Wednesday, February 18, 2009

TMS

There are two types of TMS, one delivers a single pulse of current/magnetic field and the other delivers a repetitive series of pulses (rTMS). Usually, rTMS is used for psychiatric purposes. A physical difficulty with TMS is targeting specific areas of the brain with a specific intensity. The strength of the field that reaches the brain is affected by skull thickness, distance of the coils, and shape and number of coils. The coils come in two varieties, circular and figure-eight shaped (the figure-eight allows for more specific targeting).


A current is generated in the coils of the device. This current generates a magnetic field (magnetic flux) oriented perpendicular to the coil (towards the head). The magnetic field passes through to the brain where a current opposite in direction to the stimulus occurs.

For a moving point charge q travelling at velocity v a magnetic field B is given by:


where µ0 is the permeability of free space constant.

For a current in a loop, the magnetic field is:

where R is the radius of the loop. (This equation is the Biot-Savart Law).

And (thank-you Faraday) if the strength of the magnetic flux bounded by a circuit (here, the neural network) varies, a current in the circuit which is opposite to the original current (perpendicular to the magnetic field) will be induced. How fun is that? God, I love physics.





where ф is the magnetic flux, A is the area of the circuit, and ɛ is the generated EMF.

Obviously, metal objects must be removed before TMS is started and people with non-removable implants such as pacemakers would be excluded from this treatment.

The benefit of TMS over ECT is that magnetic fields travel through body tissue without losing energy and magnetic stimulation does produce any heat or pain (because there is no loss of energy which causes things like heat and light). As well, it appears that the memory problems of ECT do not occur with TMS (there is some memory dysfunction, but this returns to normal, or near normal, more quickly).

One of the major risks of TMS is seizure. Other side effects include temporary hearing loss (from the sound of the apparatus) and headache.

In terms of psychiatric applications, studies are still relatively new and have the difficulty of not having a placebo control group. From my knowledge (and this lecture was given last year), TMS appeared to be no more beneficial than pharmacological treatment. To me, this doesn’t mean much because of each individual’s response to different treatments. Compared to ECT, preliminary results suggest TMS is less effective, but there are other contradictory reports, as well as evidence that both methods are about the same in terms of effect.

New research is being done on BP depression, with the same limitations as all the other studies, but again, results look promising.

Last year, a study was published that demonstrated rTMS had a positive impact on bulimia nervosa patients. This was attributed to the fact that bulimia and depression both have serotonin defects and that bulimia can also be treated with antidepressants.

Research is just starting on the effectiveness of rTMS for anxiety disorders, but preliminary research (and this is very preliminary) suggests there might be some positive effect.

There are also trials being conducted in the schizophrenic population.

Relapse does occur and, as with ECT, maintenance rTMS is recommended. Long term effects are unknown.

References: 1, 2, 3, 4, 5

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