Cathodal, anodal or bifocal stimulation of the motor cortex in the management of chronic pain?

The conditions of motor cortex stimulation (MCS) applied with epidural electrodes, in particular monopolar (cathodal or anodal) and bipolar stimulation, are discussed. The results of theoretical studies, animal experiments and clinical studies lead to similar conclusions. Basically, cortical nerve fibres pointing at the epidural electrode and those normal to this direction are activated by anodal and cathodal stimulation, respectively. Because MCS for the relief of chronic pain is generally applied bipolarly with electrodes at a distance of at least 10 mm, stimulation may actually be bifocal. The polarity and magnitude of a stimulus needed to recruit cortical nerve fibres varies with the calibre and shape of the fibres, their distance from the electrode and their position in the folded cortex (gyri and sulci). A detailed analysis of intra-operative stimulation data suggests that in bipolar MCS the anode of the bipole giving the largest motor response in the pain region is generally the best electrode for pain management as well, when connected as a cathode. These electrode positions are most likely confined to area 4.