Magnetic stimulators became commercially available in Sheffield (United Kingdom) in 1985. Transcranial magnetic stimulation (TMS) is currently an experimental technique and does not have an approved psychiatric indication. TMS is achieved by conducting a large current through a coil that is placed on the patient's scalp. A magnetic field is induced that passes freely though the skull and induces an electrical field in the cerebral cortex underlying the stimulating coil. TMS has been shown to preferentially activate the cortical interneurons, as opposed to the motor neurons of the cortical-spinal tract, due to the interneurons' orientation parallel to the scalp.
Activation or inhibition of the cortex has been shown to vary with the frequency of the magnetic pulses. A 20-Hz stimulation at the motor threshold (MT) over the left prefrontal cortex of depressed patients was shown to increase the perfusion of the prefrontal cortex (L > R) as well as the cingulate gyrus and left amygdala. A 1-Hz stimulation was only associated with decreases in rCBF (Speer, 2000). The intensity of the magnetic stimulation has also been shown to affect the pattern of activation. Repetitive TMS at 120 percent MT over the left prefrontal cortex produced greater local and contralateral activation than stimulation at 80 percent MT (Nahas, 2001). A negative correlation between the severity of negative symptoms in major depression and rCBF to the left dorsal-lateral prefrontal cortex has been reported (Galynker et al., 1998). Both converging lines of evidence support a hypofunction in the left prefrontal cortex in major depression that may be modified by rTMS and tentatively explain part of its antidepressant effect.
Repetitive TMS was first shown to be beneficial in the treatment of depression in a study with daily stimulation over the left prefrontal cortex at 20 Hz and 80 percent MT (George et al., 1995). Five meta-analyses of rTMS provide evidence for a beneficial acute antidepressant effect compared to placebo (see Chapter 17 for details). Attention to the stimulus parameters of frequency, intensity, and duration is indicated to avoid inducing seizures during rTMS (Wasserman, 1997).
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