Transcranial magnetic stimulation for the treatment of depression

www.expert-reviews.com ISSN 1473-7175 © 2010 Expert Reviews Ltd 10.1586/ERN.10.95 Transcranial magnetic stimulation Description of the method & devices Transcranial magnetic stimulation (TMS) involves inducing an electrical current within the brain using pulsating magnetic fields, which are generated outside the brain near the scalp. The essential feature is using electricity to generate a rapidly changing electromagnetic field, which in turn produces electrical impulses in the brain. A typical TMS device produces a fairly powerful magnetic field (~1.5–3 Tesla), but only very briefly (a fraction of a millisecond for each pulse). TMS is not simply applying a static or constant magnetic field to the brain, and differs from the other brain stimulation techniques that are either invasive (e.g., deep brain stimulation), or require a seizure for therapeutic effects (electroconvulsive therapy [ECT]). There are now entire journals devoted to the field of brain stimulation [1], and books devoted to each of the individual techniques [2], as well as in-depth overviews [3]. The interested reader is referred to these references for reviews of the other brain stimulation methods or about TMS in other clinical or research applications. This article is limited to covering TMS as a clinical antidepressant. With respect to TMS, by the year 1820, scientists had discovered that passing an electric current though a wire induces a magnetic field. In 1832, Michael Faraday demonstrated that the inverse was also true – passing a wire through a magnetic field generates an electrical current [4]. Thus, a changing magnetic field can generate electrical current in nearby wires, nerves or muscles. A static magnet will not generate a current. For most TMS applications, it is probably is the electricity induced in the brain from the pulsating magnet, and not the magnetic field itself, that produces neurobiological effects. In fact, most people assume that the neurobiological effects of most TMS applications stem from the actual depolarization of neurons, causing them to fire, and that massively subthreshold TMS, far Mark S George