Transcranial Magnetic Stimulation


Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain.  TMS uses electromagnetic induction to induce weak electric currents using a rapidly changing magnetic field; this can cause activity in specific or general parts of the brain with minimal discomfort, allowing for study of the brain’s functioning and interconnections.

A variant of TMS, repetitive transcranial magnetic stimulation (rTMS) has been tested as a treatment tool for various neurological and psychiatric disorders.

The exact details of how TMS functions are still being explored. The effects of TMS can be divided into two types depending on the mode of stimulation:

  • Single or paired pulse TMS causes neurons in the neocortex under the site of stimulation to depolarize and discharge anaction potential.  If used in the primary motor cortex, it produces muscle activity referred to as a motor evoked potential (MEP) which can be recorded on electromyography.  If used on the occipital cortex, ‘phosphenes’ (flashes of light) might be perceived by the subject.  In most other areas of the cortex, the participant does not consciously experience any effect, but his or her behaviour may be slightly altered (e.g., slower reaction time on a cognitive task), or changes in brain activity may be detected using sensing equipment.
  • Repetitive TMS produces longer-lasting effects which persist past the initial period of stimulation. rTMS can increase or decrease the excitability of the corticospinal tract depending on the intensity of stimulation, coil orientation, and frequency.  The mechanism of these effects is not clear, though it is widely believed to reflect changes in synaptic efficacy akin to long-term potentiation (LTP) and long-term depression (LTD).

TMS is generally regarded as safe, although seizures have been reported in some cases.  There have been 16 reports of TMS-related seizures (as of 2009), with seven reported before the publication of safety guidelines in 1998, and nine reported afterwards.  The seizures have been associated with both single-pulse and rTMS.  Reports have stated that in at least some cases, predisposing factors (medication, brain lesions or genetic susceptibility) may have contributed to the seizure.  A review of nine seizures associated with rTMS that had been reported after 1998 stated that four seizures were within the safety parameters, four were outside of those parameters, and one had occurred in a healthy volunteer with no predisposing factors.

Besides seizures, other risks include syncope (fainting), minor pains such as headache or local discomfort, minor cognitive changes, and psychiatric symptoms (particularly a low risk of mania in depressed patients).

Publications reporting the results of reviews and statistical meta-analyses of earlier investigations have stated that rTMS appeared to be effective in the treatment of certain types of major depression under certain specific conditions.

This is comparable to commonly reported effect sizes of pharmacotherapeutic strategies for treatment of depression in the range of 0.17-0.46.  However, that same meta-analysis found that rTMS was significantly worse than electroconvulsive therapy (ECT) (effect size = -0.47), although side effects were significantly better with rTMS.  An analysis of one of the studies included in the meta-analysis showed that one extra remission from depression occurs for every 3 patients given electroconvulsive therapy rather than rTMS (number needed to treat 2.36).  There is evidence that rTMS can temporarily reduce chronic pain and change pain-related brain and nerve activity, and TMS has been used to predict the success of surgically implanted electrical brain stimulation for the treatment of pain.

TMS uses electromagnetic induction to generate an electric current across the scalp and skull without physical contact.  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 passes unimpeded through the skin and skull, inducing an oppositely directed current in the brain that activates nearby nerve cells in much the same way as currents applied directly to the cortical surface.

The path of this current is difficult to model because the brain is irregularly shaped and electricity and magnetism are not conducted uniformly throughout its tissues.  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 the deep transcranial magnetic stimulation variant of TMS.  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.

A number of different types of coils exist, each of which produce different magnetic field patterns. Some examples:

  • round coil: the original type of TMS coil
  • figure-eight coil (i.e., butterfly coil): results in a more focal pattern of activation
  • double-cone coil: conforms to shape of head, useful for deeper stimulation
  • four-leaf coil: for focal stimulation of peripheral nerves[56]
  • H-coil: for deep transcranial magnetic stimulation

transcranial magnetic stimulationTMS butterfly coil

Design variations in the shape of the TMS coils allow much deeper penetration of the brain than the standard depth of 1.5-2.5 cm. Circular crown coils, Hesed (or H-core) coils, double cone coils, and other experimental variations can induce excitation or inhibition of neurons deeper in the brain including activation of motor neurons for the cerebellum, legs and pelvic floor.  Though able to penetrate deeper in the brain, they are less able to produced a focused, localized response and are relatively non-focal.

Images and Quoted from Source: – Transcranial Magnetic Stimulation

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