Transcranial magnetic stimulation (TMS) is a noninvasive form of brain stimulation in which a changing magnetic field is used to cause electric current at a specific area of the brain through electromagnetic induction.
An electric pulse generator, or stimulator, is connected to a magnetic coil, which in turn is connected to the scalp. The stimulator generates a changing electric current within the coil which induces a magnetic field; this field then causes a second inductance of inverted electric charge within the brain itself.
TMS has shown diagnostic and therapeutic potential in the central nervous system with a wide variety of disease states in neurology and mental health, with research still evolving.
Adverse effects of TMS are rare, and include fainting and seizure. Other potential issues include discomfort, pain, hypomania, cognitive change, hearing loss, and inadvertent current induction in implanted devices such as pacemakers or defibrillators.
TMS is non-invasive, and does not require surgery or electrode implantation. Its use can be divided into diagnostic and therapeutic applications.TMS can be used clinically to measure activity and function of specific brain circuits in humans, most commonly with single or paired magnetic pulses.
The most widely accepted use is in measuring the connection between the primary motor cortex of the central nervous system and the peripheral nervous system to evaluate damage related to past or progressive neurologic insult.
Repetitive high frequency TMS (rTMS) has shown diagnostic and therapeutic potential with the central nervous system in a wide variety of disease states, particularly in the fields of neurology and mental health, with new studies continually emerging.
During the procedure, a magnetic coil is positioned at the head of the person receiving the treatment using anatomical landmarks on the skull, in particular the inion and nasion. The coil is then connected to a pulse generator, or stimulator, that delivers electric current to the coil.
Although TMS is generally regarded as safe, risks are increased for therapeutic rTMS compared to single or paired diagnostic TMS. Adverse effects generally increase with higher frequency stimulation The greatest immediate risk is fainting, though this is uncommon. Seizures have been reported, but are extremely rare. Other adverse effects include short term discomfort, pain, brief episodes of hypomania, cognitive change, hearing loss, impaired working memory, and the induction of electrical currents in implanted devices such as cardiac pacemakers and defibrillators.
TMS uses electromagnetic induction to generate an electric current across the scalp and skull. A plastic-enclosed coil of wire is held next to the skull and when activated, produces a magnetic field oriented orthogonal to the plane of the coil. The magnetic field can then be directed to induce an inverted electric current in the brain that activates nearby nerve cells in a manner similar to a current applied superficially at the cortical surface.
The magnetic field is about the same strength as an MRI, and the pulse generally reaches no more than 5 centimeters into the brain, unless using a modified coil and technique for deeper stimulation.
This electric field causes a change in the transmembrane current of the neuron, which leads to the depolarization or hyperpolarization of the neuron and the firing of an action potential.
Deep TMS can reach up to 6 cm into the brain to stimulate deeper layers of the motor cortex, such as that which controls leg motion. The path of this current can be difficult to model because the brain is irregularly shaped with variable internal density and water content, leading to a nonuniform magnetic field strength and conduction throughout its tissues.
Luigi Galvani undertook research on the effects of electricity on the body in the late 1700s, and laid the foundations for the field of electrophysiology. In the 1800s Michael Faraday discovered that an electrical current had a corresponding magnetic field, and that changing one could induce its counterpart.
Work to directly stimulate the human brain with electricity started in the late 1800s, and by the 1930s electroconvulsive therapy (ECT) had been developed by Italian physicians Cerletti and Bini. ECT became widely used to treat mental illness, and ultimately overused, as it began to be seen as a panacea. This led to an ensuing backlash in the 1970s.
Around that time, Anthony T. Barker began exploring use of magnetic fields to alter electrical signaling within the brain, and the first stable TMS devices were developed in 1985.They were originally intended as diagnostic and research devices, with evaluation of their therapeutic potential being a later development. The first TMS devices were approved by the FDA (USA) in October 2008.