26. High intensity transcranial electrical stimulation: Brainstem fugal motor tracts may augment epidural D-waves of the cortico-spinal system

Abstract

Introduction At increasing intensities of transcranial electrical stimulation (TES) the cortico-spinal tract will depolarize deeper in the brain down to the foramen magnum, possibly causing other spine bounded tracts originating from the brainstem to depolarize too. It is hypothesized that the threshold to activate these parapyramidal (PP) axons is higher than the subcortical motor threshold of axons of the corticospinal tract (CT). In addition, one may expect that action potentials of PP axons join d-waves of the CT and augment their amplitudes. Methods A cohort of 19 patients (11 female; age mean: 51.3 ± 14y) with surgical procedures in the spinal canal under intraoperative neuromonitoring were included. TES was performed using C3 and C4 stimulation with constant voltage at 100 or 200 mcs pulse width. D-waves were measured caudal or rostral from the cord lesions. Muscular motor evoked potentials (mMEPs) were taken from the abductors pollicis brevis (APB) at cervical epidural recording levels or otherwise the anterior tibialis (AT). Voltage motor threshold parameters were derived for d-waves at subcortical level (Vthd1), at brainstem level at a latency jump of around −0.8 ms (Vthd2) and for mMEPs (VthAPB, and VthAT). The threshold ratio TRd1d2 equals Vthd2/ Vthd1. ARd1d2 is the supramaximal amplitude ratio of d1 and d2 waves. Results TRd1d2, ARd1d2, d1 and mMEP threshold values are listed below. TRd1d2 ARd1d2 Cervical level Other spinal levels Vthd1 VthAPB Vthd1 VthAT N 19 13 8 8 11 11 mean ± SD 3.09 ± 0.74 2.26 ± 0.53 31.9 ± 7.8 V 33.4 ± 7.4 V 61.8 ± 40.1 V 70.0 ± 48.6 V Paired differences Mean ± SD −1.5 ± 2.7 V −8.2 ± 16.8 V No significant differences (P = 0.05) between d1 and mMEP thresholds were found. d2 amplitudes may double or triple d1 amplitudes, whereas the mean TRd1d2 is 3.1. Conclusions At TES intensities over 3× threshold, parapyramidal axons apparently increase d-wave amplitudes and do not exclusively represent the corticospinal tract function.