Biomedical Engineering Reference
In-Depth Information
system. TMS can be used to map motor cortex representations precisely. Magnetic stimula-
tion is now used for operating-room monitoring to directly assess the central motor pathways.
Recently, rTMS has shown therapeutic potential in the control of depression [George
et al., 1996; Pascual-Leone et al., 1996]. Many groups of investigators have demonstrated
that mood can be altered by rTMS in healthy subjects and improve mood even in patients
with medication-resistant depression [George et al., 1995, 1997]. Much more clinical work
is still required to turn rTMS into a clinical alternative to current antidepressant treatments.
However, its promise is signi
fi
cant. An open trial reported comparable antidepressant
e
cacy for TMS and the most powerful of all antidepressant treatments, electroconvulsive
therapy (ECT). However, unlike ECT, rTMS does not require sedation or cause cognitive
impairment and can be administered in an outpatient setting.
At the time of this writing, commercially available magnetic stimulators are approved
by the FDA for peripheral nerve and spinal cord stimulation only, but the FDA is allowing
low-repetition-rate devices (
1 Hz) to be used by investigators for human cortical stimu-
lation without the need for an Investigational Device Exemption (IDE; see the Epilogue).
However, the FDA believes that TMS at frequencies of
1 Hz carries signi
fi
cant risk and
thus requires an IDE for studies involving rTMS of the human cortex.
A di
erent emerging area for the therapeutic use of repetitive magnetic stimulation is the
excitation of peripheral nerves for painless treatment of neurological and neuromuscular
disorders. For example, Neotonus Inc. (Marietta, Georgia) has focused its attention on the
treatment of urinary incontinence. In June 1998, the FDA approved use of their magnetic stim-
ulator to enhance peripheral nerve innervation in treating urinary incontinence in women.
Some groups, such as that of Loughborough University [Young et al., 2001], are pursu-
ing a much more ambitious goal. They are working on pulsed power energized from a pair
of parallel-connected 200-
ff
µ
F at 22 kV capacitors. The 118-
µ
H double coil induces pulses
of 500
s in duration and 400 V/m at 5 cm from the coil. This has the potential to activate
structures deep inside the body, such as the heart, bowel, bladder, spinal cord, and kidney.
The aim of this group's program is to be able to stimulate the kidneys to restart or enhance
peristaltic pumping to evacuate stone fragments left behind after extracorporeal shock wave
lithotripsy. Threshold for peristalsis with a “low-power” (by Loughborough's standards)
4.2-kJ magnetic stimulator applied directly to the kidney during surgery was reported to be
between 470 and 720 V/m, with an induced pulse width of 270
µ
µ
s. They expect that the 500-
µ
s pulse delivered by their 80-kJ stimulator will activate the unmyelinated nerve
fi
fibers in
the kidney from outside the body. This is certainly a new
field with plenty of potential. It's
easy to see that as in many other areas of biomedical engineering, progress will be made
with magnetic stimulation as ingenious solutions tackle today's engineering challenges.
fi
Safety in Magnetic Stimulation
Guidelines for safe use of magnetic stimulation have not been established conclusively. It
is obvious that magnetic stimulation should not be applied to subjects with implanted metal-
lic or magnetic objects. The magnetic
field of the stimulator attracts ferromagnetic objects
and repels nonmagnetic conductors with a force that can harm surrounding tissues. It should
also be obvious to practitioners of magnetic stimulation that an enormous magnetic pulse
can destroy any implanted electronic devices (e.g., pacemakers, implantable de
fi
fi
brillators).
ects, the experimenter should take into account
possible induction of seizures by transcranial magnetic stimulation. Also, magnetic stimulator
coils usually produce loud clicks when their windings try to expand during a pulse. The peak
sound pressure is often in the range 120 to 130 dB at a distance of 10 cm from the coil. Most
sound energy is in the frequency range 2 to 7 kHz, where the human ear is the most sensitive,
and this noise may exceed criteria limits for sensorineural hearing loss. For this reason, hear-
ing protection aids are recommended for both the experimenter and the subject.
On the less obvious side of possible side e
ff
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