NAMI HAMILTON COUNTY

What is Transcranial Magnetic Stimulation?

We asked Juliet Goldman, M.D., Ziad Nahas, M.D., and Mark S. George, M.D., of the Brain Stimulation Laboratory, Department of Psychiatry, Medical University of South Carolina, Charleston.

In the last 20 years scientists have developed many ways to observe the workings of the living human brain, but until very recently they have been able to achieve direct stimulation of precise brain regions only by placing electrodes on exposed tissue during surgery. Transcranial magnetic stimulation (TMS) is an exciting new technique that permits direct brain stimulation without surgery.

It works this way: a brief but powerful electric current is passed through a coil of wire placed near the head, creating a strong magnetic field, which in turn induces electrical currents in brain cells. Unlike electrical fields, magnetic fields pass through the scalp and skull without becoming diffused and absorbed. Thus current is delivered to the brain with great efficiency.

The strength of a magnetic field falls off rapidly, so present TMS machines can directly stimulate only the surface of the brain. TMS is a powerful tool for the study of brain functions. It can stimulate motor activity (e.g., make a thumb wiggle) or, at high frequencies and with precise focusing, alter neuronal transmission temporarily in a small region of the brain.

Researchers have used the technique to study visual pathways, language, reaction times, attention, volition, and memory. Most research on the potential therapeutic use of TMS has concentrated on depression. In the most effective known treatment for severe depression, electroconvulsive therapy (ECT), a strong electric current is passed into the brain through electrodes placed on the scalp, causing a seizure - an uncontrolled firing of neurons.

TMS delivers energy in a more controlled way. When it is used to treat depression, the coil is usually placed over the prefrontal cortex, a region behind the forehead that is the seat of judgment, planning, and decision making. It is linked to deeper parts of the brain that directly regulate mood.

Unlike ECT, TMS does not require anesthesia; the patient can talk or read during the 20-minute procedure. There are few side effects. Some patients suffer a mild headache, and seizures have occurred rarely - not at all in the last few years, since the necessary safety measures have been clarified.

Most (but not all) studies have concluded that TMS once a day for two weeks relieves symptoms of depression in some patients who have not responded to other treatments. TMS is generally found to be better than a placebo (sham stimulation) and about as effective as antidepressant drugs, although not as good as ECT. The precise mechanism of its action is unknown, but animal experiments suggest that it may bear some resemblance to the action of antidepressant drugs.

Today TMS is still regarded as an experimental treatment. To establish its value and learn more about when and how to apply it, researchers are now trying out currents of different intensities and frequencies and different ways of shaping and placing the coil. Magnetic resonance imaging (MRI) scans are proving helpful in this work.

Researchers are considering whether maintenance TMS might be effective in preventing relapse after recovery from depression. TMS may also turn out to be useful for other illnesses in which brain circuits function abnormally, such as obsessive-compulsive disorder, epilepsy, schizophrenia, and Tourette's syndrome.

Reprinted with permission from The Harvard Mental Health Letter, 164 Longwood Avenue, Boston MA 02115