Effect of transcranial direct current stimulation on in-vivo assessed neuro-metabolites through magnetic resonance spectroscopy: a systematic review

Abstract

Objective: Previous studies have examined the effect of transcranial direct current stimulation (tDCS) on the in-vivo concentrations of neuro-metabolites assessed through magnetic resonance spectroscopy (MRS) in neurological and psychiatry disorders. This review aims to systematically evaluate the data on the effect of tDCS on MRS findings and thereby attempt to understand the potential mechanism of tDCS on neuro-metabolites. Methods: The relevant literature was obtained through PubMed and cross-reference (search till June 2020). Thirty-four studies were reviewed of which, 22 reported results from healthy controls and 12 were from patients with neurological and psychiatric disorders. Results: The evidence converges to highlight that tDCS modulates the neuro-metabolite levels at the site of stimulation, which, in turn, translates into alterations in the behavioural outcome. It also shows that the baseline level of these neuro-metabolites can, to a certain extent, predict the outcome after tDCS. However, even though tDCS has shown promising effects in alleviating symptoms of various psychiatric disorders, there are limited studies that have reported the effect of tDCS on neuro-metabolite levels. Conclusions: There is a compelling need for more systematic studies examining patients with psychiatric/neurological disorders with larger samples and harmonised tDCS protocols. More studies will potentially help us to understand the tDCS mechanism of action pertinent to neuro-metabolite levels modulation. Further, studies should be conducted in psychiatric patients to understand the neurological changes in this population and potentially unravel the neuro-metabolite X tDCS interaction effect that can be translated into individualised treatment.

Opposite pattern of transcranial direct current stimulation effects in middle-aged and older adults: Behavioral and neurophysiological evidence

“…The spatial specificity of this effect also aligns well with the hypothesized mechanism of action of atDCS on the behavioral measures (see above). In fact, there is evidence indicating that tDCS modulates neuro-metabolite levels at the site of stimulation and that this translates into alterations in behavioral outcomes ( Chhabra et al, 2021 ). Notably, the “online” obtained effect on encoding processes is in accordance with the HERA (Hemispheric Encoding Retrieval Asymmetry) model, which postulates that the left DLPFC is crucially involved in the encoding of memory contents, whereas the right DLPFC is crucial for retrieval ( Tulving et al, 1994 ; Nyberg et al, 1996 ; Habib et al, 2003 ; Manenti et al, 2012 ).…”

Transcranial Direct Current Stimulation in neurogenetic syndromes: new treatment perspectives for Down syndrome?

“…In human studies using magnetic resonance spectroscopy, it was found that tDCS alters the levels of multiple neuro-metabolites, particularly glutamate and GABA, specifically but not limited to the site of stimulation (see Chhabra et al, 2021 for a review). In both young and older healthy adults and in patients with neurological and psychiatry disorders, main results confirmed that anodal tDCS induces a reduction in GABA levels (Stagg et al, 2009 ; Kim et al, 2014 ; Bachtiar et al, 2015 ; Antonenko et al, 2017 ), while cathodal tDCS led to a reduction of glutamate levels (Stagg et al, 2009 ).…”

“…In both young and older healthy adults and in patients with neurological and psychiatry disorders, main results confirmed that anodal tDCS induces a reduction in GABA levels (Stagg et al, 2009 ; Kim et al, 2014 ; Bachtiar et al, 2015 ; Antonenko et al, 2017 ), while cathodal tDCS led to a reduction of glutamate levels (Stagg et al, 2009 ). However, the baseline level of the neuro-metabolites may predict the outcome after tDCS, as well as the number of stimulation sessions (Chhabra et al, 2021 ).…”

High definition transcranial direct current stimulation of the Cz improves social dysfunction in children with autism spectrum disorder: A randomized, sham, controlled study

“…tDCS is a noninvasive, portable, safe, and painless method of brain stimulation, which works by changing cortical excitability and functional connectivity between brain regions (Nitsche & Paulus, 2000; Sellaro et al, 2016). The therapeutic mechanism of tDCS is related to polarity‐dependent neurophysiological changes induced by the target cortex, also including local changes in pH and ion concentrations in cerebrospinal fluid, as well as migration and allosteric changes of neuronal membrane proteins (Yamada & Sumiyoshi, 2021; Chhabra et al, 2021; Dipasquale et al, 2017). Current studies mainly suggest that the overall effect of tDCS is associated with changes in excitability and spontaneous discharge rates in the stimulation areas (Nitsche & Paulus, 2000; Philip et al, 2017), as well as cross‐synaptic activation in interconnected remote areas (Chib et al, 2013), with side effects being ignored (Bikson et al, 2016).…”