Intraoperative neurophysiologic mapping of the central cortical region for epidural electrode placement in the treatment of neuropathic pain by motor cortex stimulation

Neuropathic pain results from injury to the central or peripheral nervous system and can prove itself refractory to classical medical treatment by anticonvulsants and antidepressants. In such cases, motor cortex stimulation is among the neurostimulation techniques available for its symptomatic control. This technique is based on surgical implantation of electrodes over the motor cortical representation of the painful area. Image-guided navigation is used for precise identification of the motor cortex intraoperatively, but proper placement of the electrodes is usually ensured by electrophysiologic mapping. This article details the intraoperative electrophysiologic procedure that we currently use for refining electrode placement in the epidural space, including the recording of somatosensory and motor-evoked potentials (MEPs). Various procedures have been reported and some groups are using direct cortical mapping and subdural electrode placement rather than epidural. Our method is one of several proposed techniques and is mostly based on intraoperative MEP mapping in response to monopolar (anodal) epidural stimulation of the cortex. The limit of this approach is that MEPs cannot be recorded in patients with total or severe motor deficit. We have shown that intraoperative mapping of the cortical region corresponding to the painful area by recording MEPs could help select contacts to be activated for chronic stimulation. Therefore, the patients in whom intraoperative MEP mapping is possible could benefit from this technique, at least if we consider that it improves the accuracy of electrode placement and that motor cortex stimulation efficacy critically depends on this placement.