Design and Optimization of a Novel Method for Assessment of the Motor Function of the Spinal Cord by Multipulse Transcranial Electrical Stimulation in Horses

Compared to transcranial magnetic stimulation (TMS), transcranial electrical stimulation (TES) more specifically assesses the motor function of the spinal cord and excludes reproducibility errors from coil repositioning. Objective: to assess the applicability of multipulse TES in horses and retrieve optimal TES parameters to elicit muscular motor-evoked potentials in the m. extensor carpi radialis (ECR) and the m. tibialis cranialis (TC) in a scouting study. This is a prospective observational study in five healthy horses based on TES as a novel alternative to TMS to assess the motor function of the spinal cord for clinical diagnosis in search for optimal settings of stimulation parameters. After sedation, a subcutaneous anesthetic ring block was placed on the forehead around bilateral TES needle electrodes. In each step of a specific parameter optimizing protocol, one parameter was varied while leaving others at default values: TES motor threshold +30 V, n = 3 pulses/train (ppt), interpulse interval (ipi) = 1.3 ms, and 0.1 ms/phase biphasic pulses. Variable parameters were TES voltage (0-200 V), n (1-5 ppt), and ipi (0.5-4.5 ms). A multipulse facilitation factor (MPFF) quantified the motor neuron recruitment gain by multipulse stimulation. Mean latency times, MPFF, optimal ipi, and n for the ECR muscles were, respectively, 18.6 (1.26) (mean[SD]) ms, 7.1 (3.4), 1.25 (0.21) ms, 3.0 (1.4) ppt (left) and 18.4 (1.10) ms, 4.3 (1.4), 1.9 (0.7) ms, 3.5 (1.3) ppt (right) and for the TC muscles, respectively, 34.5 (0.96) ms, 53 (2.4), 1.2 (0.28) ms, 3.3 (1.0) ppt (left) and 33.4 (1.52) ms, 17.5 (21.2), 1.3 (0,17) ms, 3.3 (0.5) ppt (right). Optimal multipulse TES parameters were n = 3 ppt and ipi = 1.2 to 1.3 ms. Multipulse TES is well tolerated and an attractive alternative to TMS. Transcranial electrical stimulation is expected to be a more robust technique than TMS for evaluation of spinal motor function in horses. A better reproducibility of repeated stimulations is expected due to fixed electrodes, and a reduced sensitivity to hyperpolarizing effects of sedatives is expected.