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 little discomfort, allowing for study of the brain's functioning and interconnections. According to the United States National Institute of Mental Health, TMS "uses a magnet instead of an electrical current to activate the brain. An electromagnetic coil is held against the forehead and short electromagnetic pulses are administered through the coil. The magnetic pulse easily passes through the skull, and causes small electrical currents that stimulate nerve cells in the targeted brain region. Because this type of pulse generally does not reach further than two inches into the brain, scientists can select which parts of the brain will be affected and which will not be. The magnetic field is about the same strength as that of a magnetic resonance imaging (MRI) scan." A variant of TMS, repetitive transcranial magnetic stimulation (rTMS), has been tested as a treatment tool for various neurological and psychiatric disorders including migraine, stroke, Parkinson's disease, dystonia, tinnitus and depression.
Early attempts at stimulation of the brain using a magnetic field included those, in 1910, of Silvanus P. Thompson in London. The principle of inductive brain stimulation with eddy currents has been noted since the 20th century. The first successful TMS study was performed in 1985 by Anthony Barker and his colleagues at the Royal Hallamshire Hospital in Sheffield, England. Its earliest application demonstrated conduction of nerve impulses from the motor cortex to the spinal cord, stimulating muscle contractions in the hand. As compared to the previous method of transcranial stimulation proposed by Merton and Morton in 1980 in which direct electrical current was applied to the scalp, the use of electromagnets greatly reduced the discomfort of the procedure, and allowed mapping of the cerebral cortex and its connections.
Studies of the use of TMS and rTMS to treat many neurological and psychiatric conditions have generally shown only modest effects with little confirmation of results. However, reviews have stated that rTMS appeared to be effective in the treatment of certain types of major depression under certain specific conditions.
A meta-analysis of 34 studies comparing rTMS to sham treatment for the acute treatment of depression found that rTMS was effective with an effect size of 0.55. This is comparable to commonly reported effect of medications for treatment of depression in the range of 0.17-0.46. However, this meta-analysis found that rTMS was worse than electroconvulsive therapy (ECT), although there were significantly fewer adverse effects with rTMS. An analysis of one of the studies included in the meta-analysis found that one extra remission from depression occurred for every 3 people given electroconvulsive therapy rather than rTMS.
One trial of rTMS in depression used an active "sham" placebo treatment that appeared to mimic the sound and scalp stimulation. A review of the trial's report has questioned the adequacy of the placebo, noting that treaters were able to guess whether patients were receiving treatment with active or sham TMS, better than chance.
A comprehensive safety study of rTMS in the treatment of major depression looked at three separate groups totalling over 300 patients. It found that rTMS was associated with a low incidence of side effects, most of which were mild to moderate. Additionally, only 4.5% of patients discontinued their participation during acute treatment because of adverse events. Although TMS is generally regarded as safe, the greatest acute risk is the rare occurrence of induced seizures and syncope (fainting). 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 are primarily associated with rTMS, although they have been reported following single-pulse TMS. 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. A 2009 international consensus statement on TMS that contained this review concluded that based on the number of studies, subjects, and patients involved with TMS research, the risk of seizure with rTMS is considered very low.
Other adverse effects of TMS are:
Discomfort or pain from the stimulation of the scalp and associated nerves and muscles on the overlying skin; this is more common with rTMS than single pulse TMS.
Minor cognitive changes, and psychiatric symptoms (particularly a low risk of mania in depressed patients).
Rapid deformation of the TMS coil produces a loud clicking sound that increases with the stimulation intensity and can affect hearing with sufficient exposure, which is particularly relevant for rTMS (hearing protection may be used to prevent this).
rTMS in the presence of EEG-incompatible electrodes can result in electrode heating and, in severe cases, skin burns. Non-metallic electrodes are used if concurrent EEG data is required.
it has been shown that a current through a wire generates a magnetic field around that wire. Transcranial magnetic stimulation is achieved by quickly discharging current from a large capacitor into a coil to produce pulsed magnetic fields of 1-10 mT. By directing the magnetic field pulse at a targeted area of the brain, one can either depolarize or hyperpolarize neurons in the brain. The magnetic flux density pulse generated by the current pulse through the coil causes an electric field as explained by the Maxwell-Faraday equation,
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.
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 an action 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).
MRI images, recorded during TMS of the motor cortex of the brain, have been found to match very closely with PET produced by voluntary movements of the hand muscles innervated by TMS, to 5–22 mm of accuracy. The localisation of motor areas with TMS has also been seen to correlate closely to MEG and also fMRI.
