TMS Overview
Purpose
To provide information on the mechanisms and physiology of TMS, and its potential side effects.
Overview
Transcranial magnetic stimulation (TMS) is a safe, painless, non-invasive way of activating neurons in the cortex of the brain.
An insulated coil of wire is held gently against the scalp. A brief current is passed through this insulated coil, which generates a brief and rapidly changing magnetic field around the coil. The magnetic field passes painlessly through the scalp and skull. The magnetic field is shallow, and can induce current flow in the underlying tissues. It can which can depolarise neurons near the surface of the brain causing them to fire in unison.
When the coil is positioned over the primary motor cortex, the synchronised activation of neurons can generate a descending signal to the spinal cord. This signal can be detected when it activates muscles on the opposite side of the body, using surface electromyography (EMG). TMS can therefore provide a safe way of testing whether the descending motor pathways from the brain to the periphery are working.
There are various TMS protocols including single-pulse, paired pulse and repetitive TMS. PREP2 uses single-pulse TMS, where one stimuli is applied at a time and there is no cumulative effect. This protocol is used for assessment (not treatment) of the descending motor pathways, and is associated with the lowest risk of side effects.
The motor system
The motor system consists of the central and peripheral nervous system and the muscles of the body, with the motor cortex the highest level of sensorimotor control.
The motor cortex is divided into several distinct regions including the primary motor cortex, premotor cortex, the supplementary motor area and the cingulate motor area.
If the TMS coil is positioned over the primary motor cortex (located in the precentral gyrus of the frontal lobe) the magnetic stimulus can depolarise pyramidal neurons that form the corticospinal tract. These pyramidal neurons are mainly found in layer 5 of the motor cortex, and are excited by intracortical interneurons located in the more superficial layers of the cortex, layers 1 – 4. Activation of these pyramidal neurons by TMS most likely occurs trans-synaptically via activation of intracortical interneurons, which in turn excite the pyramidal neurons.
When pyramidal neurons fire they generate synchronised descending output from the primary motor cortex to alpha motor neurons in the spinal cord, in the form of action potentials travelling along the corticospinal tract. When the descending output from the motor cortex activates alpha motor neurons in the spinal cord, they fire action potentials that then travel along the peripheral nerves to muscles on the opposite side of the body. Surface EMG can be used to detect the arrival of these action potentials in the muscles, observed as a motor evoked potential (MEP).
Obtaining MEP status
TMS equipment can be portable in the hospital setting. This means testing can be performed at a patient’s bedside, in a procedure room, or similar location. Family members can be present during testing, if this is what the patient wants. For PREP2, testing takes about 20 minutes with the patient, with additional time before and after for setting up and putting away equipment, and documentation.
If TMS produces a MEP then the descending pathway from the primary motor cortex to the spinal cord is functional. In this case the MEP status of the patient is MEP+.
The picture below will help orientate you to EMG traces and MEPs. The picture shows an EMG recording from the extensor carpi radialis (ECR, blue trace) and first dorsal interosseous (FDI, yellow trace) muscles of the upper limb. If TMS produces a MEP then the descending pathway from the primary motor cortex to the spinal cord is functional. In this case the MEP status of the patient is MEP+. In the picture below you can see a MEP is present in both the ECR and FDI muscles and this patient would be classified as MEP+.
If TMS does not produce a MEP then the descending pathway from the primary motor cortex to the spinal cord is not functionally intact at that time. In this case the MEP status of the patient is MEP-.
In the picture below you can see an EMG trace from two muscles with no MEPs produced. If this picture was representative of all the EMG traces then the patient would be classified as MEP-.
TMS Safety
Side effects are very uncommon, but it is important to be aware of them
The risk of side effects can be minimised by screening patients for suitability for TMS using a TMS Safety Checklist. Potential side effects include syncope which is most likely due to anxiety rather than direct brain effects, TMS-associated seizure activity which is very rare for single-pulse TMS protocols, and transient headache or hearing changes (see the essential readings for more detail).
Not everyone can have TMS
The TMS Safety Checklist covers absolute contraindications and safety considerations so suitability for TMS can be determined. The patient’s physician should be responsible for reviewing the checklist and deciding whether to approve the use of TMS. Very few people (~2%) of patients are unable to have TMS.
TMS operators should be properly trained by personnel with expertise in TMS
Training should include theory and practical sessions. Operators must be familiar with the means for summoning medical or emergency assistance in their facility. If seizure or syncope are suspected during testing, TMS should be terminated immediately and first aid delivered. It is advised that people who are pregnant should not operate TMS.
Further reading
Transcranial Magnetic Stimulation: A Primer
Resources
FAQs
Is it safe for everyone to have TMS?
People should be screened for contraindications to TMS to determine if it is safe for them before any testing takes place. You can screen patients with the TMS Safety Checklist, or an equivalent checklist already in local use. The patient’s physician should be responsible for reviewing the checklist and deciding whether to approve the use of TMS. For PREP2, about 2% of patients are not suitable for TMS testing.
What does TMS feel like?
TMS is a comfortable experience for most people. Start by explaining to the patient what they can expect. For PREP2 this should include:
- that a plastic coil will be held against the side of their head to test the movement pathways from the brain to their hand and arm
- that the device makes a click noise each time it is activated
- they will feel a light tap on their scalp each time it is activated, but this won’t be painful
- they may, or may not, feel a brief twitch in their hand and arm muscles, but again this won’t be painful
How does single-pulse TMS differ from repetitive TMS?
Single-pulse TMS is where one stimulus is applied at a time, with several seconds between stimuli. When applied over the primary motor cortex single-pulse TMS is used to assess whether the motor pathways from the brain to the spinal cord are working.
Repetitive TMS (rTMS) is where multiple stimuli are applied repeatedly, at a frequency of at least once per second. Repetitive TMS can change the activity of descending motor pathways for a period of time that outlasts the period of stimulation (often 30 – 60 minutes).
Put simply single-pulse TMS can used to investigate the function of the motor pathways, but it does not modify its activity and does not provide a treatment effect.
Is single-pulse TMS like electroconvulsive therapy (ECT)?
Although both ECT and TMS involve stimulation of the brain their mechanisms, purpose, patient experience and safety profile are very different. ECT is a psychiatric treatment that involves directing an electric current into a patient’s brain via electrodes applied to the scalp to intentionally cause a series of generalized seizures. Patients are given muscle relaxants and general anaesthetic so they are not conscious during the treatment. Side effects such as memory loss are common.
By contrast TMS uses electromagnetic induction to non-invasively and painlessly activate cortical neurons. For TMS, patients are fully awake during testing and able to converse throughout the procedure. PREP2 uses single-pulse TMS which is an assessment tool, and not a treatment, and does not induce a series of seizures. Side effects are uncommon, and tend to be mild and transient.
If someone is due for TMS testing and their medical status changes what should I do?
Notify the person who is due to perform the TMS testing. They will need to review the situation and possibly revise the TMS Safety Checklist to reflect any changes. If the checklist answers change, then the physician who signed it off should be consulted to determine whether TMS testing is still appropriate.
Quiz
Click here for a practice quiz to assess your learning on the TMS Overview section.
Once the practice quiz has been completed with at least 70% correct you will be emailed a link to the final quiz for this section.
If you pass the final quiz with at least 80% correct you will be emailed a certificate of completion for the TMS Overview section. There is no limit to the number of attempts for the practice or final quiz.