Transcranial direct current stimulation (tDCS) for improving aphasia after stroke: a systematic review with network meta-analysis of randomized controlled trials

So anodal tDCS was found to be the best. Still useless, NO PROTOCOL.

Transcranial direct current stimulation (tDCS) for improving aphasia after stroke: a systematic review with network meta-analysis of randomized controlled trials

• Bernhard Elsner,
• Joachim Kugler &
• Jan Mehrholz 

Journal of NeuroEngineering and Rehabilitation volume 17, Article number: 88 (2020) Cite this article

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Summary

Background

Transcranial Direct Current Stimulation (tDCS) is an emerging approach for improving aphasia after stroke. However, it remains unclear what type of tDCS stimulation is most effective. Our aim was to give an overview of the evidence network regarding the efficacy and safety of tDCS and to estimate the effectiveness of the different stimulation types.

Methods

This is a systematic review of randomized controlled trials with network meta-analysis (NMA). We searched the following databases until 4 February 2020: Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, CINAHL, AMED, Web of Science, and four other databases. We included studies with adult people with stroke. We compared any kind of active tDCS (anodal, cathodal, or dual, that is applying anodal and cathodal tDCS concurrently) regarding improvement of our primary outcome of functional communication, versus control, after stroke. PROSPERO ID: CRD42019135696.

Results

We included 25 studies with 471 participants. Our NMA showed that tDCS did not improve our primary outcome, that of functional communication. There was evidence of an effect of anodal tDCS, particularly over the left inferior frontal gyrus, in improving our secondary outcome, that of performance in naming nouns (SMD = 0.51; 95% CI 0.11 to 0.90). There was no difference in safety between tDCS and its control interventions, measured by the number of dropouts and adverse events.

Conclusion

Comparing different application/protocols of tDCS shows that the anodal application, particularly over the left inferior frontal gyrus, seems to be the most promising tDCS treatment option to improve performance in naming in people with stroke.

Introduction

Non-invasive brain stimulation (NIBS) is an emerging approach for enhancing neural plasticity and hence rehabilitation outcomes after stroke. There are several stimulation procedures, such as repetitive transcranial magnetic stimulation (rTMS) [1], transcranial direct current stimulation (tDCS) [2,3,4], transcranial alternating current stimulation (tACS) [5], and transcranial pulsed ultrasound (TPU) [6]. Particularly for rTMS and tDCS a considerable evidence base for NIBS has emerged in the recent years.
tDCS has some advantages when compared to the other stimulation procedures: it is relatively inexpensive, easy to administer and portable, being an ideal add-on therapy during stroke rehabilitation. By applying a weak and constant direct current to the brain tDCS has the ability to either enhance or suppress cortical excitability, with effect lasting up to several hours after the stimulation [7,8,9]. Hypothetically, this technique makes tDCS a potentially useful tool to for example, post-stroke speech and language therapy (SLT) by modulating inhibitory and excitatory neuronal networks of the affected and the non-affected hemisphere [10, 11]. There are three different stimulation types.

• In anodal stimulation, the anodal electrode (+) usually is placed over the lesioned brain area and the reference electrode over the contralateral orbit [12]. This leads to subthreshold depolarization, hence promoting neural excitation [3].
  
• In cathodal stimulation, the cathode (−) usually is placed over the non-lesioned brain area and the reference electrode over the contralateral orbit [12], leading to subthreshold polarization and hence inhibiting neural activity [3].
  
• Dual tDCS means the simultaneous application of anodal and cathodal stimulation [13].
  

However, there are no clear guidelines regarding to the tDCS type, electrode configuration [14, 15], the amount of current applied and the duration of tDCS, or the question if tDCS should be applied as a standalone therapy or in combination with other treatments [16].

Rationale

There is so far conflicting evidence from systematic reviews of randomized controlled trials on the effectiveness of different tDCS approaches for improving aphasia after stroke. For example, over the past decade more than 15 randomized clinical trials have investigated the effects of different tDCS stimulation techniques for stroke, and there are at least 23 ongoing trials [17]. However, the resulting network of evidence from randomized controlled trials (RCTs) investigating different types of tDCS (i.e., anodal, cathodal or dual) as well as their comparators like sham tDCS or SLT has not yet been analyzed in a systematic review so far.
A network meta-analysis (NMA), also known as multiple treatment comparison meta-analysis or mixed treatment comparison analysis, allows for a quantitative synthesis of the evidence network. This is made possible by combining direct evidence from head-to-head comparisons of three or more interventions within randomized trials with indirect evidence across randomized trials on the basis of a common comparator [18,19,20,21]. Network meta-analysis has many advantages over traditional pairwise meta-analysis, such as visualizing and facilitating the interpretation of the wider picture of the evidence and improving understanding of the relative merits of these different types of neuromodulation when compared to sham tDCS and/or another comparator such as SLT [22, 23]. By borrowing strength from indirect evidence to gain certainty about all treatment comparisons, network meta-analysis allows comparative effects that have not been investigated directly in randomized clinical trials to be estimated and ranked [23, 24].

Objective

To summarize the evidence network of randomized controlled trials of tDCS for improving functional communication and language function after stroke, as well as its safety.

Methods

Protocol and registration

We published a study protocol, which has been registered in the PROSPERO database under the ID CRD42019135696. Our protocol adheres to the PRISMA extension statement for NMA [25].

Role of the funding source

There was no funding source for this study.

Eligibility criteria

We included studies with adults who had experienced a stroke. We compared any kind of active tDCS (anodal, cathodal, or dual, that is applying anodal and cathodal tDCS concurrently) for improving our primary outcome of functional communication and our secondary outcome of language function after stroke. Another secondary outcome was safety, measured by the number of dropouts and adverse events. We defined active tDCS as any application of direct current to the skull lasting longer than one minute. This is approximately the time it takes to fade in and fade out the sham application of tDCS in order to produce perceivable sensations on the skin similar to active tDCS [26]. We included all studies with outcome measures evaluating functional communication and for language function. We included all genuine RCTs and genuine randomized controlled cross-over trials which compared tDCS with any other intervention. We analyzed only the first intervention phase of trials with a cross-over design, and assumed between-group differences to be identical to those in trials with a parallel group design. We combined different stimulation durations, different electrical currents applied and different stimulation locations for the same stimulation type (that is anodal, cathodal, or dual tDCS) for our primary analysis. For our a priori defined subgroup analysis we chose the location of stimulation as a potential effect modifier.

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