Effects of MTS-r on Speech Production in Non-fluent Aphasia Post-ischemic Stroke Patients

Some studies have reported that low frequency repetitive Transcranial Magnetic Stimulation in patients with ischemic stroke generates left modulation of cortical excitability by facilitating, and promoting functional reorganization and facilitate the language production.

Stroke is a major public health problem, one-third of patients do not survive the first year after the vascular event, cause of severe disability and also is the third leading cause of death worldwide. Two-thirds of the survivors have neurologic deficits including aphasia (19%) generating functional disability, compromise on their daily living activities, and impact on the quality of life (93% of patients with aphasia after stroke compared with 50% of those survive without aphasia). Aphasia is an additional risk factor for depression (prevalence 5-63%), and also interfering with the functional and cognitive recovery, increasing the risk of mortality.

Generally the treatment of aphasia is based on speech therapy and drug therapy oriented comorbidities such as depression or anxiety. However, it has studied different interventions in an effort to improve the speech recovery of aphasia, as Transcranial Magnetic Stimulation (TMS), which facilitates recovery by stimulating contralesional and lesional regions. Most studies have been based on the transcallosal disinhibition theory, which consist on blocking homotopic regions through low frequency stimulation in the right posterior GFI (triangular portion) to produce a GFI disinhibition from right to left and facilitate neuroplasticity brain process.

EMT technique is based on the principle of electromagnetic induction, proposed by Michael Faraday (1831), in which posits that magnetic field depends on the stimulation time, generating an electric field that eventually causes neuronal excitability.The strength of the electric current produced in the coil between 5-10 Ka (Kiloamperes), while the strength of the induced magnetic field is about 1 to 2 Tesla, and the cortical area to stimulated is about 3 cm² and 2 cm depth. The magnetic field generated may have sufficient magnitude and density to depolarize cortical neurons tract, directly through axonal cone or indirectly through interneurons. When TMS pulses are applied repeatedly, which is known as repetitive TMS (r-TMS) produce modulation of the cortical excitability, increasing or decreasing, depending on the stimulation parameters. Those effects may persist beyond the train stimulation; defined as repetitive series of regular pulses.

Fundamental mechanisms of EMT, are not yet fully understood, because due inter-session and inter-subject variability have been observed in the modulation of induced excitability. The EMT-r may also induce changes in hormonal and neurotransmitter systems, such as, serotonin, dopamine, NMDA receptors, taurine, aspartate, serine axis and can regulate the expression of some genes; cfos, c-jung, which are important for synaptic plasticity. It has been studied several possible mechanisms involved in the functional regulation exerted by the EMT-r, such as synaptic plasticity, which is the most explored one, changes in excitability of neuronal networks, activation of feedback loops and metaplasticity, understood plasticity as synaptic plasticity. However, although but not yet, been established whether the source of the clinical benefit of any scheme of stimulation is a direct or indirect result of the modulation of the excitability, is believed to be associated with neuromodulators release and growth factors such as neurotrophic factor derived from the brain, which play an important role in the mechanism of EMT.