Introduction
Neurodevelopmental disorders are psychiatric conditions that develop early in life and involve abnormalities in the central nervous system function. Many neurodevelopmental disorders, such as attention deficit/hyperactivity disorder (ADHD), specific learning disorder (SLD), developmental coordination disorder (DCD), and autism spectrum disorder (ASD), are characterized by deficits in executive functions. Impaired executive functions in children lead to a variety of problems, including aggression, inattention, and behavioral and communication problems. Transcranial direct current stimulation (tDCS) has been proposed as a promising treatment for the rehabilitation of neurodevelopmental disorders. Studies suggest that tDCS can improve memory, attention, and executive functions through mechanisms such as changes in resting membrane potential, cerebral blood flow, synaptic plasticity, neurotransmitter levels, and brain network connectivity. The effects of tDCS on executive functions in children and adolescents are contradictory across studies, and the evidence is limited. In addition, so far, no systematic review or meta-analysis has examined the effects of tDCS on executive functions in Iranian children and adolescents with neurodevelopmental disorders. Therefore, the present systematic review and meta-analysis aimed to investigate whether tDCS can improve executive functions in Iranian children and adolescents with neurodevelopmental disorders.

Methods
This is a systematic review and meta-analysis. The related studies were searched in the Web of Science, Scopus, PubMed, Scientific Information Database (SID), and the Comprehensive Portal for Humanities. The search was conducted on studies published from 2014 to 2024 using the following keywords and strategy: (“Transcranial direct current stimulation” OR “tDCS” OR “direct current stimulation”) AND (“executive function” OR “attention” OR “working memory” OR “cognitive flexibility” OR “inhibition” OR “processing speed” OR “planing”) AND (“neurodevelopmental disorders” “attention-deficit/hyperactivity disorder” OR “ADHD” OR “autism spectrum disorder” OR “ASD” OR “specific learning disorder” OR “SLD” OR “Communication disorders” OR “intellectual disability”) AND (“children” OR “adolescents” OR “teenagers”) AND (“Iran” OR “Iranian”).
PICO-SD criteria were used to design eligibility criteria:
 1. Participants: Children (6-12 years) and adolescents (13-18 years) residing in Iran with neurodevelopmental disorders (ADHD, ASD, SLD, DCD, intellectual disability (ID), communication disorders),
 2. Interventions: Experimental group with tDCS alone (anodal protocol), 
3. Control: Control group with sham tDCS or no additional intervention,
 4. Outcome: Executive functions (working memory, attention, inhibition, cognitive flexibility, planning, and processing speed),
 5. Study design: Experimental or quasi-experimental studies with pre-test/post-test design and a control group. Two authors (Fateme Gheysari, Mehrdad Mazaheri) independently assessed the eligibility of studies. 
Articles that met the inclusion criteria were selected for full-text review. Any disagreements about the eligibility of the studies were discussed and solved by reaching a consensus. The data were analyzed in CMA software, version 2. To measure the effect of tDCS, the standardized mean difference (SMD) was calculated as the difference between groups in mean values divided by the pooled standard deviation, with a 95% confidence interval (CI). Duval and Tweedie’s trim-and-fill method and funnel plots were used to analyze sensitivity and publication bias. 

Results
A total of 536 articles were first retrieved. Irrelevant literature, review studies, duplicates, and case reports were identified and removed based on title and abstract reviews, resulting in 37 full-text articles being retrieved. Fourteen studies were excluded due to publication bias and lack of data on the mean and standard deviation of the control and experimental groups. Finally, 23 articles were included in the systematic review and meta-analysis. There were 74 effect sizes, and studies evaluated a total of 614 patients in the active tDCS, sham tDCS, and control groups. The characteristics of these studies are presented in Table 1.




The effect size of tDCS on the executive functions was 1.04 (SMD=1.04, 95% CI, 0.83%, 1.25%; P<0.001). The effect size was statistically significant, indicating a significant impact of tDCS on the executive functions of Iranian children and adolescents with neurodevelopmental disorders. Forest plot of the effectiveness of tDCS on cognitive functions of patients compared to controls is shown in Figure 1. Based on the results of the trim-and-fill method, adding eight studies to the left side of the funnel plot make the mean of the funnel plot symmetrical.



Conclusion
This meta-analysis showed that tDCS was effective in improving executive functions in Iranian children and adolescents with neurodevelopmental disorders. The current research provides preliminary evidence for the therapeutic potential of tDCS for ADHD, ASD, and childhood and adolescent dyslexia.
The included studies had low methodological quality, as they lacked randomized clinical trials. Few studies reported follow-up data, so we did not have sufficient data to assess the persistence of the tDCS effect. There was a lack of sham tDCS in some studies. Therefore, future randomized, sham-controlled clinical trials and studies with follow-up periods are needed to assess the effectiveness of tDCS on executive functions in Iranian children and adolescents with neurodevelopmental disorders. Safety and tolerability indicators and side effects were not reported in the studies to demonstrate the safety of tDCS for children and adolescents. Therefore, future studies should use a standardized adverse event questionnaire to assess tolerance and adverse events of tDCS. Additionally, the included studies varied in terms of participants’ age, gender, and level of disorder, resulting in heterogeneous data from which our results were obtained. In addition, most of the studies examined the effect of tDCS on executive functions in children and adolescents with ADHD and SLD. There is a need for future studies to examine the effects of tDCS on executive functions in children and adolescents with other neurodevelopmental disorders, especially ASD.

Ethical Considerations

Compliance with ethical guidelines
This is a systematic review and meta-analysis with no human or animal samples; therefore, the need for an ethical approval code was waived.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for profit sectors.

Authors contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflicts of interest
The authors declare no conflicts of interest.




 

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