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A bright spot on the horizon:
Transcranial magnetic stimulation in psychiatry
Matthew Kirkcaldie and Saxby Pridmore
"[Medicine is] entering an era when new methods will be available for the
modification of brain circuitry and function of normal or disordered types
by means of painless extracranial techniques that seem to be without
obvious detriment to neuronal populations ..."
- R. G. Bickford, 1987.
The brain is elusive: its most interesting qualities appear when it is tucked inside
its shell of bone, reading the world through senses and driving the body through the wide
range of human behaviour. Anatomists can describe its structure in incredible detail;
physiologists can tease out the complex chemistry of its cells, and neuropsychologists
have pieced together a broad but incomplete picture of how its functions work together.
Despite this enormous body of knowledge, the day-to-day running of the brain's activities
- and how to help when they go wrong - is still difficult to comprehend. Our knowledge is
based on accidental damage, comparisons made at autopsy, and some difficult imaging
techniques, rather than through direct interaction with the brain working inside its
enclosure.
Historically, interventions made on the brain have been fairly drastic - from holes
bored in the skull by primitive healers, through to the drugs, electrical treatments and
psychosurgery of more recent times. Psychiatry, surgery and pharmacology have combined to
alleviate or prevent many conditions which were once a death sentence, or meant a life of
misery for the sufferer. However, their techniques have often carried enormous risk, or
drastic side effects, due to the severity of the interventions used.
A new potential
One very promising avenue for influencing the living brain has emerged in the last
decade, based on the use of pulsed magnetic fields. The skull is a good insulator, and
past efforts to alter the electrical activity happening inside it have required high
voltages, with little opportunity for fine control or focus of the effects. Consider
instead how easily a magnet under a wooden tabletop can move a pin on the surface -
magnetic fields pass almost unaffected through insulators, including the skull.
It is easy in principle to get a magnetic field to produce electrical effects: simply
change the field over time, and any charge-carriers (like the ions in the cells of the
brain) will be influenced to flow, creating an induced current. However, affecting neurons
inside the head requires a lot of magnetic force to be changed very quickly, and the
technology to do this has only been around for about a decade. The first trans-cranial
magnetic stimulation (TMS) machines, capable of delivering a pulse every three
seconds, were developed as diagnostic aids for neurologists. For instance, the motor part
of the brain can be stimulated, inducing a twitch of the thumb, which tells a neurologist
that the intervening nerve pathways are intact. Machines are now available which can give
up to 50 stimuli per second (rapid-rate TMS, or rTMS) and their effects are more
interesting. Among a wide range of possibilities, it is believed that rTMS may have a
place in the treatment of some mental illnesses. It is a non-invasive technique,
apparently free of serious side-effects, capable of modifying the activity of specific
brain areas.
How it works
The magnetic fields used in TMS are produced by passing current through a hand-held
coil, whose shape determines the properties and size of the field. The coil is driven by a
machine which switches the large current necessary in a very precise and controlled way,
at rates up to 50 cycles per second in rTMS. The coil is held on the scalp - no actual
contact is necessary - and the magnetic field passes through the skull and into the brain.
Small induced currents can then make brain areas below the coil more or less active,
depending on the settings used.
In practice, TMS and rTMS are able to influence many brain functions, including
movement, visual perception, memory, reaction time, speech and mood. The effects produced
are genuine but temporary, lasting only a short time after actual stimulation has stopped.
Safety issues
Generally, TMS appears to be free from harmful effects. Research using animals and
human volunteers has showed little effect on the body in general as a result of
stimulation, and examination of brain tissue submitted to thousands of TMS pulses has
shown no detectable structural changes. It is possible in unusual circumstances to trigger
a seizure in normal patients, but a set of guidelines which virtually eliminate this risk
are available. Research continues, but TMS is certainly free of obvious side-effects like
those of electro-convulsive therapy (ECT), which still makes quite an impact on patients
despite refinements in technique.
TMS / rTMS in the treatment of mental illness
Many mental illnesses can be demonstrated to stem from the abnormal behaviour of
particular brain regions, in much the same way that diabetes is the result of
malfunctioning cells in the pancreas. It is believed that some mental disorders are the
result of nerve cells being over- or under-excitable (in other words, it is too easy or
too difficult for them to "fire" and work properly). In this context, successful
psychiatric treatment is achieved by modifying these cells' behaviour. The range of
effects produced by TMS are a clear indication of its potential to work in this way.
Of course, TMS could only be used to treat diseases whose functional causes are
understood. Recent progress in understanding the mechanisms behind depression,
obsessive-compulsive disorder, and neurological diseases like Parkinson's and
Huntington's, offers some hope in these areas. It must be stressed that most of the
excitement about TMS is based on potential rather than proven effectiveness, but research
is being conducted around the world. For instance, there is reason to believe that rTMS
could replace some ECT treatments currently used for severely depressed patients. Groups
in Germany, the United States and Israel have reported positive results from using TMS and
rTMS to treat depressed patients. The prospect of replacing ECT with a near-painless
treatment, which does not require anaesthesia, would change these people's lives
remarkably.
The authors of this article have just begun a research project, covering the use of
rTMS in depression and some of the physiology of its workings, through the Royal Hobart
Hospital and the University of Tasmania. We will keep readers posted on the progress we
make. In the meantime, interested Internet-connected people can find out more, by visiting
our resource page at
http://www.utas.edu.au/docs/healthsci/TMSresrc.html
This article was written for Open Mind,
the journal of the Tasmanian Association for Mental Health.
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Constructed by Matthew Kirkcaldie, PhD student at the
University of Tasmania.
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