorithm that allows to automatically calibrate the stimulation setup and determine required stimulation intensities for antispasticity applications for each individual treated, all within a few minutes only [10]. The spasticity-alleviating effect of transcutaneous spinal cord stimulation as a viable non-pharmacological approach was further investigated by Sandler and colleagues [11].…”