Anodic or cathodic motor cortex stimulation for pain?

Objective. In motor cortex stimulation (MCS) for central and trigeminal pain Resume leads are placed epidurally over the motor and sensory cortex. Several bipolar combinations are used to identify the cortical target corresponding to the painful body segment. The cathode giving the largest motor response is chosen for chronic stimulation. The cathode of a bipole is considered as the active electrode and the anode is assumed to be the indifferent electrode, as in spinal cord and peripheral nerve stimulation. This study aims to analyse whether this assumption is correct in MCS. Methods. A computer model of MCS was used to calculate the response of cortical nerve fibres with different orientations to cathodic and anodic stimulation. Results. A cortical fibre model parallel to the paddle of the epidural lead is depolarized when a cathodic pulse is applied and has the lowest possible threshold for its excitation. When the fibre model is turned towards 90o (normal to the paddle) the threshold stimulus for its excitation is increased until it is hyperpolarized and gets unexcitable. When an anodic pulse is applied the fibre response is opposite: a fibre model parallel or normal to the paddle is hyperpolarized or depolarized, respectively. Opposite fibre responses are also obtained when the same fibre type is either in the wall of the Central Sulcus or in the Precentral Gyrus. Discussion. The results are in accordance with experimental data (1-4). In anodic stimulation on the Precentral Gyrus pyramidal tract fibres are excited directly and at a lower threshold than in cathodic stimulation when pyramidal tract fibres are activated after a longer delay (trans-synaptically). When the active electrode is above the Central Sulcus fibre responses are inverted. Conclusion. When bipolar stimulation is applied the motor response is evoked under the anode. This electrode should be used as the cathode in stimulation for chronic pain relief. References 1. Patton HD, Amassian VE. Single- and multiple-unit analysis of cortical stage of pyramidal tract activation. J Neurophysiol 1954;17:345-363. 2. Phillips C.G., Porter R. Unifocal and bifocal stimulation of the motor cortex. J Physiol (London) 1962;162:532-538. 3. Hern EC et al. Selective excitation of corticofugal neurones by surface-anodal stimulation of the baboon’s motor cortex. J Physiol 1962;161:73-90. 4. Gorman ALF. Differential patterns of activation of the pyramidal system elicited by surface an-odal and cathodal cortical stimulation. J Neurophysiol 1966;29:547-564.