Transcranial Magnetic Stimulation and Its Use in Clinical and Basic Neuropsychiatry
Eduardo Gonçalves
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The possibility of " focal " stimulation of the brain is realisable, now, noninvasively.
Transcranial Magnetic Stimulation ( TMS ) is a tool to be used to study function's localisation, connectivity between brain's regions and pathophysiology of neurological and psychiatric disorders. Also, TMS holds therapeutical potential in the field of Neuropsychiatry. This paper reviews the basic physical principles that underlies TMS [ TMS has been referred as " lectrodeless" electrical stimulation, i.e., the Magnetic Field ( B ) acts as the medium between Electrical Current in the coil of the Magnetic Stimulator ( MS ) ( i' ) and the ( subsequent ) Induced Electrical ( Electro-ionic ) Current in the brain ( i'' ) ]. The TMS procedure consists in placing a Coil ( CO ) on the scalp; through this passes an Electrical Current ( i' ) that has been discharged from the Capacitors ( CAs ) of the MS. An automatic switch ( a thyristor ) turns on and off the circuit of the MS ( which includes the CO and the CAs ); then, it
is produced a time-varying B that lasts about 100-200 miliSecond. The "strength" of B is typically 2 Tesla ( i.e., at the same order of magnitude of the static B provided by clinical MRI ). Underneath the CO there is a neural tissue, provided with electroconductive properties, resulting, at it, an i''. This i'' causes Neural Depolarization ( ND ). Unlike Transcranial Electrical Stimulation ( TES )[ and Electroconvulsivetherapy ( ECT ) - sometimes, considered a life-saving
therapeutically procedure in some types of Depresssive Disorder - is an example of TES ] - where the skull behaves as a resistor -, with TMS, B isn't deflected or attenuated by the intervening tissue ( skin, muscle, bone ). This fact correlates with the better "focality" of TMS ( in comparison with TES ). In a therapeutically perspective, and unlike ECT, TMS doesn't require the use of anaesthesia neither the ( required ?) produced, by ECT, seizure to achieve its therapeutically goal, for instances, in clinical disorders as Depression.
In this communication, it will be reviewed, also, some clinical indications of TMS, other than Depression: Mania, Ansiety Disorders, Schizophrenia, Movement
Disorders, Epilepsy. It will be discussed its mechanisms of action, based in animal models ( of diseases ). It will be reminded some basic safety aspects concerning theuse of this technique.
Recently, more precisely, at the 06/Mar/2001, the Canadian Health Authorities recognised the therapeutically value of TMS, and decided to approve its clinical use in the treatment of Depression, with the following indications: treatment resistance, including resistance to ECT; and, age of 18-64 years old.

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Eletromagnetismo

Bioeletricidade
em Psiquiatría