Effects of Non-Invasive Neurostimulation on Autism Spectrum Disorder: A Protocol for a Systematic Review

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder marked by characteristic impairments in social communication and interaction, as well as restricted and repetitive behavior. There is a continued need for exploring effective interventions and evaluating treatment options for ASD. Transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are noninvasive neurostimulation techniques which have shown clinical benets in adult psychiatric conditions. While in ASD patients, no guideline has so-far recommended on the application of NIBS due to a lack of high-quality synthetic evidence. Therefore, objective of this study will be to systematically evaluate the evidence on clinical effects of non-invasive neurostimulation in patients with ASD.


Abstract
Background Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder marked by characteristic impairments in social communication and interaction, as well as restricted and repetitive behavior. There is a continued need for exploring effective interventions and evaluating treatment options for ASD. Transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are noninvasive neurostimulation techniques which have shown clinical bene ts in adult psychiatric conditions. While in ASD patients, no guideline has so-far recommended on the application of NIBS due to a lack of high-quality synthetic evidence. Therefore, objective of this study will be to systematically evaluate the evidence on clinical effects of non-invasive neurostimulation in patients with ASD.

Methods
We designed and registered a study protocol for a systematic review. A comprehensive search from database inception onwards will be conducted in PubMed, EMBASE and Cochrane library. Randomized and non-randomized sham-controlled studies assessing the effects of noninvasive neurostimulation in patients with ASD will be identi ed. Eligibility of citations retrieved will be independently screened by two reviewers. The risk of bias and quality of included studies will be appraised using appropriate tools. A narrative synthesis around the features of the evidences will provided. There may be a chance for metaanalysis to pool the estimates of studies included if three studies or more meet the requirements for meta-analysis. If so, a random-effects model maybe applied. And we will evaluate heterogeneity between studies using the I² statistic.

Discussion
This systematic review will provide a broad and comprehensive evaluation of the evidence on clinical effects of TMS and tDCS in patients with ASD. Our ndings will be reported according to the PRISMA guidelines and may add more con dence when healthcare professionals are making informed decisions about the choice of this therapy. Results will be published in a peer-reviewed journal.

Systematic review registration:
Submitted to PROSPERO, 20/11/2021 Background Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder marked by characteristic impairments in social communication and interaction, as well as restricted and repetitive behavior. The overall prevalence of ASD in Europe, Asia, and the United States has grown dramatically and was estimated to range from 2 to 25 per 1000 (1). Regarding the economic burden of this condition, it requires as high as $2.4 million in the United States and £1.5 million in the United Kingdom to support an individual with an ASD and intellectual disability during his or her lifespan. While for an individual with an ASD without intellectual disability, the cost of was still high, estimated to be $1.4 million in the United States and £0.92 million (US $1.4 million) in the United Kingdom (2). To date psychotherapy is the treatment of choice while only small to medium effects of improvement has been achieved. Therefore, there is a continued need for exploring effective interventions and evaluating treatment options for ASD.
In the past decade, noninvasive neurostimulation methods, including transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS), have been proposed as potential therapeutic options for modi cation of the pathological neuroplasticity (or even plasticity induction) involved in neuropsychiatric disorders including ASD (3,4). Application of noninvasive neurostimulation on child and adult populations has proved to be well tolerated and shows a favorable therapeutic pro le.
Thus, recommendations of this approach in dealing with neuropsychiatric disorders such as unipolar major depression was provided by several clinical guidelines, corresponding to a Level A of evidence (5).
While in ASD patients, no guideline has so-far recommended on the application of NIBS due to a lack of high-quality synthetic evidence.
A systemic review by Ali Khaleghi et al has narratively synthesized published articles before April 2018 (6). It demonstrated that NIBS methods could be helpful for treating some dimensions of ASD such as repetitive behavior, sociability or some aspects of executive and cognitive functions. However, a majority of studies included showed a moderate to low quality and bias could easily be identi ed. Therefore, conclusion has been drawn to emphasize on further review and analysis on randomized, sham-controlled trials. Recently, trials investigating the effectiveness of NIBS on ASD patients are continuously rising. Christina Luckhardt et al. systematically searched for randomized, sham-controlled clinical trials of tDCS before may 2020 in individuals with ASD (7). Six eligible studies were identi ed and results indicated initial support for improved cognitive and social communication skills in ASD following tDCS stimulation.
Nonetheless, the effects of TMS were not evaluated in their review. Therefore, a comprehensive review and evaluation of the up-to-date high-quality evidence in exploring effectiveness of NIBS including both tDCS and TMS in managing ASD patients, is till warranted.

