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12/01/2012
Transcranial Magnetic Stimulation
Transcranial magnetic stimulation
Transcranial magnetic stimulation (TMS) 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 the functioning and interconnections of the
brain to be studied. A variant of TMS, repetitive transcranial magnetic
stimulation (rTMS), has been tested as a treatment tool for
variousneurological and psychiatric disorders
including migraines, strokes, Parkinson's
disease, dystonia, tinnitus, depression and auditory
hallucinations.
Background
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 in Sheffield,
England Its earliest application demonstrated conduction of nerve impulses
from the motor cortex to the spinal cord,
stimulating muscle contractions. The use of magnets rather than a direct
electric current to the brain reduced the discomfort of the procedure and
research and allowed mapping of the cerebral cortex and its
connections.
Effects on the brain
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 tractdepending on
the intensity of stimulation, coil orientation and frequency. The mechanism of
these effects is not clear although it is widely believed to reflect changes in
synaptic efficacy akin to long-term potentiation (LTP)
and long-term depression (LTD).
Risks
Although
TMS is often regarded as safe, the greatest acute risk of TMS
is the rare occurrence of induced seizures and syncope. More
than 16 cases of TMS-related seizure have been reported in the literature, with
at least seven reported before the publication of safety guidelines in
1998, and more than nine reported afterwards. The seizures have been
associated with 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. 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.
Besides
seizures, other risks include fainting, minor pains such as
headache or local discomfort, minor cognitive changes and psychiatric symptoms
(particularly a low risk ofmania in depressed patients). Though
other side effects are thought to be possibly associated with TMS (alterations
to the endocrine system, altered neurotransmitter andimmune
system activity) they are considered investigational and lacking
substantive proof.
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,
§ Rapid deformation of the TMS coil produces a
loud clicking sound which increases with the stimulator intensity that can
affect hearing with sufficient exposure, particularly relevant for rTMS
(hearing protection may be used to prevent this),
§ rTMS in the presence of incompatible EEG
electrodes can result in electrode heating and, in severe cases, skin
burns.Non-metallic electrodes are used if concurrent EEG data is required.
Clinical uses
ses
of TMS andThe u rTMS can be divided into diagnostic and therapeutic uses.
Diagnostic Use
TMS
can be used clinically to measure activity and function of specific brain
circuits in humans. The most robust and widely-accepted use is in
measuring the connection between the primary motor cortex and
a muscle to evaluate damage from stroke, multiple sclerosis, amyotrophic
lateral sclerosis, movement disorders, motor neuron
disease and injuries and other disorders affecting the facial and
other cranial nerves and the spinal cord.TMS has
been suggested as a means of assessing short-interval intracortical inhibition
(SICI) which measures the internal pathways of the motor cortex but
this use has not yet been validated. Therapeutic Use
A TMS therapy device
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, 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 condition.TMS
devices are marketed for the treatment of such disorders in Canada, Australia,
New Zealand, the European Union, Israel and the United States
A
meta-analysis of 34 studies comparing rTMS to sham treatment for the acute
treatment of depression showed an effect size of 0.55
(p<.001)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.
Other
areas of research include the rehabilitation of aphasia and
motor disability after strong tinnitus,Parkinson's disease
tic disorders and the negative symptoms of schizophrenia.TMS
has failed to show effectiveness for the treatment of brain death, coma,
and other persistent vegetative states.
It
is difficult to establish a convincing form of "sham" TMS to test
for placebo effects during controlled trials 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 cerebral glucose metabolism and MEPs, which
may confound results.This problem is exacerbated when using subjective measures
of improvement. Depending on the research question asked and the experimental
design, matching this discomfort to distinguish true effects from placebo
can be an important and challenging issue.
One multicenter
trial of rTMS in depression used an active "sham" placebo
treatment that appeared to mimic the sound and scalp stimulation associated
with active TMS treatment. The investigators reported that the patients and
clinical raters were unable to guess the treatment better than chance,
suggesting that the sham placebo adequatelyblinded these people to
treatment.The investigators concluded: "Although the treatment effect was
statistically significant on a clinically meaningful variable (remission), the
overall number of remitters and responders was less than one would like with a
treatment that requires daily intervention for 3 weeks or more, even with a
benign adverse effect profile". However, 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.In this regard, the trial's report stated that the
confidence ratings for the treaters' guesses were low.
FDA actions
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 episod 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 for "the treatment of Major Depressive
Disorder in adult patients who have failed to achieve satisfactory improvement
from one prior antidepressant medication at or above the
minimal effective dose and duration in the current episode". The user
manual for the device warns that effectiveness has not been established in
patients with major depressive disorder who have failed to achieve satisfactory
improvement from zero and from two or more antidepressant medications in the
current episode and that the device has not been studied in patients who have
had no prior antidepressant medication..
In
July 2011 the FDA published a final rule in the Federal Register that
classified the rTMS system into class II (special controls) (see: Medical
device#Classification) "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.
Response to FDA
decision
Soon
after the FDA 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.
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
2012, 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 2012 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 2012 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 includedAetna, Cigna and Regence.
Medicare
In
early 2012, the efforts of TMS treatment advocates, including a Rhode Island
physician who used TMS in her practice to treat patients with depression,
resulted in the approval for the New England region of the
first Medicare coverage policy for TMS in the United
States. However, in August 2012, the Medicare administrative contractor
for the Centers for Medicare and Medicaid Services jurisdiction
covering Arkansas, Louisiana, Mississippi, Colorado, Texas, Oklahoma and New
Mexico determined that, based on limitations in the published
literature,
"... 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-appproved indication and
for all off-label uses."
American Medical
Association category codes
In
2011, the American Medical Association established three
Category I CPT® Codes to be used for the reporting and billing of therapeutic
repetitive transcranial magnetic stimulation treatment services.The three codes
effective January 1, 2012 are:
§ 90867 – Therapeutic repetitive transcranial
magnetic stimulation (TMS) treatment; initial, including cortical mapping,
motor threshold determination, delivery and management
§ 90868 – Therapeutic repetitive transcranial
magnetic stimulation (TMS) treatment; subsequent delivery and management, per
session
§ 90869 – Therapeutic repetitive transcranial
magnetic stimulation (TMS) treatment; subsequent motor threshold
re-determination with delivery and management
Technical information
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 orthogonally 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.
Coil types
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 (ie, 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
Design
variations in the shape of the TMS coils allow much deeper penetration of the
brain than the standard depth of 1.5 cm. Circular, H-shaped, 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.
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