Transcranial magnetic stimulation of the brain
Last reviewed: 23.04.2024
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The method of transcranial magnetic stimulation (TKMS) is based on the stimulation of the nerve tissue using an alternating magnetic field. Transcranial magnetic stimulation allows to evaluate the state of the conducting motor systems of the brain, corticospinal motor tracts and proximal segments of nerves, the excitability of the corresponding nerve structures in terms of the magnitude of the threshold of the magnetic stimulus necessary to obtain muscle contraction. The method includes an analysis of the motor response and the determination of the time difference between the stimulated sites: from the cortex to the lumbar or cervical roots (central time).
Indications for the procedure
Magnetic stimulation of the peripheral nerves and brain allows us to trace the condition of the motor system of the brain in clinical conditions and quantify the degree of involvement in the pathological process of corticospinal motor tracts and various parts of peripheral motor axons, including the motor roots of the spinal cord.
The nature of the violation of the processes of excitation through the central structures of the brain and spinal cord is non-specific. Similar changes are observed in various forms of pathology. These disorders include an increase in the latent time of the evoked potential, a decrease in the amplitude or lack of response to stimulation of the motor cortex of the cerebral cortex, its dispersion, and also their various combinations.
Lengthening the time of central conduction is observed during demyelination, degeneration of the corticospinal path due to pathology of motoneurons or hereditary disease, cerebrovascular disorders, glioma of the cerebral hemispheres and discogenic compression of the spinal cord.
Thus, the indication for the transcranial magnetic stimulation is the pyramidal syndrome of any etiology. Most often in clinical practice, transcranial magnetic stimulation is used for various demyelinating CNS lesions (especially multiple sclerosis ), hereditary degenerative diseases, vascular diseases, tumors of the spinal cord and brain.
[Technique of the transcranial magnetic stimulation
The patient is sitting. The induced motor potentials, when magnetically stimulated, are withdrawn by means of surface electrodes applied to the region of the motor point of the muscles of the upper and lower extremities in the standard way, similarly to the generally accepted procedure for removing the M-response during stimulation electromyography. As a stimulating electrode, magnetic coils of two basic configurations are used: annular ones having different diameters, and in the form of a number 8, also called "butterfly-shaped beds". Magnetic stimulation is a relatively painless procedure, since the magnetic stimulus does not exceed the pain threshold.
Potentials recorded during stimulation of the cerebral cortex vary according to the latency, amplitude and shape of the recorded curve. In the study of healthy people, changes in induced motor potentials during magnetic stimulation are observed in response to changing stimulation parameters (magnetic field strength, coil position) and depending on the condition of the muscles being studied (relaxation, contraction and slight voluntary motor activity).
Transcranial magnetic stimulation makes it possible to get the motor response of almost any human muscle. Subtracting the latent time of formation of the motor response when stimulating the cortical representation of the muscle and the location of the corresponding rootlet in the region of the cervical or lumbar segments of the spinal cord, it is possible to determine the time of passage of the pulse from the cortex to the lumbar or cervical roots (that is, the time of central conduction). The technique also makes it possible to determine the excitability of the corresponding nerve structures by the magnitude of the threshold of the magnetic stimulus necessary to obtain muscle contraction. The registration of the induced motor response is performed several times, and the responses of the maximum amplitude, the correct form and the minimum latency are selected.
Normal performance
When conducting transcranial magnetic stimulation, the following parameters are analyzed.
- Latency caused by motor response.
- Latency of the F-wave (in calculating the radial delay).
- The amplitude of the induced motor response.
- Time of central holding.
- Radicular delay.
- The threshold for triggering the motor response.
- The sensitivity of the structures under investigation to the magnetic stimulus.
The most pronounced lengthening of the time of central conduction is noted with multiple sclerosis. In the presence of muscle weakness, changes in the parameters of the induced motor potential and an increase in the threshold for causing motor response are detected in all patients with multiple sclerosis.
Patients with ALS also show significant changes in the functional state of the motor system, in most cases sensitivity to the magnetic stimulus decreases, the threshold for triggering the motor response increases, and the time of central conduction increases (but to a lesser extent than with multiple sclerosis).
With myelopathy, all patients report an increase in the thresholds of transcranial stimulation. The observed disorders are particularly pronounced in the presence of a coarse spastic component. In patients with spinal cord degeneration, clinically manifested by ataxia and spasticity, a decrease in the sensitivity of cortical structures to a magnetic stimulus is observed. The response to rest is often not caused even with the maximum stimulus.
When examining patients with cerebral vascular diseases, the whole range of changes in central time is observed - from the norm to the delay of response within 20 ms and total lack of potential. A lack of response or a decrease in its amplitude is a prognostically unfavorable factor, while a recordable, albeit delayed, response in the early period after the stroke is indicated indicates the possibility of restoring the function.
Transcranial magnetic stimulation is successfully used in the diagnosis of compression of the roots of the spinal nerves. In this case, the asymmetry of the central conducting time is more than 1 ms. Even more informative in the diagnosis of radiculopathy is the "radicular delay" method.