In the late 1980s a team of researchers in Grenoble, led by the neurosurgeon A. L. Benabit, introduced the technique of chronic stimulation of subcortical regions of the brain to treat movement disorders (e1, e2). This procedure, known as deep brain stimulation (DBS), involves stereotactic implantation of electrodes that then continuously emit short high-frequency electrical impulses in order to modulate functional neuronal circuits (Figure 1; eSupplement 1). The tip of each electrode contains at least four poles. Postoperatively, this permits a wide range of modes of stimulation from outside the brain. Each electrode is connected via a lead to the impulse generator, which is usually implanted under the collarbone (eSupplement 2). Figure 1 Target areas for deep brain stimulation in psychiatric diseases: Th, Thalamus; STh, subthalamic nucleus; Ac, nucleus accumbens; Cg25, subgenual area of cingulum, Figure: Jurgen Stoffels, Medizin Foto Koln Supplement 1: Effect of High-Frequency Stimulation on Brain Structures The mechanisms of action of deep brain stimulation (DBS) are not yet fully understood. Reversible functional inhibition of the stimulated target structures by depolarization blockade of the neurons around the electrode, or of the voltage-dependent ion channels on the cell membrane, constitutes one explanation (e50). The inhibition of the physiological activity of stimulated cells, which persists for about 5 minutes after the end of stimulation, correlates with the almost immediate and reversible effect of DBS on Parkinson-plus symptoms and tremor, depending on the functional status of the impulse generator (on/off). Synaptically mediated neuronal inhibition by antidromic excitation of inhibitory GABAergic afferents has also been discussed (e51). A third feasible mechanism is synaptic depression by orthodromic stimulation of efferent axons and consecutive inhibition of transmission by exhaustion of the neurotransmitter pool (e52). In dystonia and in psychiatric disorders the positive effect of DBS – like that of psychopharmaceuticals when first prescribed – often kicks in only after a number of weeks. This phenomenon cannot be wholly explained by a directly inhibiting mechanism; rather, the system evidently behaves in a plastic manner. Functional adaptation via complex and long-term modulation of neuronal systems seems to enable the effect. Functional magnetic resonance imaging, positron emission tomography (PET), and animal studies show, partially and in certain modalities, an elevation of the hemodynamic response and glucose metabolism of stimulated deep brain structures and their efferent projection areas—a sign of additional excitatory mechanisms of action (e53– e55). It can be assumed that the action of DBS rests on a large number of effects and interferences, which—depending on stimulation site, stimulation parameters, and the underlying disease—vary in importance and modulate the pathophysiological processes in various ways (e56– e58). Summarized very simply, DBS influences disordered neuronal networks and circuits by altering the distribution of excitation in the area of the target structure. The aim of DBS is to modulate and ideally eliminate the pathological signal transmission. Reconfiguration of the neuronal activity has a positive impact on the patient’s disease. Supplement 2: DBS Procedure (Planning and Operation) in the Cologne Study Group The procedure for deep brain stimulation (DBS) in patients with psychiatric disorders is analogous to that in movement disorders. Shortly before surgery the patient undergoes cranial magnetic resonance imaging with various weightings and particular modalities—the so-called planning MRI. Furthermore, after application of the stereotactic frame using sedation and local anesthesia, intraoperative computed tomography (CT) is performed. On the basis of the data obtained, and after consulting stereotactic atlases, the target is selected and the electrode pathways visualized in three dimensions. Following borehole trepanation the electrodes are implanted. Quadripolar (or multipolar) electrodes are used; by selection of the actively stimulating pole, these permit postoperative adjustment and modification of the stimulation site. A previous microdischarge, routine in DBS for movement disorders and serving to check correct positioning of the electrodes, is often performed in psychiatric diseases but is not regularly described. The localization of the electrodes can be checked by postoperative X-ray or cranial CT (and under certain circumstances also MRI). MRI is often not possible after implantation of electrodes because of the potential adverse effects. The initial parameter settings are mostly decided on the basis of empirical experience. The stimulation parameters can vary as follows: amplitude of current from 1 to 6 V, pulse duration from 60 to 200 µs, and stimulation frequency from 120 to 180 Hz. Surgery is completed in a second session with the infraclavicular implantation of the impulse generator. In DBS for the treatment of movement disorders the operation is usually performed without symptom-specific medication in order to be able to tell whether the desired effects are achieved with the intraoperative test stimulation. As yet there is no consensus whether medication should be continued or discontinued for DBS of psychiatric diseases, partly because the desired effects of DBS mostly appear only after a considerable interval. If intraoperative exposure and test stimulation are planned, however, it is advisable to reduce the dosage of specific medication as much as possible or interrupt it entirely. In Parkinson’s disease and essential tremor, DBS has proved so effective that it has been licensed as a treatment option (e3). Furthermore, there have been promising case studies on DBS treatment of certain subtypes of refractory epilepsy (e4, e5), dystonia (e6, e7), and chronic cluster headache (e8, e9). The DBS technique that has been in use for more than 20 years has become well known, and despite its invasive nature is associated with only minor adverse effects. The idea of extending DBS to the treatment of psychiatric disorders is based on the following considerations: In various cases, psychiatric adverse effects (induction of depressivity and hypomanic states) were observed in DBS-treated Parkinson’s disease patients. This gave rise to the proposal to employ DBS for primary modulation of psychopathological states (e10, e11). In recent years knowledge of the mechanisms of origin of psychiatric diseases has grown, largely due to modern imaging procedures. The underlying pathophysiological processes and disordered neuronal networks have to some extent been determined and localized. It is thus now possible to identify potential stimulation sites for DBS. The lesional procedures employed in the past as a last resort in cases of refractory psychiatric disease—anterior capsulotomy, cingulotomy, limbic leukotomy, etc.—achieved positive results (e12, e13). They do not come into question, however, because of the irreversible brain damage and their severe adverse-effect profile. The DBS technique, which is much less invasive, potentially reversible, and capable of modulation, can be applied to similar anatomic structures and may enable adjustment of the profile of action in the direction of the desired effects.