Transcranial Magnetic Stimulation (TMS) is based on electromagnetic induction to activate cortical neurons, stimulating the cerebral cortex. Assessing the excitability of the cortex will diagnose and estimate the therapeutic application of the technique for diseases such as depression, dystonia, Parkinson’s, stroke or epilepsy.
Transcranial Magnetic Stimulation: what is it and what does it consist of?
Transcranial Magnetic Stimulation (TMS) uses electromagnetic induction to activate cortical neurons. The stimulator is a capacitor that is discharged by a coil and produces a magnetic field. This, applied across the scalp, in turn induces an electric field. It is a non-invasive method to electrically stimulate the cerebral cortex to assess the excitability of the motor cortex and corticospinal tract.
It can be applied by single pulses or by trains of stimuli at varying frequencies. Single stimuli can depolarize neurons and produce measurable effects. Stimulus trains can modify cortical excitability of the stimulated area and related remote areas through functional anatomical connections and are being investigated for therapeutic application.
Barkers et al. designed in 1985 an electromagnetic stimulator with which they succeeded in stimulating, for the first time, the motor cortex through the scalp.
This stimulation technique, unlike electrical stimulation, is painless and non-invasive.
Uses of transcranial magnetic stimulation and the diseases it treats
There are two basic fields of application of transcranial magnetic stimulation: diagnostic and therapeutic. From the point of view of its diagnostic utility it is of interest in:
(a) Assessment of spinal cord conduction time. This is the conduction time from the motor cortex to the spinal alpha motor neurons. It consists of the difference between the conduction time from the cortex to the muscle and the peripheral motor conduction time. Its main application is used in the diagnosis of myelopathies secondary to disc pathology, canal stenosis or scoliosis, among others. It can also be altered in other pathologies, such as Amyotrophic Lateral Sclerosis or Multiple Sclerosis.
b) Assessment of the excitability of the motor cortex, by determining the motor threshold and silent period, which may be altered in certain pathologies affecting the central nervous system.
c) Transcranial magnetic stimulation and neurosurgery. TMS can be used for preoperative evaluation of specific areas of the brain.
The findings obtained so far can help to localize the level of the lesion along the different levels of the motor system, distinguishing between a predominantly demyelinating or axonal lesion, or predict the functional motor outcome after a lesion. The abnormalities shown by TMS are not disease-specific and the results must be interpreted in context with other clinical data.
From the point of view of its therapeutic application, the application of trains of stimuli by repetitive Transcranial Magnetic Stimulation is of particular interest in the treatment of depression. It involves applying a train of TMS pulses of the same intensity to a specific area of the brain at a frequency that can vary from one to 20 stimuli per second. Other possible indications are dystonia, Parkinson’s disease, stroke recovery and even in the treatment of refractory epilepsies.
Preparation prior to Transcranial Magnetic Stimulation
In principle, it is an innocuous exploration that does not require any preparation except for the usual neurophysiological explorations, such as cleaning and absence of creams in the stimulus and recording areas. It is advisable, however, the previous explanation by the expert in clinical neurophysiology, to reduce the initial anxiety of the patient, and even try to lower intensities.
Possible risks of Transcranial Magnetic Stimulation for the patient’s brain and contraindications
Adverse effects following single-pulse TMS on motor cortex are very rare. The potential adverse effect that has caused most concern so far has been epilepsy, although the risk is remote.
However, there are contraindications to TMS treatment. In this sense, they are absolute contraindications:
– intracerebral metallic material (aneurysm clips).
– pacemakers
– any implanted biomedical device should be questioned.
– caution with cochlear implants
Relative contraindications are:
– extensive strokes (cerebrovascular accidents)
– TBI (traumatic brain injury)
– history of neurosurgery
– family history of epilepsy
– sleep deprivation
– pregnancy (in this case magnetic stimulation at cortical level does not affect, although it should be avoided at lumbar level.
Side effects or sensations after TMS treatment
The most frequent side effect is headache, mainly due to repeated muscle stimulation. It usually starts 20-30 minutes after treatment with Transcranial Magnetic Stimulation and responds very well to usual analgesics. Finally, there is a certain risk of secondary transient hearing loss and tinnitus, so earplugs or earphones are usually used.