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Wednesday, November 26, 2014
Transcranial magnetic stimulation
Transcranial magnetic stimulation (TMS) is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain. TMS uses electromagnetic induction to induce weak electric currents using a rapidly changing magnetic field; this can cause activity in specific or general parts of the brain with little discomfort, allowing for study of the brain's functioning and interconnections. According to the United States National Institute of Mental Health, TMS "uses a magnet instead of an electrical current to activate the brain. An electromagnetic coil is held against the forehead and short electromagnetic pulses are administered through the coil. The magnetic pulse easily passes through the skull, and causes small electrical currents that stimulate nerve cells in the targeted brain region. Because this type of pulse generally does not reach further than two inches into the brain, scientists can select which parts of the brain will be affected and which will not be. The magnetic field is about the same strength as that of a magnetic resonance imaging (MRI) scan." A variant of TMS, repetitive transcranial magnetic stimulation (rTMS), has been tested as a treatment tool for various neurological and psychiatric disorders including migraine, stroke, Parkinson's disease, dystonia, tinnitus and depression.
Early attempts at stimulation of the brain using a magnetic field included those, in 1910, of Silvanus P. Thompson in London. The principle of inductive brain stimulation with eddy currents has been noted since the 20th century. The first successful TMS study was performed in 1985 by Anthony Barker and his colleagues at the Royal Hallamshire Hospital in Sheffield, England. Its earliest application demonstrated conduction of nerve impulses from the motor cortex to the spinal cord, stimulating muscle contractions in the hand. As compared to the previous method of transcranial stimulation proposed by Merton and Morton in 1980 in which direct electrical current was applied to the scalp, the use of electromagnets greatly reduced the discomfort of the procedure, and allowed mapping of the cerebral cortex and its connections.
The uses of TMS and rTMS can be divided into diagnostic and therapeutic uses.
For neuropathic pain, a condition for which evidence-based medicine fails to treat a significant number of people with the condition, high-frequency (HF) rTMS of the brain region corresponding to the part of the body in pain, is effective.
For treatment-resistant major depressive disorder, HF-rTMS of the left dorsolateral prefrontal cortex (DLPFC) is effective and low-frequency (LF) rTMS of the right DLPFC has probably efficacy. The American Psychiatric Association,:46 the Canadian Network for Mood and Anxiety Disorders, and the Royal Australia and New Zealand College of Psychiatrists have endorsed rTMS for trMDD.