THE INFLUENCE OF TRANSCRANIAL DIRECT CURRENT STIMULATION ON FUNCTIONAL RECOVERY AFTER STROKE: A SYSTEMATIC REVIEW

INTRODUCTION: Stroke is the third leading cause of death in the world and second in Brazil, whose main characteristics are global or focal disorders of brain functions, involving cognitive and mainly physical limitations, including motor, sensory changes, ataxias, apraxias and aphasias. Transcranial Direct Current Stimulation (ETCC) is a non-pharmacological treatment that has demonstrated relevant benefits in the rehabilitation of patients. OBJECTIVE : to verify the effects of ETCC on the functional capacity of post-stroke patients, focusing on efficacy and results in the face of individuals' motor disorders. METHOD : The searches were performed in the databases PeDRO, Cochrane, PubMed and MEDLINE. Eligible studies included at least one of the population outcomes ≥ 18 years, resulting from the intervention, therapeutic efficacy and Motor activity. RESULTS: Seven were selected according to the inclusion and exclusion criteria, published between 2015 and 2020. CONCLUSION: ETCC, applied in isolation, can be an effective technique to improve motor deficits after stroke, including injured upper limb function, lower limb strength, unilateral neglect, mobility and Daily Living Activities. It is suggested to carry out further studies for the standardization of a treatment protocol, as well as to verify the best moment of its application in the post-stroke patient. .


INTRODUCTION
The Cerebrovascular Accident (CVA), also known as a stroke, corresponds to an acute abnormality in the Central Nervous System (CNS), occurring when there is a decrease or blockage in the blood supply to the brain for a period exceeding 24 hours, causing lesions 1 .Considered a global public health problem and one of the most prevalent neurological diseases 2 , stroke ranks as the third leading cause of death worldwide and the second in Brazil, accounting for approximately 68 thousand deaths per year 1,3 .
Stroke has two classifications: ischemic stroke (IS), which accounts for the majority of cases, involves vascular obstruction to a region of the brain; and hemorrhagic stroke (HS), where vascular rupture leads to blood leakage into the brain or the subarachnoid space 1 .The disease's main characteristics include global or focal disturbances of brain functions, encompassing cognitive and primarily physical limitations, including motor, sensory, ataxias, apraxias, and aphasias affecting up to 80% of individuals 1,2 .The impairment can be unilateral or bilateral, resulting in hemiparesis or hemiplegia when affecting one side of the body, as well as quadriparesis or quadriplegia, affecting both the upper and lower limbs 4 .Spasticity, an alteration in muscle tone that creates resistance to movement and involuntary movements, is a significant aspect of motor changes 1 .
In-hospital management of IS involves thrombolytic therapy, dependent on a therapeutic window of up to 4 hours and 30 minutes after the onset of symptoms.For HS, treatment focuses on preventing or treating to minimize sequelae, which can occur through clinical-medication and/or surgical means 1 .
Non-pharmacological actions have also been employed to minimize cognitive and functional deficits, involving the collaboration of various professionals such as Speech Therapists, Psychologists, and Physiotherapists 1,5 .
Among non-pharmacological strategies, Transcranial Direct Current Stimulation (tDCS) stands out, demonstrating significant benefits in the rehabilitation of patients, especially when combined with adjunct therapies such as physiotherapy and cognitive training 4 .tDCS is a non-invasive technique capable of modulating cortical neuronal excitability, inducing neuroplasticity mechanisms without directly affecting neurons, reducing the possibility of adverse effects 4,6 .The application involves fixing two silicone electrodes wrapped in saline-soaked sponges and a device providing direct current, lasting 3 to 20 minutes, and low amperage ranging from 0.4 to 2mA 6 .
After a stroke, various events occur in the brain, from changes in membrane excitability to synaptic modifications and the formation of new neuronal networks 6 .Given this, the use of tDCS in stroke rehabilitation makes sense, considering the direct correspondence between the functional changes resulting from the injury and the areas stimulated by tDCS.However, there are still limitations regarding the real effects of its use in individuals affected by stroke 4 .
Therefore, the present study aimed to investigate the effects of tDCS on the functional capacity of post-stroke patients, with a focus on efficacy concerning the motor disorders of individuals.Articles that did not meet the eligibility criteria, those using combined techniques, and those published before the year 2015 were excluded, as described in Table 1.

METHOD
Data extraction was carried out by two independent researchers, and after the selection and full reading of the chosen articles, the study characteristics were summarized in a table for final analysis and the preparation of this systematic review.

