Transcranial Magnetic Stimulation (TMS) for Depression – Review of the Evidence
Major depressive disorder (MDD) is a chronic and debilitating form of mental illness that is characterised by behavioural, affective, cognitive and somatic symptoms.
Research shows that the functional neuroanatomy of depression implicates the ventromedial and dorsolateral divisions of the prefrontal cortex (PFC) amongst other deeper brain regions. 
Several lines of compelling evidence implicate these brain regions including data from imaging studies that show connections to the ventromedial PFC (VMPFC) from the hypothalamus and the ventral striatum. These connections mediate activity associated with emotions, and motivation and reward, respectively. The VMPFC also receives projections from the amygdala, which is associated with threat detection and fear conditioning.
On the other hand, dysfunction in the dorsolateral PFC (DLPFC) is also related to depression. This area receives sensory projections from the eyes as well as also being involved in cognitive or executive functioning associated with working memory, goal-orientated actions and abstract reasoning. Together, dysfunction in the VMPFC and DLPFC play important neurocognitive and neurobehavioural roles in depression. (Read more about the differences between bipolar and unipolar depression)
Antidepressants are considered first line for the treatment of depression, however, even with modern generation agents a substantial proportion of patients fail to respond to first line and subsequent trials of therapy: it is usually estimated that ~ 10-20% of patients have minimal response to multiple medication trials. In the STAR*D trial which was the largest and longest depression treatment trial, there were 4 levels of study that tested a different medication or medication combination. The study found that while about 50% became symptom-free after 2 levels; after 4 levels 30% of patients did not reach remission. 
The consensus for defining treatment-resistant depression (TRD) is non-response to at least two different antidepressants from different classes.
Transcranial magnetic stimulation (TMS) has emerged as a novel neuromodulation technique in the management of depression. In this article, we review the evidence for TMS in depression.
Kan RLD, Padberg F, Giron CG, Lin TTZ, Zhang BBB, Brunoni AR, Kranz GS. Effects of repetitive transcranial magnetic stimulation of the left dorsolateral prefrontal cortex on symptom domains in neuropsychiatric disorders: a systematic review and cross-diagnostic meta-analysis. Lancet Psychiatry. 2023 Apr;10(4):252-259