In the 1990s, it is difficult to open a newspaper or watch television and not find someone claiming that magnets promote healing. Rarely do these claims stem from double-blind, peer-reviewed studies, making it difficult to separate the wheat from the chaff. The current fads resemble those at the end of the last century, when many were falsely touting the benefits of direct electrical and weak magnetic stimulation. Yet in the midst of this popular interest in magnetic therapy, a new neuroscience field has developed that uses powerful magnetic fields to alter brain activity—transcranial magnetic stimulation. This review examines the basic principles underlying transcranial magnetic stimulation, and describes how it differs from electrical stimulation or other uses of magnets. Initial studies in this field are critically summarized, particularly as they pertain to the pathophysiology and treatment of neuropsychiatric disorders. Transcranial magnetic stimulation is a promising new research and, perhaps, therapeutic tool, but more work remains before it can be fully integrated in psychiatry's diagnostic and therapeutic armamentarium.
Example of transcranial magnetic stimulation (TMS) application. Ziad Nahas, MD, demonstrates a TMS figure-8 coil applied over the left prefrontal cortex of Ananda Shastri, PhD. Note that the subject is awake and alert, and is wearing earplugs for safety. The electromyography machine in the lower left corner (B) is used to determine the motor threshold for dosing of stimulation intensity. Several TMS devices and coils are pictured: A, Medtronic-Dantec (Copenhagen, Denmark); C, Cadwell (Kennewick, Wash) with water-cooled figure-8 coil; D, Neotonus (Atlanta, Ga); and E, Magstim (Sheffield, England).
Structural imaging may guide transcranial magnetic stimulation (TMS) placement. A coronal magnetic resonance image of a subject where the location of the TMS coil is indicated above the left hemisphere motor area. The magnetic field produced by the TMS coil when it discharges is shown in black gauss lines drawn on the brain. Combining TMS with structural imaging may allow for exact guidance of TMS coils, as well as understanding where the TMS magnetic fields are distributed in the brain. (Image courtesy of Daryl Bohning, PhD, and colleagues, Medical University of South Carolina Functional Neuroimaging Division, Charleston.)
Combining transcranial magnetic stimulation (TMS) with functional imaging reveals TMS neurophysiological effects. Four transverse positron emission tomography images from Kimbrell et al.122 The nose is at the top of the image, and the right image side is the left side of the brain. These are statistical difference images from 8 adults who had TMS applied at 1 Hz over the left prefrontal cortex (arrow, D) during one scan, and then sham during another. Prefrontal TMS at 1 Hz reduced regional brain activity (blue) locally and in remote areas such as the insula (C) and orbitofrontal cortex (A and B). Although TMS with current technology only directly effects superficial cortex, remote transsynaptic effects also occur.
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