Long-lasting improvement following tDCS treatment combined with a training for reading in children and adolescents with dyslexia

Highlights

• Two groups of dyslexic children received active or sham tDCS coupled with a training.

• Only the active group showed long-lasting benefits in reading, up to six months.

• No short-term or long-term adverse effect was documented.

Abstract

Noninvasive brain stimulation transiently modulates reading ability in individuals with dyslexia by facilitating the underactive neural pathways in them. However, its long-term effects have not been determined.

This study confirmed the ameliorative effects of multiple sessions of transcranial direct current stimulation (tDCS) combined with a training for reading on the reading abilities of children and adolescents with dyslexia and examined whether they are long-lasting.

Twenty-six children and adolescents with dyslexia received 3 20-min sessions per week for 6 weeks (18 sessions) of left anodal/right cathodal tDCS, set to 1 mA, over the parieto-temporal regions, combined with training for reading. The participants were randomly assigned to receive active or sham treatment. Reading measures (text, high- and low-frequency words, non-words) were recorded before and immediately after the treatment and 1 and 6 months later.

The long-term tolerability to tDCS was also evaluated.

The active group received long-lasting benefits in reading. Specifically, the non-word reading efficiency index improved at every time point, as did the low-frequency word reading efficiency index at 1 and 6 months after the end of the treatment. No differences emerged in the sham group. No long-term adverse effects were documented.

This study provides evidence of persistent improvements in reading in children and adolescents with dyslexia, constituting a new rehabilitative approach for the remediation of dyslexia.

Introduction

Reading is one the most important academic skills, affecting every aspect of learning. In modern literate society, the academic, social, emotional, and economic consequences of reading problems can be profound and far-reaching. Individuals with dyslexia have difficulties in reading acquisition, despite adequate instruction and intelligence and intact sensory abilities (Lyon et al., 2003). This disorder has a strong genetic component (Hensler et al., 2010) and has been estimated to affect 5–17% of children (Ferrer et al., 2010).

Due to its centrality in other types of learning, reading remains a problematic skill for many children with dyslexia, which, if left unremediated, continues to hinder academic progress and can lead to feelings of anxiety, anger, low self-esteem, and depression. Thus, it is critical to remediate reading problems over the long term in a timely and effective manner.

Training programs for reading in the remediation of dyslexia have not been effective in well-controlled studies. Some evidence is available (for a review, see McArthur et al., 2012) in support of the efficacy of phonics intervention (training on phoneme awareness and grapheme-phoneme conversion). McArthur et al. (2012) examined over 140 studies and found that only 11 used randomization or quasi-randomization to allocate participants (English-speaking children, adolescents, and adults) to an intervention or control group. Phonics training appeared to improve certain reading skills (non-word reading accuracy, word reading accuracy, and letter-sound knowledge). For other outcomes (word reading fluency, spelling, phonological output, and reading comprehension), the effects were not statistically significant.

Further, a meta-analysis by Galuschka et al. (2014) conducted an extensive literature search on the effectiveness of treatment approaches for children and adolescents with reading disabilities but identified only 22 randomized, controlled studies. The meta-analysis evaluated various treatment approaches, such as reading fluency training, phonemic awareness instructions, reading comprehension training, phonics instructions, auditory training, medical treatments, and interventions with colored overlays or lenses. The study concluded that phonics instruction was the most frequently examined remedial approach and the only approach with statistically significant efficacy with regard to reading and spelling performance in children and adolescents with dyslexia.

Although remediation that is based on decoding strategies appears to be the most effective (Gabrieli, 2009), several studies have proposed innovative interventions, such as action video games (Franceschini et al., 2013). Given the effects of video games on attention abilities (Green and Bavelier, 2012), recent studies (Franceschini et al., 2017, Franceschini et al., 2013) have shown that playing action video games improves children's reading speed and attentional skills (visual, auditory, cross-modal). These results have been confirmed in English-speaking children and adults with dyslexia (Franceschini et al., 2017, Antzaka et al., 2017).

Neuroimaging data indicate that successful remediation is the result of reading changes and is mediated by neural activity (Simos et al., 2002, Aylward et al., 2003, Temple et al., 2003). Moreover, in children and adults with dyslexia (Costanzo et al., 2012, Costanzo et al., 2013, Costanzo et al., 2016a, Costanzo et al., 2016b, Heth and Lavidor, 2015, Turkeltaub et al., 2012), directly inducing alterations in excitability in various cortical regions using noninvasive brain stimulation methods, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), effects positive changes in reading.

It is unknown whether the gains with such treatments are long-lasting. Few studies have verified the long-term effect of training for reading—documenting, for example, that improvements in reading after an intensive remediation training program for at-risk children was maintained after 2 years of follow-up (Torgesen et al., 2001).

Similarly, there are little data on the long-term effects of training with noninvasive brain stimulation-based methods. Although the changes in neuronal excitability that are induced by noninvasive brain stimulation methods are primarily short-lived, complex neurobiological mechanisms that are related to shifts in synaptic excitability have the potential to cement such changes (Cirillo et al., 2016). The actual effectiveness of brain-based treatments should be established by verifying their long-term effects. Talsma et al. (2017) demonstrated that tDCS enhances working memory performance in healthy adults immediately after a single stimulation session and 24 h later. André et al. (2016) reported the positive effects of tDCS on short-term visual memory, verbal working memory, and executive control 2 weeks after the end of the treatment in adults with mild vascular dementia. In children and adolescents with dyslexia, Costanzo et al. (2016b) found that the effects of tDCS (combined with training for reading) on reading were maintained 1 month after the intervention was concluded.

This study was aimed to confirm our previous findings in a larger group of children and adolescents with dyslexia and determine whether the effects persist for up to 6 months after the end of the treatment. The tolerability to tDCS, found to be good at the short-term follow-up (Costanzo et al., 2016b), was also verified over the long term.

Our hypothesis is that children and adolescents with dyslexia would continue to experience the positive effects on low-frequency words and non-words after 18 sessions of tDCS (Costanzo et al., 2016b) and 1 and 6 months after the end of the treatment, thus demonstrating that noninvasive brain stimulation is an effective and safe tool for the rehabilitation of dyslexia during developmental ages that has stable effects.