Repetitive transcranial magnetic stimulation (TMS) is a useful clinical tool that is effective in patients with treatment-resistant depression. This noninvasive treatment is an option for patients with depression who have not found relief from other treatments, such as psychotherapy and antidepressants.

TMS technology was developed in 1985 and has been gaining clinical interest since then. Two-thirds of TMS patients experienced either full remission of their depression symptoms or noticeable improvements.¹ An out-patient procedure, TMS does not have serious side effects.

Mechanism of Action

Approved by the US Food and Drug Administration in 2008, TMS uses an alternating current passed through a metal coil placed against the scalp to generate rapidly alternating magnetic fields. These pass through the skull nearly unimpeded and induce electric currents that depolarize neurons in a focal area of the surface cortex.

The magnetic field generated by TMS is comparable to that of a standard magnetic resonance imaging device (MRI), measured at approximately 1.5 to 3 Teslas. However, the TMS field is focal (beneath the coil), whereas the MRI field is large and fills the room housing the MRI device.

One hypothesis on how TMS works is that the stimulation of discrete cortical regions alters pathologic activity within a network of gray matter brain regions, specifically those involved in mood regulation and connected to the targeted cortical sites.² Functional imaging studies support this hypothesis by showing TMS can change activity in brain regions remote from the site of stimulation.^3,4

TMS has many molecular effects comparable to electroconvulsive therapy (ECT), including increased monoamine turnover and normalization of the hypothalamic pituitary axis.⁵ Additionally, in one neuroimaging study of depressed patients, a prefrontal serotonin deficiency at baseline normalized after treatment with TMS.

High-frequency stimulation is thought to excite the targeted neurons and is typically used to activate the left prefrontal cortex. Low-frequency stimulation appears to inhibit cortical activity and is usually directed at the right prefrontal cortex.

Consistent with this hypothesis, a review examined 66 studies in depressed patients who were treated with TMS targeting the dorsolateral prefrontal cortex. It found that high-frequency TMS generally increased regional cerebral blood flow, whereas low-frequency TMS generally decreased regional cerebral blood flow, which is reduced in a depressed brain.⁶

Indications

TMS is indicated for patients with unipolar major depression who have failed at least 1 antidepressant medication. In addition, TMS is indicated for patients who responded to a prior course of TMS.⁷

Use of TMS for treatment-resistant or refractory depression is consistent with treatment guidelines from the American Psychiatric Association, Canadian Network for Mood and Anxiety Treatments, and the Royal Australian and New Zealand College of Psychiatrists.

Patient Assessment

When conducting a patient assessment for TMS, the purpose of the evaluation is to confirm the primary diagnosis of treatment-resistant depression and determine whether the TMS intervention can be used safely. The assessment includes examinations of psychiatric history, general medical history, physical health, and mental status with emphasis upon depressive symptoms. This should emphasize risk factors for seizures and preexisting neurologic disease, such as epilepsy, intracranial masses, and vascular abnormalities.