Transcranial Magnetic Stimulation (TMS) as an Effective Treatment for Depression
Patients who have not responded to one or more antidepressant medications or have experienced adverse side effects from these medications may be good candidates for TMS. However, due to the lower effectiveness of TMS, for urgent or severely resistant cases, Electroconvulsive Therapy (ECT) remains the preferred treatment.
Definition
TMS involves the application of a rapidly changing magnetic field to the superficial layers of the cerebral cortex, which locally induces small electric currents known as eddy currents. This phenomenon was first discovered by Faraday in 1831 and later explained through Maxwell’s equations of electromagnetism. Essentially, TMS can be seen as electrical stimulation without an electrode, using magnetic fields to indirectly create electrical pulses. TMS devices deliver strong magnetic pulses via a coil placed on the scalp. Because magnetic fields are not affected by the electrical resistance of the scalp and skull, TMS allows for more precise stimulation of smaller brain areas compared to other noninvasive devices that use either alternating (ECT, CES) or direct (tDCS) electrical currents for primary stimulation. TMS is a form of noninvasive stimulation targeting specific brain regions and can be used for both research and therapeutic purposes without the need for anesthesia.
Mechanisms of Action
At sufficient intensity, the electrical currents generated by TMS can stimulate neuronal depolarization, leading to an action potential. For instance, when the TMS coil is placed over the motor strip area of the cerebral cortex, the changing magnetic field created by repetitive pulses induces local currents that cause the neurons in the motor area (M1) to fire. This action potential then travels through the polysynaptic corticospinal tract, resulting in a twitch in the muscle of the opposite hand. In summary, TMS uses magnetic fields to induce localized electrical currents in the brain, triggering the activation of neuronal circuits that can lead to observable behavioral effects. This can be demonstrated by using single TMS pulses to map the homunculus by moving the TMS coil across the cortical areas representing different muscle groups, simultaneously studying the excitability of the corticospinal system.
Side Effects, Interactions with Medications, and Other Risks
TMS is a noninvasive and generally safe procedure when conducted by a trained professional on a properly evaluated patient. However, it is not entirely without risks. In rare cases, it may cause an unintended seizure. Several factors can increase this risk, including the type of TMS, with single-pulse TMS being less likely to induce a seizure than repetitive TMS (rTMS). The dosage also plays a significant role, involving parameters such as frequency, power, train duration, and intertrain interval.
Additionally, patient-specific factors can be important. These include having a neurological disorder, such as epilepsy or a brain lesion, or using medications that lower the seizure threshold. Single-pulse TMS is generally considered low-risk for appropriately screened adults without seizure risk factors.
