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

Abstract

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.

Theoretical model of external spinal cord stimulation

“…Let us show that this time in experiments [5][6][7][8][9] is several orders of magnitude shorter than the duration of the current pulse in figure 1 (1 ms) and, therefore, can be neglected. According to [22], the conductivity of the electrolyte outside the neuron is σ ∼ 1-3 Ω −1 m −1 , but in the gray matter region it is σ ≈ 0.25 Ω −1 m −1 and in the white matter σ ≈ 0.08-0.6 Ω −1 m −1 , where the spinal nerve is located [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. The radius of motoneurons a lies within 6-10 μm for α motor neurons, within 2.5-6 μm for β motoneurons, within 1.5-3 μm for γ neuron, and within 1-2.5 μm for δ neuron [29].…”

“…An increase in the potential difference across the membrane lowers the excitation threshold of the action potential. It should be noted that in all the numerical models of neuron initiation by external currents known to us (see, for example, [10][11][12][13][14][15][16][17][18][19][20][21]), the potential ϕ i,n is calculated taking into account the average conductivity of the medium in which the neurons are located. However, as shown in this article, near a neuron, the potential depends on the radius of the neuron and, accordingly, can differ significantly from the calculated one.…”

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

“…As these amplitudes are considerably lower than saturation, we expect the probability of directly recruiting anterior roots in a therapy setting to be lower than those reported here. Nevertheless, because spine curvature can dramatically alter the probability of directly recruiting the anterior roots (Binder et al, 2021), verification of post-activation depression should be carefully confirmed in different therapeutic settings (e.g., sitting, standing, or in the position required to use a rehabilitation device).…”

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

“…In general, absolute measures of intensity are of limited use. Several factors such as electrode size (Skiadopoulos et al, 2022 ), anatomical differences (Binder et al, 2021 ), waveform (Dalrymple et al, 2023 ; Manson et al, 2020 ; Sayenko et al, 2019 ), and frequency (Sayenko et al, 2019 ) will alter the efficacy of a given current. Consequently, a train of stimulation at a given intensity (in milliamps (mA)) can induce slight paresthesia in one person but widespread motoneurone recruitment in another person.…”