Download Transcranial Magnetic Stimulation (TMS)

Transcranial Magnetic
Stimulation (TMS) & clinical
applications in neuropsychiatry
Heider SH. Qassam
MSC. PH. & TH.
Electricity ,Magnetism and
Biologic Effects
• Electromagnetism was first discovered by
Michael Faraday, who determined that a
magnetic field could be generated by running
an electric current through a wire coil.
Conversely, a changing magnetic field can
generate an electric voltage; the magnetic
field must change to have any electrical
effect.
The biological effects of pulsating
electromagnetic fields are due to electrical
rather than magnetic forces. Magnetism
generates a voltage in tissue according to
the equation:
V = n x a x dB/dt
V = Voltage
n = number of turns in the electromagnetic coil
a = area of the loop
dB/dt = The rate of change of magnetic field
with time.
What is TMS?
• Relatively new technique that induces
current in the brain by using a magnetic
field outside the skull Can induce muscle
twitch or phosphenes, or disrupt activity.
• Not an imaging technique, but allows
inferences about necessity of activity for a
given task
• An experimental treatment for some
psychiatric/psychological and neurological
disorders.
Basic mechanics
• Magnetic field used to
transport electrical
signal through the
skull (high resistance,
no impedance)
• Short pulse of current
in electromagnet
creates large
magnetic flux, and in
turn induces current.
http://www.thecamreport.com/?cat=99
Basic mechanics
http://www.mcaturk.com/,
reproduced from Magstim Company
Bryan Christie,
http://www.spectrum.ieee.org/print/3050
• Coils are generally
figure-8 shaped to
increase spatial
resolution
• Completely noninvasive,
used in human & animal
subjects
• Possible to give single
pulses (repetitive TMS
or rTMS) up to 60 Hz
and 10 s long.
Mechanism of action in the brain
• TMS causes synchronous activity of
some subpopulation of neurons,
sensitive to the orientation of the coils.
• A 50-250 ms silencing effect follows due
to sustained GABAergic effect Very
similar to the start of an epileptic seizure.
• Two important effects: direct activation of
motor/visual areas; disruption of ongoing
activity.
TMS as a therapeutic technique
UT Southwestern Medical Center,
Reprod. www.sciencedaily.com
• Single TMS over motor cortex
can produce simple movements.
• Interference with information
processing and behavior is
especially likely when TMS
pulses are delivered rapidly and
repetitively. Repeated rhythmic
TMS is called repetitive TMS
(rTMS). If the stimulation occurs
faster than once per second (1
Hz) it is referred to as fast rTMS.
• Most research on TMS has used
magnetic field intensities near
the motor threshold and,
therefore, sufficient to cause
neuronal depolarization.
•Potential clinical
applications
Mood Disorders
• The area of greatest public attention has
been the use of TMS as an antidepressant.
Several studies suggested that lowfrequency TMS over the vertex might have
antidepressant effects. Based on findings of
abnormal prefrontal function in depression
and the evidence that modulation of
prefrontal function is linked to the efficacy of
ECT.
Mood Disorders
• prefrontal TMS can exert short-term
antidepressant or antimanic effects. At the
clinical level, TMS may ultimately offer an
alternative to ECT for severe or treatmentresistant depression, particularly since the
adverse effect profile of TMS is relatively benign.
Repetitive TMS does not involve anesthesia
administration or seizure induction and has no
obvious cognitive sequelae . Another potential
use of TMS may be as an augmentation agent to
hasten clinical response in pharmacologically
treated patients
Anxiety Disorders
• In a randomized trial of left and right prefrontal stimulation in
patients with obsessive-compulsive disorder found that a
single session of right prefrontal rTMS decreased compulsive
urges for 8 hours. Mood was also transiently improved, but
there was no effect on anxiety or obsessions. Using TMS
probes, the same group reported decreased intracortical
inhibition in patients with obsessive-compulsive disorder,
reported that the condition with posttraumatic stress disorder
improved during open treatment with 1-Hz rTMS over the right
frontal cortex. similarly stimulated patients with posttraumatic
stress disorder over motor cortex and found decreased
anxiety.
Schizophrenia
• TMS has been rarely used to study
schizophrenia, with 1 report of an open
clinical series of slow rTMS resulting in
reduced anxiety .In patients with
prominent negative symptoms, found that
session of fast rTMS to the left DLPFC
was associated with slightly improved
negative symptoms.
Movement Disorders
• Therapeutic applications of TMS in
movement disorders are preliminary. Fast
rTMS of the motor cortex has been
reported to improve performance on
several motor measures in Parkinson
• Disease. Slow rTMS has been reported to
improve dystonia.
Epilepsy
• The TMS motor threshold is reduced in
patients with untreated
epilepsy.Therapeutically, there is 1 report
of potential beneficial effects of slow rTMS
in action myoclonus.
Emotion
• There is evidence that rTMS can modulate
mood systems in normal volunteers. Three
studies found that rTMS over the left
DLPFC transiently induced a mild increase
in sadness,whereas right DLPFC rTMS
produced a mild increase in happiness.
SAFETY
• Transcranial magnetic stimulation is not pleasant, and
stimulation at higher intensities and frequencies is more
painful. The pain experienced during rTMS is related to
the repetitive stimulation of peripheral facial and scalp
muscles, resulting in muscle tension headaches .These
headaches respond to treatment with acetaminophen or
aspirin. Magnetic stimulation also produces a highfrequency noise artifact that can cause short-term
changes in hearing threshold.
• This is avoided when subjects and investigators wear
earplugs.
• The most critical immediate safety concern
is that rTMS has resulted in seizures.The
TMS-induced seizures were self-limiting,
and did not seem to have permanent
sequelae. To avoid seizures the magnetic
intensity delivered with rTMS is adusted
for each individual relative to their motor
threshold.
CONCLUSIONS
• During the next several years, it will become clearer
whether rTMS has a role in the treatment of
psychiatric disorders.trials in depression have
focused on demonstrating antidepressant
properties and have not demonstrated clinical
utility. We need to know a good deal more about
the patients who benefit from rTMS, the optimal
form of treatment delivery, the magnitude and
persistence of therapeutic effects, and the risks of
treatment.. Regardless of its potential therapeutic
role, the capacity of rTMS to alter functional brain
activity should lead to important advances in our
understanding of brain-behavior relationshipsand
the pathophysiology of neuropsychiatric disorders.
References
1.
2.
3.
Jahanshahi M and Rothwell J. (2000). Trancranial
magnetic stimulation studies of cognition: an emerging
field. Exp Brain Res 131:1-9.
Pascual-Leone A, Walsh V, and Rothwell J. (2000).
Transcranial magnetic stimulation in cognitive
neuroscience – virtual lesion, chronometry, and
functional connectivity. Current Opinion in
Neurobiology 10:232-237.
Wu DA. (2005). How perception adheres color to
objects and surfaces: studies using visual illusions and
transcranial magnetic stimulation. Ph D thesis.
Available at: http://resolver.caltech.edu/CaltechETD:
etd-09282005-121349.
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