TMS for Depression

Depression disorder

The American Psychiatric Association describes major depressive disorder as ‘a medical illness which affects how you feel, think and behave; causing persistent feelings of sadness and loss of interest in previously enjoyed activities’.

Depression can lead to a range of associated problems – both emotional and physical – and usually requires long-term treatment. Unfortunately, approximately 30% of patients do not fully recover following traditional depression treatments (medication and/or psychotherapy).

Neuromodulation

TMS is a form of ‘neuromodulation’: a non-invasive, non-convulsive 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 a focused area of the brain.

TMS treatment

Transcranial Magnetic Stimulation treatment for depression involves trains of repetitive pulses (hence rTMS) which are applied over the cortical region of the brain. The strength of magnetic field in clinical use is comparable to a standard MRI scanner.

Patient response to the treatment is influenced by a number of factors including:

• Stimulus frequency
• Intensity
• Duration

Adjusting the frequency, intensity and duration allows rTMS to excite or inhibit particular neuron functions.

Putative mechanism of action

As depression is a complex clinical issue, the exact mechanism by which rTMS produces relief from depression is not fully understood – just like many other antidepressant treatments in use today.

At a physiological level, the effects of rTMS are often reported to be similar to long-term potentiation (LTP) or long term depression (LTD) of stimulated neurons. This in turn implicates changes in synaptic plasticity (the ability of synapses to strengthen or weaken over time in response to increases or decreases in their activity).

LTD and LTP are two 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 widely studied as they represent cellular correlates of learning and memory. Invoking this mechanism to explain effects of rTMS is over simplistic and requires further exploration. However, a number of studies indicate that large scale brain networks are indeed altered 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 reorganise established connectivity patterns in the brain, due to the brain’s inherent plasticity (use-dependent strengthening of existing pathways).

Research suggests that rTMS treatment can alter regional brain activity and metabolism. Applying rTMS at the left dorsolateral prefrontal cortex (DLPFC) can enhance the connectivity of this area with other regions which are crucial for regulating emotional processing.