MEPs to identify the optimal cathode position in MCS

Aim. In MCS motor evoked potentials (MEPs) are recorded to identify the pain-related somatotopic region(s) of the motor cortex. Since stimuli are applied bipolarly, MEPs may originate near the negative pole (cathode), the positive one (anode), or both. The aim is to identify the stimulus polarity effective in inducing MEPs and in relieving pain. Materials and methods. Two 4-electrode leads were placed in parallel on the dura and across the central sulcus. Identical supra-threshold stimuli were applied peroperatively by 14 bipolar electrode combinations and the magnitude of each MEP from a muscle in the pain region was noted. The largest MEPs were allocated to both the anode and the cathode of the corresponding bipole and the MEP- related anode and cathode positions were compared with those selected postoperatively to give most pain relief. Results. Data of five patients with primarily upper limb pain were analyzed. MEP-related cathodes prevailed over the premotor cortex (area 6), whereas the corresponding anodes were confined to area 4 near the central sulcus. In each patient the cathode position in chronic stimulation corresponded with the anode position related to the largest MEP. The anodes in chronic stimulation were situated posteriorly over the central sulcus or the postcentral gyrus. Conclusions. In bipolar MCS the optimal cathode position for chronic pain relief corresponds with the position of the anode giving the largest MEP of a muscle in the pain region. Applying this "rule" will accelerate the selection of the optimal bipole for therapeutic stimulation. The comparison of anodal and cathodal monopolar stimulation is currently under investigation.