Effect of movement on SEPs generated by dorsal column nuclei

Abstract

To investigate the effect of the voluntary movement on the amplitude of the somatosensory evoked potentials (SEPs) recorded by an epidural electrode at level of the dorsal column nuclei (DCN). Five patients, suffering from chronic pain resistant to pharmacological treatment, underwent an epidural electrode implant at high cervical spinal cord level (C2) for neuromodulation. After tibial nerve stimulation, SEPs were recorded from the epidural electrode contacts, from a Cz lead, and from two electrodes placed over the 12th dorsal vertebra and 4th lumbar vertebra, respectively. SEPs were recorded at rest and during a voluntary flexo-extension movement of the stimulated foot. Beyond the low-frequency SEPs, also the high-frequency oscillations (HFOs), obtained by filtering the recorded traces by means of a 1000-2000 Hz bandpass offline, were analysed. The epidural electrode contacts recorded a triphasic potential (P1-N1-P2), whose negative peak showed a latency similar to that of the P30 far-field response obtained from the scalp. The epidural potential amplitude was significantly decreased by the voluntary movement, as compared to the rest (p<0.01). A main HFO peak, identifiable at around 1200 Hz, was significantly lower in amplitude during movement than at rest (p=0.008). Our findings suggest that the epidural C2 triphasic wave is a potential arising from DCN. The low-frequency epidural SEP component is subtended by a 1200 Hz HFO, probably generated by post-synaptic events. The amplitude reduction of the DCN response during movement is possibly due to decreased excitability of the DCN neurons receiving the somatosensory ascending input.