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DEEP BRAIN STIMULATION: 20 YEARS AFTER

Life is electricity. Living cells have a potential difference across their membranes, which can be changed by applying an electrical current. Therefore the functional activity of neurons and their connections inside circuits are subject to alterations by electrical stimulation. Deep Brain Stimulation (DBS) has been developped after the advancement in stereotactic neurosurgery. The first indication of DBS was applied to intractable pain almost half a century ago. In 1987, the discovery of DBS in movement disorders by the Grenoble team led by A.L. Benabid marked the start of this wide scale procedure. The initial idea was to decrease bilateral thalamotomyinduced morbidity in the treatment of tremors, thanks to the functional nature of DBS, which enables adjustement and reversibility of electrical parameters. Spatial adjustability was also possible with the development of quadripolar electrodes. The discovery at the origin of the antitremor effect of DBS was its dependence on the frequency of stimulation: no effect at 50 Hz, possible worsening at lower frequencies, improvement at higher frequencies with a plateau reached over 130 Hz. Changes in voltage influence the spatial spreading of current around the stimulation contact, whereas pulse width has minimal effect. From this discovery was born the concept of electrical neuromodulation of brain tissue. The high frequency effect, which mimics that of lesioning has been discovered for brain nuclei other than the thalamus, namely the globus pallidus (GP) and subthalamic nucleus (STN). These latter targets have been stimulated following the advancement in basal ganglia pathophysiology in the 90s, when it has been be shown that the parkinsonian akinesia is associated with subthalamo-pallidal hyperactivity in animal models of Parkinson disease. Afterwards, the extension of the indications and stimulated targets stemed from clinical observations of secondary effects of DBS in Parkinson disease, such as the antidyskinetic effect in the GPi or the beneficial effect of obsessive compulsive disorder in the STN. This sounded the start of DBS in various types of dyskinesias, mainly dystonia and psychosurgery. So far, DBS is a therapy world-wide validated and regularly applied as the surgical procedure of choice for various types of tremors (parkinsonian, essential and symtomatic of a brain lesion) in the Vim thalamus or the subthalamic region, for the motor complications of levodopa-responsive Parkinson diseases in the STN or GPi and for primary dystonia in the GPi. The type of surgery by far the most frequently used is bilateral STN stimulation in Parkinson disease with about 50 000 operated patients. As any powerful therapy, the indications and patient management should