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Tuesday, December 2, 2014

Transcranial Magnetic Stimulation ( TMS )

What is transcranial magnetic stimulation ( TMS )?

Transcranial magnetic stimulation is a technology which, with the aid of powerful magnetic fields of the brain both stimulates, as can also be inhibited. Thus, the TMS is a useful tool in neuroscience research. In addition, transcranial magnetic stimulation is used to a limited extent in neurological diagnosis or proposed for the treatment of neurological disorders such as tinnitus, apoplexy, epilepsy or Parkinson's disease, as well as in psychiatry for the treatment of affective disorders, especially depression , but also of schizophrenia. From the first studies carried out it is not yet recognize the extent to which the rather high clinical expectations are entitled to transcranial magnetic stimulation.

History of transcranial magnetic stimulation (TMS)

First transcranial magnetic stimulations reach the physician and physicist Jacques-Arsene d'Arsonval end of the 19th century at the College de France in Paris. He used high voltage coils as they are used in electric power plants to stimulate himself and his subjects, and was able to prove such a way that a changing magnetic field induces a current flow in human tissues. This was followed, conducted primarily in self-experiments, experiments with very large coils often completely enclosed the head of the subjects. The subjects saw lively phosphenes (magnetophosphenes), and experienced cardiovascular disorders and vertigo to loss of consciousness. Recent research assume that the observed effects were not due to the stimulation of the brain, but by direct stimulation of the optic nerve and retina about.

At the University of Sheffield, the modern version of transcranial magnetic stimulation was presented by Anthony Barker 1985. It is due to the technical development of powerful capacitors and uses much smaller coils that stimulate the cerebral cortex only in a small area. The magnetic stimulation of the skull near cortex has since virtually no inconvenience to the subjects or patients and technically (in reference to Sherlock Holmes) "simplicity itself".

Technical Basics

The Transcranial magnetic stimulation (TMS) uses the physical principle of electromagnetic induction. A tangentially applied to the skull magnet coil generates a magnetic field short of 200 to 600 microseconds duration with a magnetic flux density of up to 3 Tesla. By doing so the electrical potential change in the skull near cortex causes a depolarization of neurons with triggering action potentials. The strength of this electric field coincides with the distance from the coil in a first approximation from exponential and depends upon the characteristics of the capacitor current and the coil. The current in the coil reaches more than 15 000 amperes. Be used so-called round coil and double coil. The latter consists of two circular coils each touch on the edge or overlap. Characterized the magnetic field of both partial coils is superimposed in the center part of the coil and therefore increased. Double coils are called because of their shape as eight or butterfly coil.

Are in electrical engineering at common magnetic stimulation generally distinguished from monophasic, biphasic circuits. A resonant circuit is powered by a powerful capacitor and a strong current compatible rectifier switch (thyristor) closed. After half an oscillation, the current direction of the resonant circuit reverses (the current "strike back"). In the single-phase circuit, the capacitor will change after a quarter of vibration its polarity, and therefore can not be charged back by the vibrating stream. Instead, the current vibration is intercepted via a rectifying diode and an electrical resistance and exponentially reduced. In the biphasic circuit, however, the capacitor from the back oscillating current is recharged via a rectifying diode on submaximal capacity and switched off after a half oscillation. Therefore, in the coil results in the single-phase circuit, an exponentially decaying current in the circuit, a biphasic current is similar to a damped sine full oscillation.


The magnetic stimulation causes the brain to trigger action potentials. The exact mechanism 1985 is still not understood despite intensive research since the introduction of the method in detail.

From a certain magnetic field strength, a sufficiently strong electric field is generated in the skull near the cerebral cortex to depolarize neurons. This depolarization is most likely to take place on axon. Runs the induced electric field in the running direction of the axon, the magnetic field strength required is the smallest. The magnetic field strength, which is currently required to cause an effect on the neuron, called in neurophysiology excitation threshold. Nerve endings, branchings and bends especially have a particularly low arousal threshold.


Transcranial magnetic stimulation is used in neuroscience research in neurology and psychiatry. Of scientific interest is mainly short-term disturbance of a small region of the brain, to investigate their physiological function. So you can trigger with the magnetic stimulation over the motor cortex muscle twitching, over the visual cortex can phosphenes, but also produce scotomas. The rTMS of brain regions that are responsible for language may lead for a few minutes to the deterioration of verbal skills of the participants.

