Day #210: Review – Cervical trans-spinal direct current stimulation: a modelling-experimental approach
I lied! I did know what today was going to be on, it’s the fifth critical review paper. Since my PI gets a copy, so do you! To be honest, I need to create a category for these reviews (Update: I did make a category, Critical reviews), but for now, my first looking at elbow spinal stretch reflexes is here. My second where I review modulating spinal cord excitability with a static magnetic field here. The third where I review modulating the H-reflex while walking in spinal cord injury populations. Lastly, my fourth on Motoneuron excitability during voluntary muscle activity in a spinal cord injury population can be found here. That said, let’s take a look at my latest review.
Transcutaneous spinal direct current stimulation (tsDCS) is an emerging non-invasive technique for neuromodulation of the spinal cord circuitry. Because this technique is so new, understanding how the current produced flows through the body will help researchers find optimal placement for the electrodes used. The tradeoff between computational requirements and model complexity is difficult because simplifying a model may reduce its predictive ability, whereas increasing complexity can be computationally intensive.
This study tries to balance model accuracy and model complexity by creating a predictive model, then testing the predictions with human subjects. To do this, researchers recruited 10 healthy (4 male) right-handed subjects between the ages of 22 and 40 years old. The subjects underwent three sessions of tsDCS using a new montage predicted via the model. Each session consisted of three conditions, anodal stimulation, cathodal stimulation, and sham with at least one week between sessions to avoid carry-on effects. Transcranial magnetic stimulation (TMS) was used to produce motor evoked potentials (MEPs) and bipolar stimulation of the right median nerve at the wrist was used to produce sensory evoked potentials (SEPs). Half of the subjects were also tested using H-reflex and F-waves, which were used to infer lower motor neuron (LMN) excitability. F-waves were recorded using right ADM supramaximal stimulation of the ulnar nerve at the wrist. H-reflex was recorded using a bipolar montage over the flexor carpi radialis (FCR) muscle and stimulated by delivering 1 ms rectangular stimuli in the median nerve at elbow.
The model predicted a C3-T7 montage would produce the highest current density and e-field magnitudes with maxima at the C6-C7 spinal segments, this was placement was used in the experiments. While they found no difference between sham, cathodal, or anodal SEP’s, using Bonferroni corrected pairwise comparisons they did find significant differences between cathodal and sham conditions (p = 0.023). They also found MEP latency in ADM decreased with cathodal tsDCS. Both findings agree with previous observations that cathodal-dependent LMN facilitation is reported.
From a research standpoint it’s interesting to see a study that creates a model and tests the predictions of that model. Typically, models are reported on using previously known data to determine accuracy. New predictions from that model are not often tested. One point of interest is the suggested C3-T3 montage used. Typically, cervical stimulation is done by placing the anode directly under the cervicomental angle along the midline of the anterior neck which causes the muscles of the neck to contract when stimulation is being applied. Should this montage work as well or better, cervical tsDCS would be more tolerable for subjects.
One major concern for the study was the small sample size. The authors address this concern and recommend further testing. They also address the simplifying assumptions used in the model and suggest modifying the model based on experimental data to better explain some of the variability in observations. Overall this study was well done, and the limitations addressed adequately. While a larger sample size could have improved the study and strengthened the statistical findings, future improved models will help the entire field mature and will greatly benefit people living with spinal cord injury.
Fernandes, S.R., Pereira, M., Salvador, R. et al. Cervical trans-spinal direct current stimulation: a modelling-experimental approach. J NeuroEngineering Rehabil 16, 123 (2019). https://doi.org/10.1186/s12984-019-0589-6