What is Transcranial magnetic stimulation (TMS)?

The transcranial magnetic stimulation (TMS) is a form of non-invasive neurostimulation which is founded on the principle of electromagnetic induction. The basic idea is to induce temporary changes at the cerebral level and is based on FARADAY's Law, which is very trivially that of electromagnetic induction.

The Transcranial Magnetic Stimulation a.k.a. TMS is a brain stimulation technique which in recent years, has also been tested in the study and in the treatment of addictions.

TMS allows you to stimulate or inhibit the cerebral cortex in a non-invasive way. It employs a magnetic pulse, focused in a limited portion of space, which is delivered through a stimulation coil. The magnetic flux is generated by the extremely rapid passage of electric current in the coil. Once generated, this magnetic impulse passes through the scalp and the bone of the skull until it reaches the brain. Any magnetic field induces an electric current in a conductor that is placed inside it (Faraday's principle of electromagnetic induction), so the magnetic pulse of the TMS induces an electric current on the surface of the brain when it reaches it.

In the area affected by the arrival of the magnetic field, the neurons will then be activated in an artificial way by the electric current generated by the field. TMS causes temporary and local interference with normal brain activity and, therefore, with the processing processes that are carried out by the brain area affected by the stimulation (Bonfiglioli & Castiello, 2005). The basic structure consists of a current pulse generator and a coil. The subject undergoing a session with TMS is seated in a comfortable chair and is alert. At the moment of the impulse emission, the coil is kept in a fixed position on the surface of the subject's head. As currently available treatments for drug addictions are limited and only moderately effective, researchers are driven to seek new therapeutic modalities. The studies that have evaluated the efficacy of repetitive TMS (rTMS) in the treatment of addiction fit into this context: although the number of these studies is still small, they provide evidence and support for undertaking further brain stimulation studies in the treatment of addictions.

How transcranial magnetic stimulation works

The transcranial magnetic stimulation consists of a series of coils or electrodes that are positioned on the head. The coil supplies electrical energy that generates a magnetic field in the brain for a short period of time (transient injury), which inhibits the cognitive functions of the stimulated area.

The magnetic field produced manages to pass the scalp and reach the brain and the underlying brain structures, inhibiting their functioning. The coil is positioned on the head at the brain region of interest. The variation of the magnetic field (2.5 Tesla of the intensity of 200 μs and duration of 1 ms) evokes a flow of electric current that interferes with normal brain activity, causing a depolarization of neurons. The magnetic stimulus produces a sound response, similar to a series of clicks and a tingling sensation on the scalp. Neuronal depolarization can lead to the activation of groups of neurons, both close to and distant from the directly stimulated area. Furthermore, the stimulation of the area takes place transynaptically that is, starting from the thinnest fibers and then reaching the thicker ones of the first layers of the cortex.

Deeper structures can also be stimulated using relatively large coils. However, the induced electric fields are always higher the closer the coil is to the area of ​​interest.

Typical magnetic field pulse parameters required to depolarize nerves include a rise time of the order of 100μsec, a peak of the order of 1 Tesla, and magnetic field energy of several hundred joules. The circuits used to generate the magnetic field pulses are usually based on a capacitor discharge system with typical coil currents, peaking in the range of several kilowatts and discharge voltages down to a few kilovolts. The relatively high voltage is needed to give the desired rapid increase in current in the inductance of the stimulating coil.

Therapeutic areas of transcranial magnetic stimulation

The transcranial magnetic stimulation is used in the field of neuroscience to study the underlying functions of certain brain areas. The subjects participating in the experiment are asked to perform a cognitive task and, at the same time, a coil, positioned on a specific brain area, inhibits the functionality of the area in question. In this way, the performances obtained are recorded, knowing that they will be deficient, confirming the cognitive function performed by the stimulated area.

Furthermore, transcranial magnetic stimulation is used in the clinical setting. In fact, it allows, in general, the improvement of the cerebral circulation and cognitive functions.

In the mid-nineties, it was found, quite accidentally, that patients with neurological pathology who were undergoing repetitive TMS (rTMS) for diagnostic purposes, and who had an associated mood disorder, could present an improvement of the depressive picture. These observations initiated the use of transcranial magnetic stimulation as a therapeutic treatment in the neuropsychiatric field. In fact, the TMS, if used repetitively at high or low frequencies, is precisely defined as rTMS, can induce and modulate the phenomena of neuronal reorganization, and is able to facilitate or inhibit in a decisive way the neuronal circuits responsible for a certain function or a certain symptom. The transcranial magnetic stimulation, again, has allowed increasing the information on the pathophysiology of ADHD. Stimulation with transcranial magnetic stimulation of the fronto-striatum-cerebellar circuit together with cognitive training allows obtaining an improvement of cognitive abilities and a reduction of symptoms.

The transcranial magnetic stimulation is used in psychiatry for the treatment of depression and a range of symptoms related to ' anxiety and consumption; in Neurology, it is useful for post-stroke cognitive rehabilitation or head trauma, tension muscle headache, Parkinson's and Tinnitus (Tinnitus).

For the aforementioned disorders, administration protocols are used in which transcranial magnetic stimulation is applied several times over time in addition to psychotherapy. In any case, transient side effects such as headaches or migraines were found in the long run.

So does it interact with human tissues?

The brain has its own electrical functioning; when we generate a magnetic field and, consequently, electrical impulses, these interact and modulate the electric current already present in the brain.

The TMS has a magnetic field generator, which is called a Coil, which is placed on the head at the point to be stimulated relative to the symptom of the reference pathology.

There are various types of single, double, or repetitive impulse stimulation. 

What can this type of treatment cure?

The pathology provides for a different application, for example, two different COILs, two different stimulations, which are H1 and H7, which hit two different areas.

The H1 which is used in major depression, while the other helmet (H7) which mainly goes to stimulate the anterior cortex which works very well for OCD (when you have repeated obsessive thoughts and the person has to put in the act of behaviors to eliminate anxiety, is the example of the person who has to go home 12 times to check that he has closed it).

We always use H1 or H7: the frequencies, the number of impulses, the duration of the impulse are possibly modified, but these are purely technical things.

The whole brain is therefore not stimulated but only the useful part based on the present pathology.

For example, in the case of addictions, we use the H7 of the obsessive-compulsive disorder, so we go to stimulate the anterior cerebral cortex: modulating the inhibitory part of our network of motivation and reward, with the result that people have less desire to use of that substance. This works the same on smoking, cocaine, as in bulimia and anorexia. If you are quitting smoking and someone offers you a cigarette, it may be that even during the treatment, you smoke it the same, but you still lose the desire to look for it.

Author: Vicki Lezama