"Large Field" Transcranial and Peripheral Magnetic Stimulation

At the beginning of the twentieth century, neurologists frequently used electricity, in the form of faradaic current (battery-powered D.C. current and transformed into asymmetric alternating current by magnetic induction, to treat hysterical paralysis. This technique, called "Faradisation", was applied at the level of paralysed limbs, with some efficiency.

The major problem with this technique was its painful character. The current must first pass through the skin before reaching the peripheral muscles and nerves. The skin is very resistant, resulting in a loss of power and pain. This technique was particularly used during the First World War, due to an epidemic of functional neurological phenomena, with some efficacy but in a particularly barbaric way (see "World War I" in the menu Patients). Thereafter, this method was largely neglected.

In the 1980's, a first transcrânniene magnetic stimulation device (TMS) was developed, allowing a large part of the cortical surface of the cerebral convexity to be stimulated, in a non-painful way, with a circular coil. The principle is as follows: a capacitor discharges into a circular coil to produce a variable magnetic field. This magnetic field crosses the cranial box without resistance and induces a circular current in the cerebral cortex which, due to its constitution (several billion neurons and their axons), is assimilated to a closed circuit. According to Faraday's law, a variable magnetic field induces an electric current in this closed circuit, which, in the case of TMS, will activate cortical neurons. The first use of this type of stimulation was aimed at diagnosis. Each stimulation, resulting in a diffused muscular contraction of the limbs, allows a latency calculation to explore the final driving path. In a second phase, TMS was and still is very much used therapeutically but in a "focal" way, i.e. stimulating a few cm² of cerebral cortex with a figure of eight coil.


Coil in the shape of a figure of eight

This type of stimulation is only possible for pathologies where there is an identified target. This is the case for depression (prefrontal dorsolateral cortex) and neuropathic pain (motor cortex). Treatment is done in the form of sessions of approximately 4000 stimulations on the target at a frequency of 5 to 20Hz (activator). The treatment usually consists of a series of five sessions over five consecutive days and then monthly reminder sessions.

For functional neurological disorders, the networks are complex (see Assumptions on origins of disorders) and it is probably for this reason that the SMT "focal" does not work. In the absence of a target, a "large Field" SMT with a circular coil is more logical.



Circular Coil

A session usually consists of about sixty transcranial stimulations at 1 Hz at the engine threshold. Peripheral stimulation at 3 Hz on deficient or painful regions has additional efficacy. This magnetic stimulation crosses the skin in a painless way and creates deep currents that will stimulate the nervous nets which, using a retrograde path, can influence the corresponding control systems and improve the functional neurological symptoms. Peripheral magnetic stimulation can be considered as the modern form of the faradisation of the neurologists of the early twentieth century, more effective and above all much less painful.

With paralysis, a single session can suffice. In fibromyalgia, three sessions separated by three weeks are often necessary to determine if the patient responds to this type of treatment. It will be necessary to do maintenance sessions every 1 to 3 months. The therapeutic effect can be immediate (see Fibromyalgia in the Patient drop-down menu) or, more usually appear after a few days. But this effect is transient, hence the need to repeat the sessions. When the patient responds positively to the first stimulation sessions, there will never be a decrease in efficacy after the following sessions. Over time, we can often, but gradually, space the sessions.