A comparison between the effectiveness of transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) on corticospinal excitability in chronic stroke: A review of literature

A B S T R A C T Background: Stroke is deﬁned as abrupt onset neurological disorder of vascular aetiology. Stroke represented 1.2% of total deaths in India. Upper extremity paresis is a leading cause of functional disability after a stroke as it causes difﬁculty in everyday life. Decreased corticospinal excitability of lesioned hemisphere is a well recognized neurophysiological consequence after stroke. The level of excitability of affected hemisphere correlates with motor function and predicts recovery. Transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) have gained popularity in the stroke rehabilitation literature. Aims and Objectives: To compare the effects of transcranial direct current stimulation and repetitive transcranial magnetic stimulation on corticospinal excitability in chronic stroke. Materials and Methods: A thorough electronic search was made through various databases such as PubMed, ScienceDirect and Google Scholar. The total number of articles included were 20 of which 8 were RCTs, 2 systematic reviews, 3 literature reviews, 4 experimental studies and 3 crossover trials. Conclusion: tDCS has therapeutic applications in restoring the interhemispheric balance between the unlesioned and lesioned hemisphere, hence alleviating symptoms such as apraxia, spatial neglect, and gait. tDCS can also be used along with other peripheral intervention techniques such as occupational therapy, robot-assisted arm therapy and physical therapy to accelerate recovery in chronic stroke patients. On the other hand, rTMS has been used as an evaluative, prognostic (along with CT scan and MRI) and rehabilitative tool in common practice to assess disability in chronic stroke patient.


Introduction
The WHO 2005 clinically defines stroke as 'the rapid development of clinical signs and symptoms of a focal neurological disturbance lasting more than 24 hours or leading to death with no apparent cause other than vascular origin'. Based on the duration, stroke can be divided into hyperacute which involves the first 6 hours, acute involving 6-48hours, sub-acute 48hours -weeks and chronic stroke weeks to months based on the onset of stroke symptoms. 1 Stroke is clinically divided into two broad categories that define its pathophysiology: Ischemic stroke accounts for 50%-85% and hemorrhagic accounts for 1% -7% of all strokes worldwide.
In developing countries, stroke is the first leading cause of disability. Recent rapid socioeconomic changes have led to changes in people's lifestyle, leading to workrelated stress and altered food habits, raising the risk of hypertension. Dalal P.M et al. 2 conducted a 2-year study from January 2005 to December 2006 which revealed that 456 subjects of which 238 were males and 218 females, had the first-ever stroke, indicating an annual incidence in subjects of 25 years and above of 145/100,000 persons for males it is 149/100,000 persons and for females, it is 141/100,000 persons. WHO estimates suggest that by 2050, 80% stroke cases in the world would occur in low and https://doi.org/10.18231/j.jsip.2020.008 2456-7787/© 2020 Innovative Publication, All rights reserved. 57 middle-income countries mainly India and China of which brain stroke is the third largest killer in India after a heart attack and cancer and the second largest in the world. 3 Several neurological functions are impaired by stroke most commonly motor disability contralateral to the side of lesion. 4 Reduced cortical excitability of the lesioned hemisphere is a well known neurophysiological consequence following stroke. The level of excitability of lesioned hemisphere correlates with the motor function and predicts recovery. 5 Motor recovery after stroke is directly related to neural plasticity. Various strategies have been developed to enhance motor recovery. Repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) can induce changes in human cortex excitability. In rTMS, a wire-coil shaped in a figure of 8 fashion is placed over the scalp to generate a transient local magnetic field. High-frequency rTMS increases cortical excitability and low-frequency rTMS inhibits cortical excitability. The pulsed magnetic field enters the brain and creates an electric current that flows through the neurons resulting in neuronal depolarization. On the other hand, tDCS modulates the neuronal membrane potential through polarizing currents by weak constant direct current, hence influencing the levels of excitability and modulating the spontaneous firing rate of neurons. Anodal tDCS increases the excitability of the stimulated cortex, whereas cathodal tDCS decreases the excitability of the stimulated cortex. 4 The purpose of noninvasive brain stimulation is that the modulation of cortical excitability may induce neural plasticity and/or interfere with maladaptive neural activation that subsequently weakens motor function and impedes motor recovery.
Since the mechanism, underlying non-invasive brain stimulation remains to be elucidated and insufficiently recognized, the study aims to provide a comprehensive overview of the role of non-invasive brain stimulation to enhance corticospinal excitability and use-dependent plasticity in chronic stroke.

Materials and Methods
Data were collected for this literature review from the following sources 1) Pub Med, 2) Google Scholar, 3) Science Direct, 4) Brain Journals. Only English articles published after year 2005 concerned with tDCS and rTMS and chronic stroke were used for the study. Articles of other languages published before the year 2005 were excluded from the study.

Procedure
A structured literature search was done using databases such as Pub Med, ScienceDirect, Pedro, Brain journals, APTA etc. Keywords used are tDCS, rTMS, chronic stroke, corticospinal excitability. Boolean concept of using and was used for a more specific search. Articles based on these keywords were not found on databases such as Pedro and APTA. Vancouver method was used for reference.

Discussion
This study provides substantial evidence that both transcranial direct current stimulation and repetitive transcranial magnetic stimulation can be used as a potential neuro rehabilitative tool in mitigating motor deficits and reestablishing the functional independence in chronic stroke patients. On comparison, tDCS due to its compatibility has therapeutic applications in restoring the inter-hemispheric balance between the unlesioned and lesioned hemisphere, hence alleviating symptoms such as apraxia, spatial neglect, dysphagia and gait. tDCS can also be used along with other peripheral intervention techniques such as occupational therapy, robot-assisted arm therapy and physical therapy to accelerate recovery in chronic stroke patients. On the other hand, rTMS has been used as an evaluative, prognostic (along with CT scan and MRI) and rehabilitative tool in common practice to assess and address disability in chronic stroke patients.
There is the dearth of literature suggesting the simultaneous use of both these novel techniques to shape adaptive brain processes following a stroke in such a way that sustained success is achieved in the amelioration of symptoms.   Double-blind, crossover randomized trial The subject participated in the crossover experiment in two series each comprising of two sessions, an intervention session (dual tDCS/sham was applied during motor skill learning of the paretic upper limb) and retention session (imaging session 1week later).

6/11
Continued on next page

Conclusion
Both tDCS and rTMS are promising adjuvants in mitigating motor deficits post stroke and promoting recovery, and hence should be included in regular clinical practice under supervision of trained physiotherapist.

Source of funding
None.

Conflict of Interest
None.