Clinical applications are limited mostly to single pulses to the motor cortex or repetitive stimulation:

-The triggering of muscle contractions by stimulation of the motor cortex is used diagnostically in neurology. It leads to electrical potentials (motor evoked potentials; MEP), which are relatively easy to derive with electrodes. Of the brain and spinal cord Certain diseases such as multiple sclerosis cause changes in MEP, which is therefore an important diagnostic support. Also of diagnostic interest is the change of thresholds in various neurological disorders such as migraine or epilepsy. Also the use of psychotropic drugs or drug leads to changes in the threshold that can be measured with the transcranial magnetic stimulation.

-The rTMS can induce a tolerance (habituation) to the stimulation, which may result in the stimulated region to a longer-term change in the activity of the cerebral cortex. So you can deteriorate the mobility of volunteers through TMS of the motor cortex for a few minutes. May also alter the activity of the prefrontal cortex, what one is trying to use in the treatment of depression in psychiatry. The antidepressant effect should persist in patients treated for a few days, but is not sufficiently scientifically. In contrast to electroconvulsive therapy (ECT), no double-blind study in which rTMS on reasonable grounds possible: For adjustment you take (depending on the study) 100 - 110% of the flux density, the precentral the threshold over the primary motor cortex to the dentate flexion of the little finger corresponds (motor threshold). With rTMS trying - without the risks of ECT - treatment-refractory depression with a frequency of 10 Hz (corresponding to the alpha rhythm of brain waves in the relaxed state) in various "trains" (sequences) to deal with different number of sessions. In the case of schizophrenia using a stimulation frequency of about 1 Hz.

In scientific research, the range of applications is greater.

A principal problem in the stimulation by TMS represents the spatial resolution. It is unclear to what extent connected regions are stimulated by the stimulation of a target region. Thus, it is difficult to draw conclusions about the exclusive role of a stimulated brain area. Another problem arises from the fact that TMS stimuli in terms of their intensity are not yet standardized. This is partly due to the poor replicable magnetic field strength, which depends not only on the respective coil, but also on the stimulator connected to it and in addition, the combination of the two. On the other hand, the standardization of stimulation by means of the above-mentioned relation to motor threshold is questionable, since this limit does not show any correlation within the same head in other brain regions. One does not know how much a particular area was stimulated, even if the motor threshold is specified as a reference. In the application of the stimulation protocols outlined below, there are often conflicting results, which can vary from study to study, as well as from subject to subject. The complex structures of the brain thought to be affected by different protocols in many ways, so that precise statements about the effectiveness of individual protocols so far are not possible:

-Using single pulses can be influenced temporally well-defined and controlled areas of the brain. This allows certain processing steps (eg in the visual system) to interfere directly and thus time to determine these processing steps (relative to stimulus presentation) exactly. The disadvantage of the single pulse is its low energy so often be disturbed only very weak stimuli in their processing or disorder is very small.
-With a double pulse (paired pulse), much of the temporal precision is maintained, the impact on the neural processing is much greater.
-Tetanic stimulation (theta-burst stimulation) has proved in the past because of the capability of long-term potentiation be useful to improve neural connections in their strength. Tetanic stimulation consists of several short bursts (of 50-100 Hz for 100-1000 ms) defined by a longer time interval (seconds) are separated from each other in time. Brain regions are thought to be part of a network if, after tetanic stimulation greater synchronicity of their activity than before.
-Repetitive stimulation (rTMS) is used similarly in research, such as in clinical use.
Another possibility, which in turn may consist of any of the listed applications is the simultaneous stimulation of different brain areas with two or more coils in order to study the influence of the areas of each other or their role in a network accurately.

Transcranial magnetic stimulation side effects

Subjects and patients who are facing a transcranial magnetic stimulation should appeal to their doctor the risks and side effects. The risks and side effects described here can only provide an overview. The doctor will need to decide in each case whether a person in the TMS is suitable or not.

Since the introduction of magnetic stimulation 1985 hardly any side effects have been observed. The most common side effects are transient headache, occurring mainly in Mitstimulation of muscles. The most serious disease is very rare trigger an epileptic seizure during rTMS. Why were developed strict application of rules for the TMS in a consensus in 1998. Newer protocols with stronger effect as the tethane stimulation are not yet reflected in this consensus, thus reducing risks of this stimulation have been virtually incalculable.

More especially rare, side effects must be found by the more careful and long-term monitoring during and after application of transcranial magnetic stimulation (TMS) in research and clinical practice. For this reason, this list might not be complete.

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