Transcranial magnetic stimulation (TMS) is a type of brain stimulation therapy that has four different subtypes, or modalities. That is, there are four different manners in which TMS is administered.
The four methods are: 1) Single-pulse TMS, 2) Repetitive TMS, 3) Paired-pulse TMS, 4) Alternating current TMS. The rTMS technique is commonly used as most patients will undergo more than one session of TMS, and during the session they will receive multiple, or repetitive pulses of stimulation.
Let’s begin with the simplest method, Single-pulse TMS. This method occurs when a magnetic coil is placed against the scalp and a patient receives just one magnetic pulse. The single-pulse is often administered over the motor cortex area of the brain because stimulation to this area provides an overt muscle response in the patient. The smallest amount of energy that elicits a motor response is called the motor threshold (MT). Once a MT has been noted, a physician may use this number as an approximate estimate of how much stimulation, or how strong of a pulse to deliver to the patient. In sum, the MT provides a “starting point” for determining the most therapeutic dose of stimulation for the patient. Once the starting dose is established, a physician will likely administer one of the other three modalities of TMS.
Repetitive TMS (rTMS) commonly follows single-pulse TMS after the MT has been established. During rTMS, multiple pulses over time are delivered. As mentioned above, this method is common, as most patients will undergo multiple sessions of TMS. Both the strength and the time interval between the pulses can be varied and properly determined for each patient. Depending on the therapeutic protocol the clinician creates, pulses can be spread out far enough apart to allow for the neurons to respond and function accordingly. One common example of rTMS is called theta burst TMS, which involves administering three consecutive pulses and is followed by a delay before the next sequence of pulses. Variations of the theta burst method can be administered and tailored to best-fit individual patient needs.
The paired-pulse method also involves placing a magnetic coil against a region of the scalp, but this time delivering two pulses. The first pulse, sometimes called the “conditioning pulse” is given, and then a second pulse follows and it is often this second pulse that acts on the neurons.
Although the stimulation is thought to primarily affect the neurons just underneath the coil, the reality is the brain contains so many neurons that are in close proximity, that each neuron has the ability to influence others. Hence, when neurons underneath the coil are activated (or inhibited), they can affect other neurons, which could be located within various areas of the brain. Even neurons that are located close to the cortex (outer layer) of the brain could affect the functioning of neurons located deeper within the brain. Therefore, one goal of administering paired-pulse TMS is that it allows for assessment of how one brain area may be affecting another. Further, pathways of neurons, such as the motor cortex pathways that are involved in movement can be examined with the paired-pulse method. Gaining an understanding of where altered neuron function may be occurring can help aid in managing symptoms of psychiatric conditions.