So just a review and NOTHING ON CREATING PROTOCOLS FOR APHASIA! Useless.
Transcranial Direct-Current Stimulation and Behavioral Training, a Promising Tool for a Tailor-Made Post-stroke Aphasia Rehabilitation: A Review
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Introduction
Aphasia is an acquired language disorder resulting from damage to the portions of the brain which are responsible for language comprehension and formulation. The most common causes of this disorder include vascular lesions, encephalic traumatic injury, and brain tumors (Marangolo and Caltagirone, 2014), with a prevalence of 250,000 cases in the United Kingdom and 1 million in the United States (Crinion, 2016). Aphasia may also be associated with other degenerative, inflammatory, autoimmune or parasitic disorders. About 0.7 to 3% of people presenting with multiple sclerosis also show aphasic symptoms (Naro et al., 2021).
Although damages to specific brain areas and their connections mainly occur in the left hemisphere, functional magnetic resonance imaging (fMRI) studies such as the one carried out by Thompson and den Ouden (2008) showed that in some cases the dominant language areas can be located in the right hemisphere.
Aphasia can involve different levels of language processing with impairments in both oral and written comprehension and production. Most patients who experience aphasia show some degree of spontaneous recovery within the first two to three months, due to a functional neural reactivation and reorganization. The most important factors that determine recovery are the lesion size and location, the type and severity of aphasia, the treatment received and, to some extent, the nature of early hemodynamic response (Watila and Balarabe, 2015).
From the second half of the twentieth century, different rehabilitation perspectives and therapeutic interventions for aphasia rehabilitation have been developed. The most recommended treatment for this disorder is Speech and Language Therapy (SLT). However, it is argued that SLT would lead to moderate effects, even when administered at high intensity. For this reason, over the last few years, new strategies have been implemented to enhance the effects of traditional rehabilitation.
These effects depend on the metaplasticity, which “refers to activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD)” (Abraham and Philpot, 2009). Changes in the pathomechanisms underlying psychiatric and neurological disorders are possible by acting on metaplasticity (Cantone et al., 2021). Non-Invasive Brain Stimulation Techniques (NIBS) can be a beneficial tool to transiently modulate cortical excitability and lead to lasting changes after the stimulation time (Fisicaro et al., 2020). Non-invasive brain stimulation techniques facilitate the activation of single brain areas, or the inhibition of other ones whose hyperactivation could have a maladaptive effect on cognitive recovery (Simonetta-Moreau, 2014). One of the main neuromodulation tools is Transcranial Magnetic Stimulation (TMS), which generates magnetic field pulses under the scalp. A single impulse leads to a short-term effect, while a sequence of stimulation on the same region of interest can generate long-term effects. These can either be inhibiting or excitatory, depending on the stimulation frequency (Fisicaro et al., 2020). For instance, repetitive TMS (rTMS) can successfully treat both motor and non-motor symptoms in stroke patients, including depression, which often affects the rehabilitation process after a stroke (Fisicaro et al., 2019). Both for safety and cost issues, tDCS is often preferred over TMS.
The main strength of tDCS is its feasibility associated with relatively minor side effects, if safely and properly administered. Another strength is that it shows promise as an effective and versatile neurostimulation tool. It has the potential to be a treatment for several conditions characterized by an alteration of the cerebral cortex activation. Indeed, it has been proven to have beneficial effects on both neuropsychiatric and neurological disorders, such as mood disorders, substance abuse, Alzheimer’s and Parkinson’s disease, multiple sclerosis, as well as post-stroke motor and cognitive impairment (Lefaucheur et al., 2017). Additionally, tDCS can be applied in sham mode, making it easier to carry out a single-blind study (Nitsche et al., 2003). Therefore, tDCS represents one of the most promising tools for the treatment of aphasia (Biou et al., 2019). It does not directly induce an action potential, but it delivers a continuous current flow at a low intensity (1/2 mA) instead. Transcranial Direct-Current Stimulation requires two electrodes, an anode and a cathode, which are generally placed on the scalp. Depending on the polarity and the consequent positioning of the electrodes, the experimenter can obtain a depolarizing effect, thus favoring neuronal firing (anodic tDCS), or a hyperpolarized effect by decreasing the discharge rate (cathodic tDCS) (Liebetanz et al., 2002). Because of the electrode size, tDCS allows the stimulation of large cortical areas, with a consequent reduction of stimulation focality. The effects of tDCS are variable and depend on the stimulation duration, the current density, the characteristics of the neuronal tissue involved and the current flow direction, which can move from the anode to the cathode or vice versa (Chase et al., 2020). Non-invasive brain stimulation is an important resource in neuropsychological rehabilitation, however, its application is not risk-free, as most non-invasive current induction tools. The ultimate goal of applying tDCS in rehabilitation is to re-establish an interhemispheric balance by promoting functional brain reorganization and facilitating relearning (Simonetta-Moreau, 2014).
To date, scientific literature offers a comprehensive overview of the several therapeutic treatments used for the rehabilitation of aphasia. Unfortunately, these many specific training methods lead to moderate effects. Therefore, a number of techniques have been implemented over the years in support of speech therapy and neuropsychological rehabilitation to promote a faster and more effective recovery. As tDCS is the most widely used method in rehabilitation, this review aims at examining those studies which associate rehabilitation with tDCS, and investigate its effectiveness (Marangolo, 2017).
Summary of Findings
Several articles were selected and analyzed for this review. PubMed, PsycInfo and Cochrane were consulted for the systematic search of the relevant articles. As for keywords, different combinations of the terms “aphasia,” “speech impairment,” “Broca’s aphasia,” “non-fluent aphasia,” “tDCS,” “transcranial direct current stimulation,” “non-invasive brain stimulation,” “cognitive rehabilitation,” “neurorehabilitation,” “aphasia training,” “cognitive training,” “language recovery” were used. The whole research process started from an accurate analysis of the most quoted and detailed reviews available on this topic. Out of the 37 most relevant reviews and meta-analysis, only 33 specifically analyzed tDCS studies on aphasic patients with acquired cerebral lesions (Figure 1). The experimental studies were subsequently extracted from these reviews. From a total of 93 studies analyzed, 46 were considered the most relevant. The main exclusion criterion was the type of aphasia: only tDCS studies conducted on a sample of aphasic patients at the chronic phase with an acquired cerebral lesion were included. Studies on other types of aphasia (e.g., primary progressive aphasia) or carried out earlier than six months from the damage were excluded. Moreover, it was decided to include only studies enrolling a minimum of three subjects, thus removing single-case studies from the total count of papers. The conclusions drawn from those studies could have been weak and not significant enough for the purpose of this study. All experimental studies using TMS or other brain stimulation techniques other than tDCS were not included as well. Out of the 79 articles included in this review, 26 were reviews, 7 meta-analysis and 46 were experimental studies (Supplementary Table 1).
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