Background: The characterization of neurological and neurodegenerative conditions, such as Amyotrophic lateral sclerosis (ALS), relies on biomarkers that can quantify sensorimotor dysfunction. Potential candidate biomarkers associated with spinal cord dysfunction have received little attention so far, especially very high frequency oscillation which may be generated (in part) by long sensory tracts. Previous studies1,2 have measured low-amplitude high-frequency wave evoked potentials (LHWs) recorded invasively at cervical spinal levels. Invasive neuroelectric recordings from the posterior location of the spinal cord have reported changes in the characteristics of LHW responses in the patients with neuropathic pain. In this study, we explore the possibility of using non-invasive recording techniques to evaluate Spinal-LHWs using both the anterior and posterior electrode locations at the cervical spinal cord level.
Objective: To investigate the characteristics of evoked LHW components recorded (anteriorly and posteriorly) at C6 cervical spinal level using the surface ring electrode placement system3 in response to median nerve stimulation.
Methodology: Data from 10 young healthy participants were collected. Non-invasive surface electrodes were placed on the neck in accordance with the ring electrode placement system at C6 vertebral level (Cv6) to record the neuro-electrophysiological signals (sampling rate of 8kHz). A total of 1400 evoked responses (trials) were recorded in response to the median nerve (MN) stimulation at wrist (1.5 X Motor Threshold, 2Hz). The recorded signals were pre-processed to remove artifacts, bandpass filtered between 350-2000Hz, and spatially filtered to increase the signal-to-noise ratio. The high-frequency evoked potentials (LHW) were obtained by averaging the resulting signals across all trials.
Results: The LHW responses were observed in response to the MN stimulation at anterior cervical (AC) and posterior cervical (PC) electrode locations. The recorded peak amplitude (mean ± SD) was Cv6-PC: 0.081 ± 0.029 (µV), Cv6-AC: 0.26 ± 0.11 (µV). The peak amplitude recorded at AC was greater than the amplitude measured at PC (p = 0.0011, paired t-test).
The observed peak (mean ± SD) and onset latencies (mean ± SD) were Cv6PC: 11.5 ± 2.4 (ms), Cv6-AC: 10.5 ± 0.97 (ms) and Cv6PC: 6.1 ± 0.81 (ms), Cv6-AC: 6.2 ± 0.58 (ms) respectively. The duration of the evoked responses was Cv6-PC: 9.0 ± 1.9 (ms), Cv6-AC: 7.9 ± 2.0 (ms).
Discussion: The preliminary results on young healthy participants indicate that LHW responses can be recorded with non-invasive techniques at cervical levels. Furthermore, the response measured at anterior location had higher amplitude than the response measured at posterior location. This could indicate that long-sensory tracts (eg. spinothalamic tract) located anteriorly might be responsible for LHW response, supporting Prestor1 et. al. The non-invasive LHW assessment could uncover potential biomarkers associated with sensorimotor dysfunction in the spinal cord.