by Dr Sean David

In the 1800’s, world renown English physicist Michael Faraday discovered the principles of electro-magnetic induction. Fast forward to the 21^st century, Faraday’s discovery was harnessed into the clinical practice of transcranial magnetic stimulation (TMS) for the treatment of mood disorders. Based on Faraday’s Law, TMS is able to stimulate brain neuronal circuits with tiny electrical currents induced by a changing magnetic field.

In Singapore, the practical application of TMS is employed in the form of repetitive transcranial magnetic stimulation (rTMS). In rTMS, magnetic pulses are delivered in trains at certain frequencies. “Fast” (high frequency e.g. 10Hz) stimulation increases cortical excitability for treatment of depression. Whereas “slow” (low frequency e.g. 1Hz) stimulation reduces cortical excitability for treatment of anxiety disorders.

Furthermore, TMS can be targeted at focused regions of the cortex for superior precision treatment of specific conditions e.g. rTMS at 1Hz to the right orbitofrontal cortex (OFC) reduces intrusive obsessions in obsessive-compulsive disorder (OCD).

Mood disturbances such as depression are increasingly understood as disorders of connectivity in neural networks linking cortical and subcortical grey structures of the brain. Functional brain imaging has shown dysfunction in cortical regions such as the dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC), as well as deep grey matter structures including the amygdala, nucleus accumbens, hippocampus and hypothalamus.

These brain circuits are pivotal for executive functioning, regulation of emotions, reward processing and preservation of memory and cognition. They also link the nervous system to the endocrine system, which mediates the body’s response to stress.

Evidence suggests that TMS induces neuroplastic changes in these circuits. Neuroplasticity is the ability of the brain to reorganize itself by forming new neural connections. TMS helps readjust neurotransmitter (e.g. serotonin and dopamine) levels in a variety of brain regions. TMS also appears to exert a neuroprotective effect on the brain.

Research has shown that TMS decreases brain inflammatory factors reducing oxidative stress on the brain. TMS also boosts the levels of brain-derived neurotrophic factor (BDNF), encouraging neuronal growth in regions such as the hippocampus which is vital for learning and memory. It is postulated that the anti-depressant properties of TMS may also help in normalizing the body’s neuroendocrine stress response system.

rTMS has achieved its place on international treatment guidelines as an augmentation treatment modality to be strongly considered in treatment resistant depression. It is reported that 30-40% of depressed patients may have inadequate response to anti-depressant medication treatment. The direct neuronal effects of rTMS may explain why rTMS may work for this group of patients.

In May 2022, the U.S. Food and Drug Administration (FDA) approved the use of the NeuroStar TMS system as an adjunct for treating adult patients suffering from OCD. Promising research is ongoing for the clinical application of TMS in treatment of Post-Traumatic Stress Disorder (PTSD), addictions, chronic pain, insomnia and many other neurological disorders.

With further advancements in TMS research and the incorporation of TMS in routine clinical practice, there is strong hope for recovery and regaining of optimal functioning for patients afflicted by complex neuropsychiatric conditions.

References

1. Transcranial Magnetic Stimulation, Clinical Applications for Psychiatric Practice. 2018 American Psychiatric Association Publishing, First Edition.
2. The Science of Transcranial Magnetic Stimulation, William M. Sauvé, MD; and Lawrence J. Crowther, Meng. Psychiatric Annals, Vol44, No.6, 2014.
3. Repetitive transcranial magnetic stimulation increases serum brain-derived neurotrophic factor and decreases interleukin-1b and tumor necrosis factor-a in elderly patients with refractory depression. Xiangxiang Zhao, Yanpeng Li, Qing Tian, Bingqian Zhu and Zhongxin Zhao. Journal of International Medical Research 2019, Vol. 47(5) 1848–1855.
4. What is repetitive transcranial magnetic stimulation and how does it actually work? Paul Fitzgerald, Professor of Psychiatry, Monash University. The Conversation AU, published May 13 2021.