The Science of Transcranial Magnetic Stimulation (TMS)
History of Transcranial Magnetic Stimulation (TMS)
TMS was first developed in 1985 by Barker and his collegues in Sheffield. Here is a picture of the first machine.
Subsequently, researchers came up with modalities to deliver multiple pulses in a short interval which came to be known as repetitive TMS (rTMS)
TMS has been FDA approved since 2008 for Major Depression
It has been in Australia for about 8 years, but only in larger private mental health facilities.
The major change now is that Medicare has listed TMS for use in outpatient settings. This change came into effect 1 month ago after years of pressure on Medicare due to clear evidence of TMS’s effectiveness in treating Depression and other mental health conditions.
The Science of TMS
TMS uses an alternating current passed through a metal coil placed against the scalp to generate rapidly alternating magnetic fields.
This passes through the skull nearly unimpeded and induce electric currents that depolarize neurons in a focal area of the surface cortex
The magnetic field generated by TMS is comparable to that of a standard magnetic resonance imaging (MRI) device (approximately 1.5 to 3 Tesla); however, the TMS field is very focal (beneath the coil).
TMS stimulates regions of the brain that are underactive in patients with depression, anxiety disorder and chronic pain. Specifically the Dorsolateral Prefrontal Cortex with respect to depression and anxiety.
During TMS treatment, magnetic s fields timulate neural activity in the underactive areas. When this is done tens of thousands of times over a course of 4-6 weeks, lasting changes in brain circuity occur.
The exact mechanisms of action for TMS is unknown. One hypothesis is that stimulation of discrete cortical regions alters pathologic activity within a network of grey matter brain regions that are involved in mood regulation and connected to the targeted cortical sites.
Both ECT and TMS have been shown to increased monoamine activity (seritonin in particular) and assist in the normalisation of the Hypothalamic pituitary axis.
The effect on TMS varies based on a number of factors. These include whether a high frequency of magnetic stimulation is used. When this occurs the targeted neurons are stimulated. We generally used high frequency stimulation when target the left prefrontal cortex in the treatment for Depression.
In contrast low frequency stimulation appears to inhibit cortical activity (and is usually directed at the right prefrontal cortex). Inhibitory stimulation to the Right DLFPC is an alternative treatment used in depression. This can be useful in patients who are at a high risk of seizure (Eg. depending on their medication). Inhibitory stimulations greatly reduce any risk of seizure.
In a systematic review by Noda et al, the review examined 66 studies in depressed patients who were treated with TMS targeting the dorsal lateral prefrontal cortex and found that high frequency TMS generally increased regional cerebral blood flow and that low frequency TMS generally decreased regional cerebral blood flow
The intensity of the treatment is the other main parameter we can change to ensure efficiacy.