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Guide to magnetic RTMS therapy

Jun 14 2007, 11:54 AM EST

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.

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.

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.

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.