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Transcranial magnetic stimulation

Currently, transcranial magnetic stimulation presents significant interest to clinicians and researchers all over the world. The physical basis that stimulates nerve tissue by using alternating magnetic field is based on fundamental discoveries in the field of physics and medicine. In the beginning of 20th century studies showing the influence of alternating magnetic field on the human brain were already reported (in 1896, d'Arsonval described the process of phosphenesis - a patient's subjective perception of flashes of light in the eyes that occur when the patient's head is placed in a coil that generates an alternating magnetic field). Half a century later, Barlow (1947) and Colin (1959) showed that phosphenesis is associated with the stimulation of not only the retina of the eye but also the visual cortex of the brain by the magnetic fields. Positive results had also been obtained by stimulation of the peripheral nerves through the alternating magnetic field.

Nevertheless, at that time, a number of significant drawbacks, which existed in the solenoids produced a pulsed magnetic field that made the method practically inapplicable. A magnetic stimulator, which had enough power to excite the motor cortex through the cranium and cause muscle contractions in the limbs, was first created in 1985. This was the beginning of research into the potential of TMS in the treatment of various kinds of diseases relating to the nervous system - including the consequences of stroke.

TMS in stroke rehabilitation

The physiological basis of the method is the depolarization of nerve cell membrane, which takes place due to the generation of an electric field in the depths of tissue. It should be noted that carrying out cortical stimulation with the use of an alternating magnetic field is one of the most physiological techniques. It can be explained that exactly those neurons activated which step up first during the arbitrary movement.

Typically, TMS is used to diagnose acute disorders of cerebral circulation. The most informative method for this goal is the analysis of the induced motor response (IMR). During the acute stage of a stroke, IMR is absent in more than half of all cases. To locate the focus of the stroke, TMS can be performed already in the first hours after detecting the disease. Repeated TMS can be performed on the 3rd, 7th and 14th days, which enables predicting the course of the disease and facilitating the restoration of lost functions.

IMR is absent in more than 50% of cases if ischemia is present in the cerebral cortex. There may be (in 30% of cases) IMR of increased latency and low amplitude in case of subcortical lesions. Increasing of IMR most often be observed on condition of mixed or subcortical foci - in particular, with hemorrhagic strokes. The absence of IMR within the basin of the middle cerebral artery, thalamus and in the region of the posterior thigh of the inner capsule is explained by the greatest vulnerability of the conducting pathways due to the compactness of their location. The increase in CMCT or the absence of IMR is also explained by the vascular lesion of the medial part of the trunk, which corresponds to the anatomical location of the cortico-spinal pathways in this region.

In case of ischemic foci located in the lateral part of the trunk, the function of the cortico-spinal pathways is preserved. In some cases, ipsilateral IMR is observed in stroke. Structures that participate in its generation are uninhibited in the pathological conditions, whereas they are in a state of inhibition in the norm. In this case, ipsilateral IMR can be recorded both from the unaffected and the affected hemisphere.

An analytical study of ipsilateral IMR may be a prognostic criterion for the rehabilitation recovery of lost functions. An indicator, which is called the "period of silence" is very sensitive in stroke. The duration of this period increases with minimal disorders in the brain that are ischemic in nature and not accompanied by marked structural or clinical changes.

The detection of a silent period duration dependence on the strength of the background tension of the muscle tissue indicates a high risk of spasticity. Analysis of the unaffected hemisphere IMR often reveals its increased excitability, while the excitability of the affected hemisphere is significantly reduced. This state of things is, on the one hand, the consequence of pronounced disturbances in the brake interactions of the hemispheres, and on the other hand is a favorable prognostic sign.

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