Repetitive transcranial magnetic stimulation (rTMS) is a technique that uses powerful, focused magnetic field pulses to stimulate the neurons of the brain. The first TMS devices came into use in 1985, and the first studies of rTMS as a treatment for depression were published shortly afterward in the early 1990s.
Health Canada first approved rTMS as a treatment for major depression in 2002. In the United States, the Food and Drug Administration approved an rTMS device as a treatment for medication-resistant depression in 2008. At present there are estimated to be over 1000 rTMS centers in operation around the world.
An rTMS device has a stimulation unit that generates brief pulses of strong electrical current. The stimulation unit is connected to a hand-held electromagnet called a coil, which contains loops of wire, typically wound in the shape of a figure of 8. The most commonly used rTMS coils are roughly the size of two half-bagels side by side.
The coil converts the electrical current into magnetic field pulses that are as strong as those in an MRI scanner, but focused into an area just a few centimeters across. When the coil is placed against the surface of the head, the magnetic fields are powerful enough to pass through the scalp and skull to stimulate the area of the brain that lies directly underneath.
Types of rTMS Approach: Excitatory and Inhibitory
Single pulses of TMS can excite the neurons in the target brain region, causing them to fire the tiny electrical discharges (known as action potentials) that they use to communicate with one another. For example, applying a pulse of TMS to the regions of the brain that control movements (called the motor cortex) can cause the neurons to send signals down the spinal cord to the appropriate body part. The muscles in that part of the body then contract, producing brief movements of the hand, foot, or other parts of the body.
To achieve longer-lasting effects, pulses of stimulation need to be applied repetitively over time – this is the “R” in rTMS. rTMS can be used to either increase or decrease the activity of the targeted brain region, depending on the pattern of stimulation that is applied. There are many different stimulation patterns that can be used for rTMS, each with slightly different effects on brain activity. At the UHN rTMS clinic, four different types of stimulation are currently in use:
High-frequency stimulation (10 pulses per second) has an excitatory effect on neural activity. Over time, this form of stimulation strengthens the connections between neurons and increases the activity of the target region. This can be useful in treating disorders that involve underactivity in certain brain areas.
Low-frequency stimulation (1 pulse per second) has an inhibitory effect on neural activity. Over time, this form of stimulation reduces the strength of connections between neurons and decreases the activity of the target region. This can be useful in treating disorders that involve overactivity in certain brain areas.
Theta burst stimulation (TBS) is a newer form of stimulation that more closely mimics the natural rhythms of activity in the neurons of the brain. The stimulation consists of short bursts of stimulation at high frequencies (50 pulses per second), with the bursts themselves being applied 5 times per second.
First used in 2005, TBS has been shown to produce stronger and longer-lasting changes in brain activity, while requiring much less time to administer. One kind of TBS, called continuous TBS (cTBS), can generate potent inhibitory effects with just 40 seconds of stimulation. Another kind of TBS, called intermittent TBS (iTBS), can generate potent excitatory effects with just under 4 minutes of stimulation.
cTBS and iTBS may eventually prove to be more effective than the older forms of stimulation in treating neurological and psychiatric disorders. However, since they are newer forms of rTMS, they are not yet backed by the same weight of evidence as conventional low-frequency and high-frequency rTMS, which have been studied extensively over the last 20 years.