A bright spot on the horizon:
Transcranial magnetic stimulation in psychiatry
Matthew Kirkcaldie and Saxby Pridmore
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.
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