Transcranial Magnetic Stimulation (TMS) Options

[Shawn]

this is from an article I found online, to give an overview of the TMS technique, which is several years old now. I found the info at http://www.musc.edu/tmsmirror/intro/layintro.html . There's also a very useful link over TMS at http://www.biomag.hus.fi/tms/Thesis/dt.html


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.

[jbramen]

More about TMS...

I hate to sound like a synic, but I am not sure that TMS will have any widepread use as a treatment tool. It is limited in that it can only target the brain surface. Much brain pathology is located in deep tissues. Also, I don't know that it will ever replace ECT. While little is known for sure about the underlying reasons for ECT's effectiveness, it is thought that a widespread resetting of synchronous neural firing is at its core. It is also believed that much of the brains synchrony is set my connections to the thalamus, a subcortical region not likely to be effected by TMS any time soon.

the other problem with TMS is that it is messy. For most parts of the brain, it remains unclear what underlying cortex has been effected by the pulse. However, areas such as motor cortex control known behaviors, so human subjects can be assessed and the experimentor can know what part of the brain has been effected by stimulation. Of course, not all cortical regions can be tested in this way.

Now this isn't to say that TMS doesn't have its uses. It is often used to study "lesions." TMS can overexcite neurons and put them into a refractory period, where they are unable to respond to further inputs for a short period of time. This has led to very interesting research on topics like mirror neurons. Basically, work in this area has demonstrated that motor cortex is important for sensory perception. We are able to percieve the actions of others by means of "empathy," or by using the same neurons to percieve an action as we do to create an action, in much the same way as we do for emotions and perhapse language.

For more on this subject, see:

http://www.cbd.ucla.edu/bios/iacoboni.html