In January 2007, an advisory panel of the United States Food and Drug Administration (FDA) did not recommend clearance for marketing of an rTMS device, stating that the device appeared to be reasonably safe but had failed to demonstrate efficacy in a study of people with major depression who had not benefitted from prior adequate treatment with oral antidepressants during their current major depressive episode. The panel agreed that "unblinding was greater in the active group, and considering the magnitude of the effect size, it may have influenced the study results." However, the FDA determined in December 2008 that the rTMS device was sufficiently similar to existing devices that did not require a premarket approval application and allowed the device to be marketed in accordance with Section 510(k) of the Federal Food, Drug, and Cosmetic Act In March 2014, the FDA cleared this device "for the treatment of Major Depressive Disorder in adult patients who have failed to achieve satisfactory improvement from prior antidepressant medication in the current episode".
In July 2011, the FDA published a final rule in the Federal Register that classified the rTMS system into Class II (special controls) "in order to provide a reasonable assurance of safety and effectiveness of these devices". The rule identified the rTMS system as "an external device that delivers transcranial pulsed magnetic fields of sufficient magnitude to induce neural action potentials in the prefrontal cortex to treat the symptoms of major depressive disorder without inducing seizure in patients who have failed at least one antidepressant medication and are currently not on any antidepressant therapy". An FDA guidance document issued in conjunction with the final rule describes the special controls that support the classification of the rTMS system into Class II.
Soon after the FDA initially cleared the device, several members of Public Citizen stated in a letter to the editor of the medical journal Neuropsychopharmacology that the FDA seemed to have based its decision on a post-hoc analysis that did not establish the effectiveness of rTMS for the treatment of depression. The writers of the letter expressed their concern that patients would be diverted from therapies such as antidepressant medications that have an established history of effectiveness.
In December 2013, in a decision that was partially based on a post hoc analysis for the primary end point for effectiveness, the FDA permitted the marketing in the United States of a TMS device intended for the treatment of migraine headache. The FDA classified the device into Class II (special controls). The FDA defined this type of device as being one that "delivers brief duration, rapidly alternating, or pulsed, magnetic fields that are externally directed at spatially discrete regions of the brain to induce electric currents for the treatment of migraine headache".
Health insurance considerations
Commercial health insurance
In July 2011, the Technology Evaluation Center (TEC) of the Blue Cross Blue Shield Association, in cooperation with the Kaiser Foundation Health Plan and the Southern California Permanente Medical Group, determined that TMS for the treatment of depression did not meet the TEC's criteria, which assess whether a technology improves health outcomes such as length of life, quality of life and functional ability. The TEC's report stated that "the meta-analyses and recent clinical trials of TMS generally show statistically significant effects on depression outcomes at the end of the TMS treatment period. However, there is a lack of rigorous evaluation beyond the treatment period", which was, with a few exceptions, one to four weeks. The Blue Cross Blue Shield Association's medical advisory panel concluded that "the available evidence does not permit conclusions regarding the effect of TMS on health outcomes or compared with alternatives."
In 2013, several commercial health insurance plans in the United States, including Anthem, Health Net, and Blue Cross Blue Shield of Nebraska and of Rhode Island, covered TMS for the treatment of depression. In contrast, UnitedHealthcare issued a medical policy for TMS in 2013 that stated there is insufficient evidence that the procedure is beneficial for health outcomes in patients with depression. UnitedHealthcare noted that methodological concerns raised about the scientific evidence studying TMS for depression include small sample size, lack of a validated sham comparison in randomized controlled studies, and variable uses of outcome measures. Other commercial insurance plans whose 2013 medical coverage policies stated that the role of TMS in the treatment of depression and other disorders had not been clearly established or remained investigational included Aetna, Cigna and Regence.
There is no national policy for Medicare coverage of TMS in the United States. Policies vary according to local coverage determinations (LCDs) that Medicare administrative contractors (MACs) for the Centers for Medicare and Medicaid Services (CMS) make for geographical areas over which they have jurisdiction. CMS presently has ten to fifteen MAC jurisdictions that each cover several U.S. states.
LCDs for individual MAC jurisdictions can change over time. For example:
In early 2012, the efforts of TMS treatment advocates resulted in the establishment by a MAC with jurisdiction over New England of the first Medicare coverage policy for TMS in the United States. However, a new MAC for the same jurisdiction subsequently determined that Medicare would not cover services for TMS performed in New England on or after October 25, 2013.
... the evidence is insufficient to determine rTMS improves health outcomes in the Medicare or general population. ... The contractor considers repetitive transcranial magnetic stimulation (rTMS) not medically necessary when used for its FDA-approved indication and for all off-label uses.