Objectives
This study will systematically review available data on past, ongoing and upcoming studies using TMS or tDCS as a therapeutic intervention in patients with ASD diagnosed by a solid method. This review will cover the following aspects: 1. Evaluating the effects of TMS and tDCS on core symptoms in patients with ASD.
2. Evaluating the effects of TMS and tDCS on neurocognition or psychiatric comorbidities other than core de cits.
3. Summarizing stimulation parameters that have been used in current TMS and tDCS administration and explaining how have these parameters affected results?
4. Assessing the risk of bias and quality of the current evidence regarding this issue.

Methods
The present protocol is being reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P) statement (8). This protocol has been submitted within the International Prospective Register of Systematic Reviews (PROSPERO) database (date: 20/11/2021). Any amendments made to this protocol when conducting the study will be outlined in PROSPERO and reported in the nal manuscript. When this systematic review and meta-analysis being completed, it will be reported in accordance with the reporting guidance provided in the PRISMA statement (9). Since there is no human or animal experiment, the study will not require an ethical approval.

Eligibility criteria
Studies will be included according to the following criteria: participants, interventions and comparators, outcome(s) of interest and study design.

Participants
Patients diagnosed with autism spectrum disorder based on a valid method will be included. The

Interventions
Noninvasive neurostimulations including both transcranial direct current stimulation and transcranial magnetic stimulation will be included. Variation of tDCS like transcranial alternating current stimulation (tACS) will also be included. As for TMS studies, we will include low-frequency rTMS (LF-rTMS), highfrequency rTMS (HF-rTMS), intermittent theta burst stimulation (iTBS), continuous theta burst stimulation (cTBS), paired associative stimulation (PAS), as well.

Comparators
Sham stimulation. For tDCS, to make sure the participant was blinded to the procedure, the power indicator which was visible to the participants was lit up for both active and sham stimulation. However, in the sham stimulation condition, the current was discontinued normally after 30 seconds (14). For TMS, the sham stimulation was delivered with the coil tilted one-wing 90° off the head, which is a valid sham condition commonly used in double-or single-blind, sham-controlled trials (15).

Outcomes of interest
Based on our preliminary searches, a variety of effect measurements have been used to evaluate the outcomes of noninvasive neurostimulation. These outcomes will be measured pre-and post-intervention. We will categorize these measurements into two major kinds, as test/scale/tasks and objective outcome measurements. 2. Objective outcome measurements includes the metabolites levels, the functional magnetic resonance imaging (fMRI), and Electroencephalogram (EEG).

Study design
We will consider randomized controlled trials, nonrandomized control trials, open-label trials, crossover trials. Cohort, case-control, case series, and case reports will also be searched for but not be included in the nal paper. Only studies published in English will be included. No limitations will be imposed on publication status (unpublished studies will be eligible for inclusion) or study conduct period.

Study selection
Studies will be selected based on the following inclusion criteria: (1) Sham-controlled designed studies comparing the effects of any noninvasive neurostimulations with that of a sham stimulation group; (2) Studies enrolling patients with solid diagnosis of autism spectrum disorder; (3) Studies providing outcomes measuring the therapeutic or side effects. Exclusion criteria will be: (1) Studies reported in language other than English; (2) Studies published in the form of editorial, comments, or conference abstract, which details of PICOS were not reported; (3) Studies comparing the responses of patients with ASD to noninvasive neurostimulations with that of other human subjects.
Records from database searches will be exported into EndNote and duplicates removed. Two reviewers will independently screen titles and abstracts for potentially eligible studies. The reviewers will then independently screen the full text of potentially eligible studies. Reasons for exclusions will be reported.
At each stage of the review process, disagreements will be resolved by discussion or, if not achieve consensus, consulting a third review author acted as an arbitrator. The reviewers were not blinded to names of authors, institutions, outcomes, or journals.

