Yaning Zang
Yongni Zhang2†
Jiabao Guo
Shanshan Gu- 1Department of Kinesiology, Shanghai University of Sport, Shanghai, China
- 2School of Health Sciences, Duquesne University, Pittsburgh, PA, United States
- 3Centre for Regenerative Medicine and Health, Hong Kong Institute of Science & Innovation, Chinese Academy of Sciences Limited, Hong Kong, Hong Kong SAR, China
- 4Department of Rehabilitation Medicine, The Second School of Clinical Medicine, Xuzhou Medical University, Xuzhou, China
- 5Department of Physical Therapy, University of Toronto, Toronto, ON, Canada
- 6Department of Musculoskeletal Pain Rehabilitation, The Fifth Affiliated Hospital of Zhengzhou University, Zhengzhou, China
Background and Objective: There is vast published literature proposing repetitive transcranial magnetic stimulation (rTMS) technology on the motor cortex (M1) for the treatment of neuropathic pain (NP). Systematic reviews (SRs) focus on a specific problem and do not provide a comprehensive overview of a research area. This study aimed to summarize and analyze the evidence of rTMS on the M1 for NP treatment through a new synthesis method called evidence mapping.
Methods: Searches were conducted in PubMed, EMBASE, Epistemonikos, and The Cochrane Library to identify the studies that summarized the effectiveness of rTMS for NP. The study type was restricted to SRs with or without meta-analysis. All literature published before January 23, 2021, was included. Two reviewers independently screened the literature, assessed the methodological quality, and extracted the data. The methodological quality of the included SRs was assessed by using the A Measurement Tool to Assess Systematic Reviews (AMSTAR-2). Data were extracted following a defined population, intervention, comparison, and outcome (PICO) framework from primary studies that included SRs. The same PICO was categorized into PICOs according to interventions [frequency, number of sessions (short: 1–5 sessions, medium: 5–10 sessions, and long: >10 sessions)] and compared. The evidence map was presented in tables and a bubble plot.
Results: A total of 38 SRs met the eligibility criteria. After duplicate primary studies were removed, these reviews included 70 primary studies that met the scope of evidence mapping. According to the AMSTAR-2 assessment, the quality of the included SRs was critically low. Of these studies, 34 SRs scored “critically low” in terms of methodological quality, 2 SR scored “low,” 1 SR scored “moderate,” and 1 SR scored “high.”
Conclusion: Evidence mapping is a useful methodology to provide a comprehensive and reliable overview of studies on rTMS for NP. Evidence mapping also shows that further investigations are necessary to highlight the optimal stimulation protocols and standardize all parameters to fill the evidence gaps of rTMS. Given that the methodological quality of most included SRs was “critically low,” further investigations are advised to improve the methodological quality and the reporting process of SRs.
Introduction
Neuropathic pain (NP) is an ongoing and challenging condition due to its high morbidity rate of 7–10% in the general population; it usually results from lesions in the somatosensory nervous system, including the peripheral or central nervous system (Colloca et al., 2017). NP negatively impacts patients’ quality of life by reducing functional mobility, activities of daily living, and participation in social roles, which may lead to psychological problems (Kalia and O’Connor, 2005; Gromisch et al., 2020). The initial treatment applied to NP is generally pharmacotherapy, such as use of antidepressants, anticonvulsants, and opioids (Finnerup et al., 2015). However, even with complex treatment regimens, the results of pharmacological approaches remain unsatisfactory, and some may lead to adverse events, such as toxicity, gastrointestinal events, or increased risk of addiction or drug abuse (Papanas and Ziegler, 2016; Urits et al., 2019). Therefore, non-pharmacological interventions, which are considered safe and effective, have been used to treat NP. Repetitive transcranial magnetic stimulation (rTMS) technology is widely accepted at present as a non-pharmacological intervention for treating NP.
