Deep transcranial magnetic stimulation

Deep transcranial magnetic stimulation (Deep TMS) is a non-invasive technique that can induce increases or decreases in excitability of large populations of neurons in deep areas of the brain.^[1] Deep TMS is used for the treatment of drug-resistant major depressive disorder, without the need for hospitalization or anesthesia.^[2]

Medical uses

Depression

Deep TMS has shown promise as an add-on to antidepressants for the treatment of unipolar and bipolar depressive relapses in small studies, although it is not a mainstream therapy.^[3]

Traumatic brain injury (TBI)

Deep TMS has shown mixed results in the treatment of cognitive and motor deficits from traumatic brain injury. ^[4]

Stroke

Deep TMS has been used in stroke patients, where it has shown mixed results in improving motor symptoms.^[4]

Alzheimer's diseases

Deep TMS has shown promise in the treatment of Alzheimer's disease, although rigorous clinical trials are lacking.^[5]

Safety

Deep TMS is a relatively new technique and all experiments, for any disorder, also test the safety of the method as part of the approved experimental protocol. So far, Deep TMS has been shown to be generally safe and tolerable, presenting some minor side effects.^[2]

H-coil design principles

Deep TMS utilizes the basic principles of TMS but expands them from the use of a single focal stimulation source to a summation of several simultaneous operating elements.^[2] Deep TMS coils (termed H-coils) can overcome disadvantages of other coils, such as their inability to induce deep stimulation without over-stimulation of cortical regions and facial nerves.^[6] To achieve that:

• Coils include a flexible base shaped to fit the human head. The base, which is the part of the coil closest to the head, must be optimally fitted to the scalp at the desired region.
• The wires leading currents in a direction opposite to the preferred direction (the return paths) should be located far from the base and the desired brain region. This enables a higher absolute electric field in the desired brain region.
• The induced electric field in the desired deep brain region is obtained by optimal summation of electric fields, induced by several coil elements with common direction locations around the skull.
• Coils must be positioned to produce a considerable field in a direction tangential to the surface, which should also be the preferable direction to activate the relevant neuronal structures (considering the direction of the targeted neuronal bundles).^[2][6][7]

To affect different areas of the brain (depending on the targeted disorder), specific versions of H-coils are installed within a helmet placed on the subject's head. For example, the H1-coil induces greater stimulation over the left prefrontal cortex (PFC); the H2-coil induces bilateral stimulation over the PFC; and the Haut-coil induces bilateral stimulation over the medial PFC.^[2]

Sham TMS coils

In sham-controlled clinical TMS studies, which test the placebo effect of the treatment, both patients and operators should be "blinded" to the type of stimulation (i.e., active or a sham treatment). However, it is difficult to perform rTMS treatment in a double blind design because the person delivering the TMS can easily become aware of the treatment condition.^[8] In the Deep TMS system used for clinical trials, the sham coil is placed inside the same helmet encasing the active TMS coil, and electronic system controls which of the two coils is connected to the stimulator in a certain session.^[6] The operation is performed by a magnetic card specific to each patient. This way both the patient and the operator may be unaware of the operation mode.^[6] The sham coil mimics sound and scalp sensations induced by the active coil without having any stimulating activity.^[2]

History

Deep TMS technology is a type of transcranial magnetic stimulation developed by Abraham Zangen and Yiftach Roth starting in 2001, and a patent was filed by the American National Institutes of Health.^[9] In January 2013, the United States Food and Drug Administration permitted the manufacturer to market a Deep TMS device that was indicated for the treatment of depressive episodes in adult patients suffering from major depressive disorder who failed to achieve satisfactory improvement from previous anti-depressant medication treatment in the current episode.^[10]

Deep TMS was tested for a variety of disorders, including, bipolar disorder, addiction, posttraumatic stress disorder, Parkinson's disease, and schizophrenia with a typical protocol consisting of 15–20 sessions, each lasting 15–20 minutes, over a course of three to four weeks.^[2]

References