Intra-operative spinal cord evoked potentials during cervical and lumbo-sacral microsurgical DREZ-tomy (MDT) for chronic pain and spasticity (preliminary data).

We have undertaken the intra-operative study of spinal cord surface evoked potentials in patients operated upon for pain and/or spasticity using the microsurgical DREZ-tomy (MDT) procedure. The goals of this work were 1) to collect data on spinal cord evoked potential components and 2) to analyze the effects of MDT on spinal cord physiology. The MDT consists of a therapeutic lesion in the ventro-lateral aspect of the dorsal root entry zone, directed to the activatory circuitry, and aiming at returning the dorsal horn physiology towards inhibition. Averaged evoked potentials to peripheral nerve electrical stimulations were obtained from various loci on the surface of the dorsal columns of the cervical and lumbo-sacral spinal cord in 19 patients, using a small uninsulated silver ball electrode. An initial far-field positivity was found, corresponding to a compound action potential in the proximal part of the brachial (or lumbo-sacral) plexus. Pre-synaptic compound action potentials were identified, most often composed of multiple successive sharp peaks. A post-synaptic field potential generated in the dorsal horn was recognized. The MDT caused an immediate and irreversible decrease of amplitude down to a disappearance of the dorsal horn potential. This decrement was proportional to the amount of operated cord segments. In contrast, there has been a relative post-MDT sparing of the pre-synaptic action potentials originating from the operated cord segments, and the scalp contralateral parietal N 20 has been only reversibly affected by the therapeutic lesion. We thus argue for a specific involvement of dorsal horn physiology by the MDT, with a relative sparing of the dorsal column system.