Principles of therapeutic use of transcranial and epidural cortical stimulation

Among the alternatives to drugs in the treatment of neurological and psychiatric disorders, neuromodulation techniques, including brain stimulation, have been used increasingly this past decade. Cortical targets are especially appealing, because they are easily accessible by noninvasive or invasive methods. Applicable techniques include repetitive transcranial magnetic stimulation (rTMS), transcranial electrical stimulation using pulsed or direct current, and epidural cortical stimulation (ECS) with surgically implanted electrodes. In contrast to deep brain stimulation in movement disorders or electroconvulsive therapy in depression, the efficacy of cortical stimulation to treat neurological or psychiatric disorders has not been yet clearly demonstrated. However, encouraging results have been reported in neuropathic pain (for ECS) and depression (for rTMS). In this review, we will consider some principles and mechanisms of action of these methods. First, it must be noted that fibers of intracortical or cortico-subcortical networks are more prone to be activated by the stimulation than cell bodies of local cortical neurons. Hence, the site(s) of action may be distant from the site of stimulation. In addition, various parameters of stimulation (such as stimulation frequency, intensity, or electrode polarity) and the configuration of the induced electrical field greatly influence the nature of the recruited circuits, and therefore, the overall efficacy. Finally, clinical changes may be delayed and prolonged beyond the time of stimulation, complicating programming algorithms in the case of implanted stimulation device. All these features need to be taken into account when considering cortical stimulation as a method of treatment.