Transcranial Magnetic Stimulation – from origin to treatment tool

Last update: May 15, 2023
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Reading time: 5 minutes
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By Brain Matters

Where it all started, from magnetism to electricity.
Let’s start with a brief story, Michael Faraday was raised in a poor farmer's family, his father being an apprentice farrier. As a 14-year-old Faraday himself acquired an apprenticeship with a bookbinder and bookseller. During his years in this apprenticeship, he read many books and developed an interest in science, with a focus on electricity. He would soon write notes about the books he read and started conducting scientific experiments himself. We now see Faraday as one of the key figures in the science of electricity; even Albert Einstein said he had a picture of Faraday on his study wall.

Faraday discovered that we can create electricity through magnetism, this is where Faraday’s law of induction comes in. The principle of this law is that running a current through loops of metal wire (such as a magnetic coil), will create a temporary magnetic field, which in turn induces an electrical field. This principle is the working mechanism used for among other things  the induction cooker, the pickup elements in electrical guitars and Transcranial Magnetic Stimulation (TMS). For TMS the coil(s) are placed over the scalp, causing the electromagnetic pulse generated to travel through the skull into the brain. In our brain, the electrical field created by the pulse leads to an action potential. To put this into context, say that you want to move your index finger, a certain area in your motor cortex is responsible for this finger movement when you want to move it voluntarily. We can now position the TMS coil over this area and by firing a strong enough pulse, create an action potential that will lead to your index finger tapping once. 

From cool finger tapping toy to treatment
Alright, very neat, we can make you tap your finger by activating the coil. But how come TMS is now seen as a very potential tool for patient treatment of various disorders? This is because research has shown that using TMS repetitively with set parameters can induce certain beneficial, long-lasting effects on the brain. Firstly, we need to get back to the finger action, since there is a method by which the TMS user can see direct feedback from the given pulse. If the pulse strength is too low, there will be no finger movement. The larger the pulse strength, the bigger the movement and the more likely that other close areas are also activated as well, causing movements in for example other fingers, the wrist, or the elbow. What we want to do now is find the threshold, the intensity with which a pulse will cause a movement of the finger 50% of the time. With this threshold we can then calculate the intensity we want to use for treatment. This is because when we use TMS for treatment, we often stimulate a brain area which does not give a directly observable response (such as more frontal lying areas related to cognition).

While the pulses above threshold will cause neuronal activation in the brain, receiving multiple pulses in a short amount of time will have an effect depending on the parameters. For example, receiving 10 Hertz (Hz) stimulation (meaning 10 pulses per second) will cause an excitatory effect in the targeted area, while receiving 1 Hz stimulation will cause an inhibitory effect in the area. Receiving multiple pulses like this  is called repetitive TMS (rTMS). The effects generally increase with more pulses being delivered and more sessions being conducted. Repeated excitatory or inhibitory effects of rTMS can change the plasticity of the brain and create a lasting effect.

In the Netherlands, rTMS is covered by health insurance as treatment for depression if 2 other treatments (either therapy or medication) haven’t had enough effect. To give an indication of the effectivity of rTMS treatment for depression, a study by Carpenter and colleagues (2012) found that patients suffering from Major Depressive Disorder and who had persisting symptoms despite a pharmacological antidepressant intervention responded to the rTMS (responding means a significant reduction in depression symptoms) in 41.5-56.4% of the cases and had remission of symptoms in 26.5-28.7% of the cases meaning they no longer met the criteria for diagnosis. These response and remission rates were based on patient reports, the rates reported by their clinicians were higher. The effects of treatment were relatively stable over a 12-month period. I think this study gives a clear example of how impactful rTMS can be. Achieving these results in a group that did not respond to antidepressant treatment is more than encouraging.  

While depression is currently the only disorder for which rTMS treatment is covered by health insurance, its potential for treating other disorders is big. Currently there is ongoing research on using it to treat obsessive compulsive disorder, craving symptoms due to addiction, PTSD or to use it after stroke to improve recovery; and there are many promising findings. I think it’s a matter of finding the right parameters and predictors for treatment efficacy and then it can be used in a much wider variety of symptoms. If you’re thinking this all sounds wonderful, but electricity reminds me of electroconvulsive therapy (ECT), please do read on!

The safety & side effects of rTMS
I often hear people who are not familiar with TMS mention ECT or they compare it to that. I think it’s important to emphasize that TMS and ECT are nothing alike. While ECT is endured under anesthesia, rTMS is not painful and sessions are generally perceived as harmless and painless. I often compare the feeling of a pulse on the scalp to a soft tick of a finger or pen. rTMS is a non-invasive treatment method, it’s safe and the side-effects are relatively small when compared to those of medication. The most severe side-effect is the occurrence of a seizure, which is almost impossible with the right screening before treatment, one of the factors which might warrant against usage of rTMS is a (family) history of epilepsy. 

Finishing note
I work with TMS myself and see it as a promising tool for treating a variety of disorders. There is still much to be discovered but there are many researchers and institutions working on the road ahead. The specificity TMS can have makes it a strong (potential) alternative for medication usage which often influences the whole body in some proportion. While I think it can be an alternative in many disorders, I also think the combination of rTMS with cognitive behavioral therapy or pharmacotherapy shouldn’tbe underrated. While I think this now covers the basics of TMS, having a visual impression or real-life experience can really help to get an idea of it. I want to finish with a list of some videos I recommend which might interest you:

The Potential Side Effects of rTMS | Transcranial Magnetic Stimulation (TMS) - Dr.Martjin Arns: https://www.youtube.com/watch?v=2ON6KM8AHK8 

An interesting video to showcase how strong the effects of TMS can be (more research oriented). Michael Mosley has areas of his brain turned off - The Brain: A Secret History - BBC Four: https://www.youtube.com/watch?v=FMR_T0mM7Pc

Simple and visual explanation video of TMS: https://www.youtube.com/watch?v=pfy0t5Yapco 

Author: Kobus Lampe
References:

  • Carpenter, L. L., Janicak, P. G., Aaronson, S. T., Boyadjis, T., Brock, D. G., Cook, I. A., Dunner, D. L., Lanocha, K., Solvason, H. B., & Demitrack, M. A. (2012). Transcranial magnetic stimulation (TMS) for major depression: a multisite, naturalistic, observational study of acute treatment outcomes in clinical practice. Depression and anxiety, 29(7), 587–596. https://doi.org/10.1002/da.21969 
  • Michael Faraday Wikipedia page. (2022, November 17). In Wikipedia. https://nl.wikipedia.org/wiki/Michael_Faraday
  • Rossi, S., Antal, A., Bestmann, S., Bikson, M., Brewer, C., Brockmöller, J., ... & Hallett, M. (2021). Safety and recommendations for TMS use in healthy subjects and patient populations, with updates on training, ethical and regulatory issues: Expert Guidelines. Clinical Neurophysiology, 132(1), 269-306.
  • Schwippel, T., Schroeder, P. A., Fallgatter, A. J., & Plewnia, C. (2019). Clinical review: The therapeutic use of theta-burst stimulation in mental disorders and tinnitus. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 92, 285-300.
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