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.

Motor cortex stimulation: role of computer modeling.

Motor cortex stimulation (MCS) is a promising clinical technique used to treat chronic, otherwise intractable pain. However, the mechanisms by which the neural elements that are stimulated during MCS induce pain relief are not understood. Neither is it known which of the main neural elements, i.e. cell bodies, dendrites or fibers are immediately excited by the electrical pulses in MCS. Moreover, it is not known what are the effects of MCS on fibers which are parallel or perpendicular to the cortical layers, below or away from the electrode. The therapy and its efficacy are less likely to be improved until it is better understood how it may work. In this chapter, we present our efforts to resolve this issue. Our computer model of MCS is introduced and some of its predictions are discussed. In particular, the influence of stimulus polarity and electrode position on the electrical field and excitation thresholds of different neural elements is addressed. Such predictions, supported with clinical evidence, should help to elucidate the immediate effects of an electrical stimulus applied over the motor cortex and may ultimately lead to optimizations of the therapy.

Modelling motor cortex stimulation for chronic pain control: electrical potential field, activating functions and responses of simple nerve fibre models.

This computer modelling study on motor cortex stimulation (MCS) introduced a motor cortex model, developed to calculate the imposed electrical potential field characteristics and the initial response of simple fibre models to stimulation of the precentral gyrus by an epidural electrode, as applied in the treatment of chronic, intractable pain. The model consisted of two parts: a three-dimensional volume conductor based on tissue conductivities and human anatomical data, in which the stimulation-induced potential field was computed, and myelinated nerve fibre models allowing the calculation of their response to this field. A simple afferent fibre branch and three simple efferent fibres leaving the cortex at different positions in the precentral gyrus were implemented. It was shown that the thickness of the cerebrospinal fluid (CSF) layer between the dura mater and the cortex below the stimulating electrode substantially affected the distribution of the electrical potential field in the precentral gyrus and thus the threshold stimulus for motor responses and the therapeutic stimulation amplitude. When the CSF thickness was increased from 0 to 2.5 mm, the load impedance decreased by 28%, and the stimulation amplitude increased by 6.6 V for each millimetre of CSF. Owing to the large anode-cathode distance (10 mm centre-to-centre) in MCS, the cathodal fields in mono- and bipolar stimulation were almost identical. Calculation of activating functions and fibre responses showed that only nerve fibres with a directional component parallel to the electrode surface were excitable by a cathode, whereas fibres perpendicular to the electrode surface were excitable under an anode.