The principle of transcranial magnetic stimulation (TMS)
The principle of TMS was translated into practice in 1985 when equipment was constructed which was able to create strong changing electromagnetic fields lasting less than a second and which could be used to induce electric current in the nervous system. The overall effect of TMS depends on the site and extent of the area of nervous system stimulated by the equipment.
Applying TMS in diagnosis
TMS is a fast, non-invasive, painless functional diagnostic method which is now in use all over the world for the diagnosis of nervous system disorders. One of the most spectacular effects of TMS is stimulation of motor pathways. The result can be seen immediately because muscles are induced to contract. By monitoring electrical activity in the muscles when they contract (the motor evoked potential, we can address several important questions in the diagnosis of nervous system diseases: -Is the motor pathway intact ? Can muscle contraction be induced by TMS ? How much time elapses between stimulation to the muscle movement ? Where is the lesion in the spinal cord ?
Therapeutic application of TMS in the treatment of central nervous diseases
It was observed in 1993 that repetitive transcranial magnetic stimulation (rTMS) could decrease anxiety and improve depressive moods. The use of rTMS was soon introduced for the treatment of depression.
RTMS in patients with Parkinson’s disease for 7 days can decrease the slowness of movement, with the consequence that the subject can eat faster and is able to dress with less assistance. He or she can stand up, turn over in bed and walk further with longer steps than before treatment. The increased speed of movement develops gradually over the next weeks. The effect of rTMS lasts for an average of 4 months, when it should be repeated. The painful cramps caused by the disease or the side effects of certain drugs can often be reduced by the end of the treatment week. The present treatment protocol has an influence on balance instability - the main symptom in Parkinson-like diseases – which can be significantly reduced. Repeated treatments with rTMS can slow the development of Parkinsonian symptoms compared with the group of patients taking drugs alone.
The main symptom of patients with stroke is the weakness (paresis) in one half of their body. The muscle tone is increased (spasticity). RTMS applied for one week can decrease the muscle tone for several months and help to start active movement of the paretic limb. Functional improvement can be achieved if the rTMS is repeated for a week on three different occasions during a six-month period. The effect of rTMS is independent of the duration of stroke.
Treatment using rTMS for a week can decrease the painful cramps in patients with spinal cord injury. It can also induce active movement in paretic extremities. According to animal studies rTMS promotes the development and reorganization of serotonergic pathways.
The most frequent questions:
Is treatment with rTMS painful ?
No, because there is not enough electric current under the skin to be able to stimulate pain sensitive receptors.
Can rTMS be focused on a restricted neuron population (neuronal network) ?
Partly. The 8-shaped coil is appropriate for that objective, although rTMS is less appropriate for highly focused applications than direct electric stimulation. The effect of rTMS depends on the position of the coil. Action potentials can be induced in nerves if the changing electromagnetic field crosses the neuronal cell body-axon axis perpendicularly.
What is the site of action of rTMS?
Stimulation with low frequency (1-3 Hz) can decrease the excitation of cortico-cortical connections and increase the release of dopamine in the striatum. The effect of rTMS depends on the parameters of application. Slowing the progression of Parkinson's disease is explained by the enhanced formation of new nerve cells (neurogenesis) in the subventricular zone. Animals with nigrostriatal lesions showed cell proliferation expressed by tyrosine-hydroxylase after rTMS.
In which research field is the transcranial magnetic stimulator used ?