Selectivity of a non-invasive electrical stimulation system for neurophysiological assessment of posterior root reflex responses

Abstract

Transcutaneous spinal cord stimulation is a non-invasive, hugely useful approach for different neuromodulation applications. With this stimulation technique single muscle responses can be elicited for execution of reflex studies e.g., or a train of stimuli can be used for activation of the central pattern generator, facilitating spinal cord injured individuals in generating gait cycles. For the latter a selective control of separate muscles is necessary. Here, information of the targeting spinal cord segments is of high relevance. Though it is possible to apply single stimuli creating muscle twitches for orientation in the spinal cord, it would be more timesaving to fasten a vertical electrode array along the spine. Thus anticipate excited structures by stimulation of a certain electrode level. Also for applications executed in the supine or prone position it would be a benefit knowing which structures and segments at and within the spinal cord are treated through a special stimulation position. The main goal was to reach comprehensive knowledge about the activated muscle-groups quadriceps (Q), hamstrings (HAM), tibialis anterior (TA) and triceps surae (TS) through transcutaneous spinal cord stimulation (tSCS) in three different body positions namely supine, standing and prone. Because of their independent innervation pattern, Q and TS were investigated regarding selective control of each muscle. For this purpose an electrode array, consisting of 21 ECG electrodes (three per level with seven stimulation heights), was designed for reaching the posterior respectively the anterior roots of the lumbosacral region. Beside the selectivity, the activation threshold, the resulting amplitude and the differentiation of the evoked potentials according to their origin in the afferent respectively the efferent fibres were key aspects of this work, defining `best- stimulation sites with a low threshold, a high afferent component and high amplitudes for every muscle. Ten subjects were investigated through stimulation of the corresponding spinal segments above the T11 - L2 vertebral bodies. Selective behaviour of the muscles Q and TS could be verified in most of the subjects in the supine and standing body position at and near threshold intensity. At higher intensities this changed to a predominant stimulation. In the prone body position a selective behaviour of Q was prevalent. With mainly afferent induced responses in the supine, the efferent ones increased from standing to the prone body position. Also the amplitudes were different with highest values in the supine, smaller in the standing and lowest in the prone position. Threshold intensities lowered from cranial to caudal. Extensive data were collected which can be useful for application of such an array to serve for assistance in orientation and the treated structures within the spinal cord even by changing the body position to supine, standing or prone.