Safety of TMS and rTMS

Introduction

1. If used properly, single-pulse TMS has no known harmful side effects. TMS has been used since 1985 and today some 3,000 stimulators are in use.
2. If used properly, also repetitive TMS (rTMS) is thought to be safe. Beginners should consult literature and competent personnel, since rTMS an cause seizures.
3. It is extremely important for the future of TMS/rTMS that the experimenters document all harmful effects and the stimulation parameters that produced them.
4. Magnetic stimulation is not approved for cortical stimulation in the USA. See information on regulatory affairs.

The following text outlines the safety studies, guidelines and known effects of TMS and rTMS. A list of references to safety studies is given in the end. Consult literature before experimenting with TMS. International Federation of Clinical Neurophysiology (IFCN) has published an information brochure that covers TMS-related safety aspects generally.

Contraindications

Protocols should always exclude patients and volunteers with

intracranial metallic or magnetic pieces - The magnetic field present in TMS will generate forces on objects exposed to it: magnetic objects will be attracted and nonmagnetic repelled. The force may be substantial, but decreases quickly with decreasing cross-sectional area and conductivity of the object and with distance from the coil.

pacemakers, or any other implanted devices - The magnetic field pulse will disturb nearby electronic devices.

Safety of TMS and rTMS

Seizure induction: Single-pulse TMS has produced seizures in patients, but not in normal subjects. rTMS has caused seizures in patients and in normal volunteers.

Hearing loss: During TMS there will be a loud clicking sound from the coil. The peak sound pressure is 120-130 dB 10 cm from the coil. Most sound energy is in the frequency range 2-7 kHz where the human ear is the most sensitive. The noise may exceed criteria limits for sensorineural hearing loss.

Heating of the brain: Heating of the brain is unlike to cause deleterious effects. Theoretical power dissipation from TMS is few milliwatts at 1 Hz, while the brain's metabolic power is 13 W.

Engineering safety: TMS equipment operates at lethal voltages of up to 4 kV. It is hence important not to keep coffee cups or ice bags on the stimulator. The maximum energy in the capacitor is about 500 J, equal to dropping 100 kg from 50 cm on one's feet. An electrical engineer with experience of low-power electronics only should keep his/her hands behind the back and ask a power electronics expert to do the job.

Scalp burns from EEG electrodes: Mild scalp burns in subjects with scalp electrodes can be easily avoided using, e.g., small low-conductivity Ag/AgCl-pellet electrodes.

Effects on pulse rate, blood pressure, serum prolactin and cortisol levels and hormone levels: Many tests, including blood pressure, pulse rate, hormone levels and serum prolactin and cortisol levels, have revealed no statistically significant changes. The same is true for psychometric tests. Naturally, naming and verbal fluency tasks are disturbed during TMS.

Spontaneous EEG: Spontaneous EEG following TMS has been found to be normal; yet, EEG will fail to uncover transient changes like development of mild or additive cellular dysfunction. Monitoring of the EEG during TMS could, at least in principle, be used to stop the experiment if abnormalities appear.

Long-term effects: It is believed that harmful effects of TMS are related to the induced electric field, since the body tissue is transparent to low-frequency magnetic fields.

Original content by Jarmo Ruohonen, updated and converted to HTML 4.0 by Jussi Nurminen