Transcranial Magnetic Stimulation (TMS)

What is TMS? 

It is a magnetic stimulation technique that uses a time-varying magnetic field to act on the cerebral cortex, generating induced currents to alter the action potentials of cortical nerve cells, thereby affecting brain metabolism and neural activity.

It is a painless, non-invasive, safe, and reliable method of stimulating the central and peripheral nervous systems.

How it works?

 1. Capacitor discharges into the stimulation coil.

0.01s - several kV 1-6tesla

2. Penetrates the skull and acts on the cerebral cortex.

3. Alters brain metabolism and neural activity.电容向刺激线圈放电

Excitatory effect

High-frequency stimulation （＞1Hz）

Inhibition effect

Low-frequency stimulation (≤1Hz)

Treatment effects of the TMS therapy are as the followings:

※Regulates the excitability of the cerebral cortex

※Promotes synaptic connections and repairs incompletely damaged nerve cells

※Alters cerebral blood flow and glucose metabolism rate

※It affects the secretion of neurotransmitters in brain tissue (such as glutamate, GABA, serotonin, and dopamine), 

※Stimulates the secretion of neurotrophic factors, 

※Interferes with brain function, inducing neural network oscillations.

Neuroplasticity

The most basic, important, and active plasticity of the nervous system is synaptic plasticity, which is the foundation of human learning, memory, disease, aging, nerve growth, development, and repair.

Extensive network of connections in the cerebral cortex

The cortex contains approximately 500 million cortical pillars, ranging in diameter from 200 to 500 μm, and is composed of six layers of cells. Internally, neurons are interconnected, including afferent nerve fibers, interneurons, and efferent nerve fibers, forming the basic network for local information processing. It can generate rhythmic oscillations.

Stimulation mode

1.Single-pulse mode

2.Repetitive mode

3.Paired mode

4.TBS mode Theta burst Stimulation

What does TMS actually do to the brain?

TMS works by using a small coil placed on your head. It sends magnetic pulses into your brain. These pulses pass through your skull and create tiny electrical signals in your nerve cells underneath. Depending on the pulse pattern, TMS can help wake up underactive areas or calm overactive areas.

Coil

Round shape coil: Large stimulation area, suitable for MEP detection.

8 shape coil: Strongest magnetic field in the center, suitable for targeted therapy.

A figure-eight shape coil, consisting of two coplanar coils carrying opposite currents, produces an electric field with two types of peaks: a main peak located at the junction of the two coils and side peaks on either side. The value of the main peak is approximately twice that of the side peaks. This characteristic makes the focusing ability of a figure-eight coil far superior to that of a round shape coil.

TMS treatment parameter settings

1. Motor Threshold (MT): The minimum stimulus required to elicit a motor evoked potential (MEP) in the contralateral fingers when the motor cortex is stimulated. It is categorized into absolute intensity, active motor threshold (AMT), resting motor threshold (RMT), and peripheral motor threshold. It represents cortical excitability and is often the basic unit of stimulus intensity. The threshold varies from patient to patient.

2. Stimulation Intensity: 80%-120% MT is safe and effective, based on percentage of MT.

3. Stimulation Frequency (Hz): The number of stimuli per second (0.1Hz-100Hz). ≤1Hz is low frequency (inhibitory effect), >1Hz is high frequency (excitatory effect). In clinical treatment, high frequency is generally ≥5Hz.

Course of treatment: The number of consecutive treatments, 10 times constitutes one course of treatment. Generally, 2-3 courses of treatment are required. The dosage may be reduced or the number of treatments may be extended depending on the condition.

Purpose of TMS positioning

Because different brain regions govern different bodily functions, effective treatment can only be achieved by treating the corresponding brain region. Functional area localization is suitable for brain regions with well-defined functions and easily detectable stimulation effects.

The parietal lobe integrates information from our senses, interpreting our touch (somatosensory perception) and monitoring the relative positions of the body and its limbs (proprioception).

The occipital lobe decodes visual signals. Visual information travels from the retina through the thalamus to the occipital lobe, where it is decoded to tell us the location, manner, and content of what we see.

The frontal lobe is the largest lobe of the cerebral cortex. It learns about environmental, memory, and emotional information and uses this information to make decisions.

The temporal lobe houses memory, emotions, and language comprehension. It is also crucial for recognizing objects, places, and people.

Q&A

1. How long does TMS typically last？

Deep TMS treatments typically last 12 weeks and include two phases: acute and continuation. The first stage involves 20 sessions over 4 weeks, similar in structure to rTMS. The second phase consists of 2 sessions per week for an additional 8 weeks.

2.What are the side effects?

TMS is well tolerated and associated with very few side-effects. The most common side-effect (reported in about half of patients) is headaches. These are usually mild and diminish over the course of treatment.

About one-third of patients may experience painful scalp sensations or facial twitching with TMS pulses. These also tend to improve over the course of treatment. Adjustments can be made immediately in coil positioning to reduce discomfort.

The TMS device produces a loud clicking noise. Due to this earplug are required during the treatment. There is no evidence to suggest permanent changes to hearing if earplugs are worn during treatment.

The most serious risk of TMS is seizures. However, the actual risk of a seizure occurring during TMS is exceedingly low. At Montana VA Health Care System, we follow up-to-date safety guidelines that are designed to minimize the risk of seizures.