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Between 20-40% of people with depression do not recover following standard treatments such as medication and psychotherapy. Transcranial magnetic stimulation is a treatment used for adult patients with major clinical depression when antidepressants have repeatedly failed to control their symptoms. The American Psychiatric Association (2013) describes major depressive disorder as a medical illness that affects how you feel, think, and behave causing persistent feelings of sadness and loss of interest in previously enjoyed activities. It can lead to a variety of emotional and physical problems and usually requires long-term treatment.

Background

Transcranial magnetic stimulation (TMS) is a form of neuromodulation, a non-invasive, nonconvulsive technique used to stimulate neural tissue. The procedure involves placement of an electromagnetic coil to deliver a short powerful burst of magnetic field (1.5-2 Tesla) through the scalp to induce electric current in the brain. TMS is a new treatment for psychiatric disorders but as been well established in neuroscience research experiments and in clinical application for neurological conditions.  it has been used as an investigative tool in neuronal diseases.

Treatment

The stimulation can be delivered either as a single pulse or repetitive pulse, known as Repetitive Transcranial Magnetic Stimulation (rTMS). TMS treatment involves repetitive pulses applied in trains of stimulation over a target area over the cortical region of the brain. The response to the treatment is influenced by factors such as stimulus frequency, intensity and duration, and its ability to excite or inhibit particular neuron functions. The strength of the magnetic field for clinical application is similar to that of a standard Magnetic resonance imaging (MRI) scanner.

Putative mechanism of action

Like many other antidepressant treatments that are available in clinical practice, the exact mechanism by which rTMS produces relief from depression is still unknown. At a physiological level the effects of rTMS are frequently reported to be similar to long-term potentiation (LTP) or longterm depression (LTD) of stimulated neurons which in turn implicates changes in synaptic plasticity.

Synaptic plasticity is the ability of synapses to strengthen or weaken over time in response to increases or decreases in their activity. LTD and LTP are forms of long-term synaptic plasticity that apply to a long-lasting experience-dependent change in the efficacy of synaptic transmission (i.e. amplitude of synaptic potentials).

They have been studied as they represent cellular correlates of learning and memory. Invoking this mechanism to explain effects of rTMS is over simplistic and needs exploration. Studies indicate that large scale brain networks are changed in patients with depression, and the degree of the change in their connectivity predicts the severity of depression.

Repetitive stimulation of focal nodes of these networks can result in reorganisation of connectivity patterns in the brain due to the brain’s inherent plasticity (use-dependent strengthening of existing pathways). Research suggests that rTMS results in changes in regional brain activity and metabolism, and applying rTMS at the left dorsolateral prefrontal cortex (DLPFC) can enhance this region’s connectivity with other regions that are crucial for regulating emotional processing.