Data extraction
Two authors will extract data independently from the included articles, using a pre-developed form adapted from the Cochrane data collection form for intervention reviews and extracted data form

Risk of bias in individual studies
Page 7/13 We will use the revised Cochrane risk-of-bias 2 (RoB-2) tool to assess the quality of each trial (18). Two reviewers will independently score each trial, each quality item will be graded as low risk, high risk, or unclear risk. We will resolve any discrepancies by consulting a third review author. We will assess the risk of bias for the 7 domains: randomization sequence generation (selection bias), allocation concealment (selection bias), blinding of participants and personnel (performance bias), blinding of outcome assessment (detection bias), incomplete outcome data (attrition bias), selective reporting, and other bias.
The included trials will be graded as low quality, moderate quality or, high quality based on the evaluation. To evaluate the possibility of publication bias, we will use funnel plots for analyses that contained more than 10 studies.

Data synthesis
We will perform a narrative synthesis around the features of existing evidences investigating the effects of noninvasive neurostimulation on ASD patients. All eligible trials will be summarized in narrative form.
Tables will be constructed based on the information extracted (Table 1). These tables will include key study characteristics such as study design, population, diagnosis tool, randomization methods, intervention parameters, sham stimulation methods, outcomes, etc. There may be a chance for meta-analysis to pool the estimates of studies included if three studies or more meet the requirements for meta-analysis. If so, a random-effects model maybe applied. And we will evaluate heterogeneity between studies using the I² statistic. We planned not to pool data where there was considerable heterogeneity (I² ≥ 75%) that could not be explained by the diversity of methodological or clinical features among studies.

Subgroup analyses
If there are su cient data provided by the quali ed studies, we will do meta-analyses strati ed by: age; sex; stimulation site, randomization procedures, stimulation intensity, montage, duration, number of stimulation session. We will use the test for subgroup differences available in RevMan 5 to determine whether there was evidence for a difference in treatment effect between subgroups.

Sensitivity analyses
Potential reasons for heterogeneity will be explored in sensitivity analyses; the pre-speci ed subgroup analyses, if feasible, will be examined to determine potential reasons for any observed statistical heterogeneity.

Strength of evidence
The overall quality of evidence for all outcomes will be evaluated using the Grading, Recommendations, Assessment, Development and Evaluation (GRADE) framework, estimating individual risk of bias, metabias, precision, consistency, directedness and the magnitude of effect (19). These indicators will determine the certainty of the estimated effect, which will be rated as either very low, low, high or very high.

Discussion
In this review, we aim to synthesize the current evidence addressing the effects of noninvasive neurostimulation on patients with ASD. To minimize bias, our review will only include eligible studies with sham-controlled design. In addition, the quality of the included studies will also be assessed based on their randomization sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other bias. After these pre-de ned procedures, data extracted from all the included studies will then be synthesized. Therefore, it will provide information about what the effects of NIBS on core symptoms, neurocognition, and psychiatric comorbidities in patients with ASD. In addition, stimulation parameters that have been used in current TMS and tDCS administration and how have these parameters affected results will also be summarized. The ndings may have valuable implications for multiple stakeholders including patients, health care professionals, health system decision-makers and researchers working in noninvasive neurostimulation. There is a dilemma that researchers have found noninvasive neurostimulation as a promising approach for ASD while no guideline has so-far recommended on the application of noninvasive neurostimulation due to a lack of high-quality synthetic evidence. The use of our expected ndings may add more con dence when healthcare professionals are making informed decisions about the choice of this therapy. Questions may also be raised in our review for a further investigation in related to using NIBS in patients with ASD. At last, anticipated limitations include the paucity of high-quality trials and high heterogeneity of data hampering quantitative analyses.

Declarations
Ethical approval and consent to participate: There is no human or animal experiment, thus the study will not require an ethical approval or a consent to participate.
Consent for publication: This manuscript does not contain any individual person dataand all authors agree on the publication of the manuscript.