However, the same MAC subsequently determined that Medicare would cover TMS for the treatment of depression for services performed within the MAC's jurisdiction on or after December 5, 2013,
The NICE has issued guidance to the NHS for TMS for the following two indications:
Treatment of severe depression (IPG 242)
Treating and preventing migraine (IPG 477)
1. TMS for treatment of severe depression
The NICE evaluated TMS for severe depression (IPG 242) in 2007. The Institute subsequently considered TMS for reassessment in January 2011 but did not change its evaluation. The Institute's recommendation states:
Current evidence suggests that there are no major safety concerns associated with transcranial magnetic stimulation (TMS) for severe depression. There is uncertainty about the procedure's clinical efficacy, which may depend on higher intensity, greater frequency, bilateral application and/or longer treatment durations than have appeared in the evidence to date. TMS should therefore be performed only in research studies designed to investigate these factors.
2. TMS for treating and preventing migraine
In January 2014, the NICE reported the results of an evaluation of TMS for treating and preventing migraine (IPG 477). The Institute's recommendation states:
Evidence on the efficacy of TMS for the treatment of migraine is limited in quantity and for the prevention of migraine is limited in both quality and quantity. Evidence on its safety in the short and medium term is adequate but there is uncertainty about the safety of long-term or frequent use of TMS. Therefore, this procedure should only be used with special arrangements for clinical governance, consent and audit or research. ....
TMS - Butterfly Coils
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.
The design of transcranial magnetic stimulation coils used in either treatment or diagnostic/experimental studies may differ in a variety of ways. These differences should be considered in the interpretation of any study result, and the type of coil used should be specified in the study methods for any published reports.
The most important considerations include:
the type of material used to construct the core of the coil
the geometry of the coil configuration
the biophysical characteristics of the pulse produced by the coil.
With regard to coil composition, the core material may be either a magnetically inert substrate (i.e., the so-called ‘air-core’ coil design), or possess a solid, ferromagnetically active material (i.e., the so-called ‘solid-core’ design). Solid core coil design result in a more efficient transfer of electrical energy into a magnetic field, with a substantially reduced amount of energy dissipated as heat, and so can be operated under more aggressive duty cycles often mandated in therapeutic protocols, without treatment interruption due to heat accumulation, or the use of an accessory method of cooling the coil during operation. Varying the geometric shape of the coil itself may also result in variations in the focality, shape, and depth of cortical penetration of the magnetic field. Differences in the coil substance as well as the electronic operation of the power supply to the coil may also result in variations in the biophysical characteristics of the resulting magnetic pulse (e.g., width or duration of the magnetic field pulse). All of these features should be considered when comparing results obtained from different studies, with respect to both safety and efficacy.
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
H-coil: for deep transcranial magnetic stimulation
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 produce a focused, localized response and are relatively non-focal.
Devices available for transcranial magnetic stimulation include:
Coils: This is the main component of a TMS system and the part applied directly to the head. A coil can be of different types.
Stimulators: The stimulator is the machine delivering high intensity pulses of electrical current in the coil to produce electromagnetic induction in the brain. It allows setting all important stimulation parameters and defining complex patterns of pulses to be delivered to the brain. In case of rTMS, the stimulator often contains a cooling system to evacuate the heat produced by repetitive pulses of current.
Neuronavigation systems: Neuronavigation is a technique originally used in neurosurgery. It makes uses of a software system able to load MRI and possibly fMRI data to localize stimulation spots directly in a 3D reconstruction of the brain. Combined with optical motion tracking systems focusing on the head, neuro-navigation provides computer-assisted TMS allowing for personalized stimulations. In traditional TMS indeed, the coil is positioned based on anatomical landmarks on the skull (including, but not limited to, the inion or the nasion), thereby deriving the location of stimulation spots from the anatomical position of the brain in the head.
Coil positioning systems: positioning systems help to keep the coil in place for the whole duration of a TMS session. Such systems can be simple static coil holders or computer-controlled robotic arms. Static holders need to be manually adjusted at the stimulation site. Robotic arms are controlled by neuronavigation to adjust the coil position automatically.
It is difficult to establish a convincing form of "sham" TMS to test for placebo effects during controlledtrials in conscious individuals, due to the neck pain, headache and twitching in the scalp or upper face associated with the intervention. "Sham" TMS manipulations can affect cerebralglucose metabolism and MEPs, which may confound results. This problem is exacerbated when using subjective measures of improvement. Placebo responses in trials of rTMS in major depression are negatively associated with refractoriness to treatment, vary among studies and can influence results. Depending on the research question asked and the experimental design, matching the discomfort of rTMS to distinguish true effects from placebo can be an important and challenging issue.
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