Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a method of inducing a flow of electrical current within brain cells (called neurons) by the external application of a magnetic field. In this respect, it can be thought of as having some similarities to electro-convulsive therapy (ECT). ECT also induces electrical activity in the brain but by the direct application of an electrical current which induces an epileptic seizure. Unlike ECT, TMS does not lead to a generalized seizure and requires no general anesthetic.

The patient is normally seated and is fully conscious. A special cap is placed on his/her head that has the markings on it for the coil positions. Electrodes are placed on the scalp, which monitor the motor evoked potentials produced by observable muscle movement. The first part of the procedure is called mapping. During this, the patient's motor threshold is established. The motor-threshold is taken to be a measure of the person's cortical excitability and allows calibration for the strength of the stimulus needed. Current investigations are concentrating on establishing treatment parameters with regards to the strength of stimuli, duration of stimuli trains and inter-train intervals.

At present, TMS is still in its very early stage of evaluation as a potential treatment for depressive conditions. Research studies have reported mixed results in the treatment of major depression. TMS currently remains as an experimental treatment with more studies needed to examine the duration of its effect and its efficacy as compared to ECT. It is possible that TMS will become an adjunct to medication treatment in depression or an alternative if drug treatment fails.

Single pulse TMS was first developed in the mid-1980s as an investigative tool in neurology. It was discovered that if a magnetic field was applied to various parts of the brain it would either stimulate or inhibit various areas and could, therefore, change some functions of the brain. Neurologists were able to use this tool to map the functions associated with various parts of the cerebral cortex. It was also discovered that different TMS frequencies appeared to have different physiological effects, for example, lower frequencies could be inhibitory and higher frequencies activating. Very high-energy fields were found capable of inducing a seizure. In people who have uncontrollable epilepsy, neurologists have been able to use this technique to physically pinpoint an epileptic focus prior to the person undergoing surgery to remove this focus. In studies of language localization it was also noted that if TMS was applied to the frontal region of the brain on the dominant side that the subject experienced a change in mood. This has led to the interest in the subject amongst psychiatrists.

TMS is based on the physical principle that an electrical coil is moved through a magnetic field an electric current will be induced and flow through the coil. If the coil is kept stationary and the magnetic field is switched on and off, the same thing will occur.

This is the process by which an electrical generator works. The principle underlying this phenomenon is called focal electromagnetic induction or Faraday's law. In TMS, however, the brain can be considered to be the coil in which electrical current is induced to flow within neuronal circuits. To produce this effect a TMS machine is used to deliver a localized magnetic field that is rapidly switched on and off. The more rapidly this is done the more powerful and sustained is the current in the neuronal circuits. This procedure is called repetitive transcranial magnetic stimulation or rTMS.

In TMS an insulated coil conducting an electric current is placed on the surface of the scalp. By changing the current passing through the coil, magnetic fields are produced which can pass through electric insulators such as skin and bone. Neurons up to 2 cm below the coil, in the cerebral cortex of the brain, can be activated. The currents that are produced in these neurons by these rapidly changing magnetic fields can interrupt or facilitate neuronal functioning.

Are there any negative side effects of TMS?

TMS is considered to be a safe procedure as long as treatment guidelines on stimulus strength, intensity and duration are adhered to.

Some individuals have described a mild discomfort or tingling of the scalp after application of the technique. Mild headaches have also been noted. These normally respond to simple pain relief (e.g. with paracetamol). At present, the only serious side effect that has been reported (in neurological applications) has been the induction of a seizure when a high energy level of stimulation has been used. Because the coils make a rapid and sustained clicking noise as they discharge, wearing earplugs has been recommended. No long-term adverse physical effects have been found. Some of the potential benefits that TMS may have over the more conventional ECT are its apparent lack of effect on memory and other cognitive functions.