Customized Application of tDCS for Clinical Rehabilitation in Alzheimer’s Disease
Ikusi/ Ireki
Data
2021Egilea
Rodella, Claudia
Cespón, Jesús
Repetto, Claudia
Pellicciari, Maria Concetta
Rodella C, Cespón J, Repetto C and Pellicciari MC (2021) Customized Application of tDCS for Clinical Rehabilitation in Alzheimer’s Disease. Front. Hum. Neurosci. 15:687968. doi: 10.3389/fnhum.2021.687968
Laburpena
Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by cognitive-behavior
deficits, which strongly impact daily-life activities (Weintraub et al., 2012). Currently, the
limited efficacy of pharmacological treatments has encouraged researchers to develop nonpharmacological
interventions, such as cognitive training and non-invasive brain stimulation
(NIBS) treatments, designed to prevent or delay cognitive impairment (Cass, 2017; Cespón et al.,
2018).
In recent years, there has been growing interest in evaluating the use of NIBS to improve
cognitive functioning in healthy and pathological aging (Hsu et al., 2015; Cappon et al., 2016),
and to integrate this modality in dementia rehabilitation programs (Prehn and Flöel, 2015). In
particular, some experimental (Ferrucci et al., 2008; Boggio et al., 2012; Marceglia et al., 2016) and
meta-analytical (Hill et al., 2016; Indahlastari et al., 2021) studies have supported the clinical utility
of transcranial direct current stimulation (tDCS). The potential of tDCS lies on its mechanisms
of action. Specifically, tDCS has local impacts on the GABA/glutamate balance (Stagg et al., 2009),
which has been found to be altered in AD patients (Guerra et al., 2011). It also influences functional
connectivity, synchronization, and oscillatory activities in prefrontal cortex (Keeser et al., 2011), a
region substantially affected by AD. In addition, tDCS may have non-neuronal effects, as almost
all tissues and cells are sensitive to electric fields (Ruohonen and Karhu, 2012). For instance, tDCS
could potentially modulate the inflammatory response and the conformation of beta-amyloid and
other pathological proteins (Toschi et al., 2009), involved in the progression of AD.
Nevertheless, several studies have found very little or null effects (e.g., Horvath et al., 2015)
of tDCS on various cognitive domains. These results have been attributed to high inter-study
(Pellicciari and Miniussi, 2018) and inter-individual variability in response to tDCS (Li et al., 2015),
even though the sources of inter-individual variability were not clearly identified. Importantly,
although the sheer number of parameters (e.g., polarity, intensity, location, electrodes size) that
can be varied could represent a weakness of these protocols, they alsomean the application of tDCS
is highly customizable. Consequently, exploring inter-individual differences and how these might
influence the effects of tDCS has become crucial. In the following section, we point to Brain Reserve
(and the related grade of brain atrophy), Cognitive Reserve and baseline performance measures
as potential sources for the inter-individual variability reflected in results obtained after applying
tES protocols.