Transcranial magnetic stimulation (TMS), a technique to locally stimulate the brain by placing magnetic coils at carefully selected points on the head, is becoming an important tool for studying brain function and organization, and may offer new techniques for treating central nervous system conditions such as depression, experts say. TMS has been approved for treatment of depression in Canada, but is still considered experimental in the United States.
In June, three world-class TMS investigators traveled to Quebec City to address the 24th annual meeting of the Bioelectromagnetics Society. In a plenary session, Shoogo Ueno, professor of biomedical engineering at the University of Tokyo; John Rothwell, Sobell Institute of Neurology, London, U.K., and Eric Wassermann of the neurology institute at the U.S. National Institutes of Health, Bethesda, Md., discussed current knowledge on localized magnetic brain stimulation, field calculations, functional brain mapping, nerve excitation models and applications for cognitive recovery of injured hippocampal neurons in an animal model, and touched on principles of repetitive, or rapid-rate TMS (rTMS), a related technique.
TMS uses a strong, static magnetic field to stimulate an electric current in small groups of brain or nerve cells. Researchers working with TMS and rTMS are mapping and exploring the function and timing of signals along neural pathways. By temporarily altering brain activity through TMS and rTMS in experimental settings, some research groups have already discovered extraordinary new insight into language processing, mood, the nature of conscious awareness, and more. For example, using a figure-eight magnetic coil strategically placed on a conscious volunteer subject’s head, Ueno and colleagues have obtained functional maps of the human motor cortex where nerves controlling the hands, arms and feet are located.
Experiments show that a magnetic field on the order of 1 Tesla applied for 0.1 to 0.2 milliseconds has a potential for therapeutic application in Parkinson’s disease, Ueno said. Wassermann added that the noninvasive technique has been an extremely safe method for studying healthy, normal brain responses.
In animals, Ueno and colleagues have observed repair of damaged hippocampus tissue in rats treated with a 1.2 Tesla pulsed magnetic field for 48 hours, suggesting the tantalizing possibility that people with hippocampal tissue damaged by epileptic seizures, for example, may someday benefit from rTMS techniques for brain tissue repair or recovery.
In England, Rothwell and colleagues note that until TMS and rTMS techniques were perfected in recent years, “much of what we know had come from indirect behavioral measures” such as a muscle twitch in the rare patient with an implanted electrode. Now, however, exciting new information is being uncovered Ñ not only about effects in the brain but in distant structures known to receive anatomical connections from the stimulated brain area.
“TMS is the best tool we have for directly influencing the activity of the human brain,” Rothwell said at the Quebec City meeting. He also discussed combining TMS and rTMS with other modalities Ñ EEG, fMRI and PET Ñ to “give an extraordinarily detailed picture of how stimulation affects function both at the site and at a distance in the body.”
In Quebec City, Wassermann discussed some of the strengths and drawbacks of each of the various brain activity measures Ñ BOLD MRI, PET, EEG Ñ used along with TMS in order to expose connections between the stimulated areas and other brain regions. For example, PET has the ability to examine the whole brain at once, which allows scientists to note changes in blood flow with TMS stimulation. Such studies “are beginning to provide new and scientifically interesting data,” Wassermann remarked.
TMS Emerging As a Useful Tool in Treatment of DepressionMore recently, Sheila Dowd, associate professor of psychiatry at the University of Illinois-Chicago, reported in August at the annual meeting of the American Psychological Association that TMS appears to relieve depression as effectively as electro-convulsive therapy (ECT). “Many severely depressed patients do not benefit from or tolerate existing treatments,” she and colleagues noted.
But in ten to 20 treatments with a 10-Hz, 110 per cent motor threshold magnetic field applied to the left dorsolateral prefrontal cortex for a total of 10,000-20,000 stimulations over about a month, people with severe depression reported about equal improvement as measured by the 24-item Hamilton Depression Rating Scale compared to those who received ECT (bitemporal, 4-12 treatments), according to Dowd. It is not clear exactly why the magnetic stimulation is effective for severe depression, but most of the 15 people randomly assigned to receive TMS rather than ECT in Dowd’s experiment said they preferred the magnetic field therapy anyway.
This is probably due in part to the fact that side effects are usually limited with use of TMS. In this case, Dowd cited “a few reports of minor headache and some redness where the electrode-like coils were placed against the skull.”
With the early success of experiments by Dowd and others, many observers believe that larger studies of a role for TMS in treating depression are warranted.