RESULTS
Out of 12,571 articles found, 20 were selected and fully read.Thirteen were discarded for not meeting eligibility criteria, leaving seven articles for inclusion (Figure 1).

Yes
There is evidence of improved function in MS.

MS function.
Evidence on improveme nt in MS function, revealing a doseresponse relationship *.
Table 1 presents the characteristics and key findings of the articles.In general, six out of seven studies supported tDCS as a potentially effective therapy 7,8,[10][11][12][13] .

DISCUSSION
This systematic review revealed a low number of clinical studies in this topic.Although the selected studies had robust methodologies, the results were heterogeneous in their outcomes, making it challenging to draw definitive conclusions.One methodological issue to be noted regarding the use of tDCS is the standardization of application parameters, which varied across studies,  showed a smaller effect size (Hedge's g = 0.18, 95% CI = [-0.30,0.66], P = 0.07) 13 .Therefore, it suggests that the evidence is of low quality, and more evidence is needed for tDCS to be introduced in fall prevention intervention 9 .
To date, there is no Level A recommendation achieved for any clinical indication 14 .However, From a marker of microstructural defect in affected motor pathways, it was evident that patients with extensive corticospinal tract lesions responded poorly to inhibitory stimulation (cathodal) of the unaffected hemisphere, while patients with smaller lesions responded well 15 , as there may be a change in the balance between neuron excitation and inhibition, and this hyperexcitability may be a sign of resetting neuron activity in the affected area due to homeostatic mechanisms 16 , providing a conducive environment for axonal sprouting signal in the infarcted area 16 .In this regard, Chhatbar et al. (2016), in their Meta-analysis, showed that bihemispheric tDCS use can simultaneously decrease (cathodal) and increase (anodal) neural activity in the affected and unaffected areas, respectively.
This demonstrates that there is still no standardization of how to apply the technique, and the patient's profile may influence the choice of protocol.In this sense, Fregni et al. (2020) show us that parameters, density, and duration of stimulation are factors that change the outcome, and there is no systematic measurement of how these factors interact, so it is also important to consider the severity of the stroke when planning such factors 7 .Similar findings were found by other authors, suggesting the need for standardization of tDCS application protocols for result reliability 5,6,17 .
An interesting finding reveals the use of tDCS to improve unilateral neglect, characterized by the inability to notice, associate, or respond to events from the contralateral hemibody or hemispace to the brain injury 18 , or even as a disorder of attention, perception, and action post-stroke 18 .As demonstrated in the study This study has limitations that must be acknowledged.The included studies used different and distinct evaluation methods, as despite the eligibility criteria being described, the articles did not have all variables equal and presented reasonably homogeneous results [7][8][9][10][11][12][13] , leading to restrictions in analyzing post-tDCS intervention effects, inducing an isolated result from each article.However,

Figure 1 .
Figure 1.Flowchart of the methodological Search.

by
Oliveira et al. (2014), where tDCS use showed statistically significant improvements in unilateral neglect compared to a control group18 .These studies align withLi et al. (2018) andElsner et al. (2017), showing a significant improvement in mobility and lower limb strength11 and ADLs 12 .However, these same studies did not observe significant improvements in walking speed, endurance, and balance11 and improvement in the affected upper limb function12 .Regarding falls,Denissen et al. (2019) showed little certainty that tDCS use could prevent falls or even reduce their occurrence, thus characterizing it as low-quality evidence9 .

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al. Brazilian Journal of Implantology and Health Sciences Volume 6, Issue 2 (2024), Page 2279-2284.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al. Brazilian Journal of Implantology and Health Sciences Volume 6, Issue 2 (2024), Page 2279-2284.

Table 1 .
Details of the Studies.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al. Brazilian Journal of Implantology and Health Sciences Volume 6, Issue 2 (2024), Page 2279-2284.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al. Brazilian Journal of Implantology and Health Sciences Volume 6, Issue 2 (2024), Page 2279-2284.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al.
7regni et al. (2020)showed a Level B recommendation with anodal ipsilesional and cathodal contralesional tDCS in subacute stroke.It also presented a Level C recommendation for Bilateral montage, with evidence that tDCS is possibly effective for motor rehabilitation in subacute stroke7.

The Influence of Transcranial Direct Current Stimulation on Functional Recovery after Stroke: A Systematic Review Fonseca
et. al. Brazilian Journal of Implantology and Health Sciences Volume 6, Issue 2 (2024), Page 2279-2284.