The rTMS technique uses a transient high-intensity magnetic field acting on the cerebral cortex to generate induced currents. It alters the action potential of cortical nerve cells, depolarizes neurons in the targeted brain region, and ultimately leads to neuroplastic changes (Paulus et al., 2013). Stimulation target, frequency, and number of sessions are considered critical variables for analgesic efficacy. In terms of stimulation target, the primary motor cortex (M1) is the most commonly used target of stimulation for clinical treatment and has been the most extensively studied. In the 2020 guidelines for rTMS (Lefaucheur et al., 2020), the M1 was recommended as Level A evidence (definitive efficacy) for the treatment of NP. However, clinical promotion is still limited to some extent due to the heterogeneity of treatment protocols, such as frequency and sessions, and effectiveness among various studies. The European Society of Neurology encourages studies to collect and summarize evidence on the factors affecting these techniques (Cruccu et al., 2016). SRs are a common method for synthesizing research evidence. Nonetheless, SRs tend to address more specific research and practice questions and cannot provide a comprehensive overview of rTMS for NP. For example, the following research gaps are unknown: (1) SRs focus on a specific stimulation type or specific pain type (such as pain after spinal cord injury, post-stroke, and diabetic neuropathy), while research on other pain types needs to be developed. (2) The frequency, duration, and other parameters of interventions collected by the SRs varied, and the large amount of evidence with a lack of summarization and classification may lead to clinicians’ confusion. (3) Differences in the quality between individual trials and SRs contributed to the heterogeneity of the evidence. The same primary study may be included in different SRs, which may yield various conclusions due to varying inclusion criteria.
A novel approach to evidence synthesis research called evidence mapping (Grant and Booth, 2009; Haddaway et al., 2016; Miake-Lye et al., 2016) has been developed. Evidence mapping is designed to provide an overview of a research area by including published SRs. Evidence mapping uses published SRs as units of analysis. In the population, intervention, control, and outcome (PICO) framework, evidence mapping extracts and categorizes these data from primary studies, which are included in the SRs. On the basis of the classification criteria, the obtained PICO is integrated into different PICOs, and the contribution of the number of primary studies related to that classification is also calculated to summarize the current interventions (Petersen et al., 2015). The characteristic of the evidence mapping method is to overcome the limitations of primary studies by using the selection of studies, effect size analysis, and bias evaluation of SRs. Considering that the quality of SRs affects the credibility of the evidence, the same primary studies may be included in SRs of different quality. Various conclusions may be drawn due to different inclusion criteria, such as random and double-blind bias. Therefore, evidence mapping uses AMSTAR-2 to evaluate the quality of SRs and the credibility of the results of SRs (Ballesteros et al., 2017; Madera Anaya et al., 2019). Evidence mapping can be translated into two visual products, namely, tables (general information tables and study-specific characteristic tables) and bubble plots (multidimensional composite presentation of classification criteria, quantity, and quality of evidence), which also provide a descriptive narrative summary of the results (Bragge et al., 2011; Haddaway et al., 2016).
Evidence mapping aims to summarize, identify, and analyze the current available evidence in SRs regarding rTMS on M1 for NP. Collecting and integrating data from primary studies on the basis of SRs provide breadth of evidence. Assessing the quality of SRs provides strength of evidence. This information is provided in a user-friendly manner that helps identify research gaps and assist evidence users in the decision-making process.
Materials and Methods
Study Design
Evidence was mapped on the basis of the methodology proposed by Global Evidence Mapping (Bragge et al., 2011). The study process was divided into four phases (Figure 1: Core tasks performed to map evidence).
Figure 1. Core tasks performed to map evidence.
Boundaries and Context of Evidence Mapping
Studies and guidelines related to NP were referred, and an expert with research background in NP was consulted to frame the evidence map. With the help of the expert, the specific terminology of the search strategy was confirmed and the possible evidence users (pain, neurology, psychiatry, anesthesiology, and rehabilitation) involved were discussed. On the basis of the above information, the eligibility criteria have been established for inclusion in the study. Studies containing rTMS for NP were considered eligible. Studies on patients with NP were included, whereas experimental subjects that were animals or healthy people were excluded. The intervention should be rTMS, and the comparison could be rTMS, sham rTMS, other treatments of relieving pain, or no treatment. The outcome should be pain measured with various clinically validated tools [e.g., Visual Analog Scale (VAS), Numerical Rating Scale (NRS), Short-Form McGill Pain Questionnaire, and Brief Pain Inventory]. Studies that did not address intervention outcomes, such as those that aimed to explore NP-related pathophysiology and focus on cost-effectiveness, were excluded. Studies that reported other outcomes (e.g., fatigue, motor function, spasticity, sensory function, and cognition) but pain were also excluded. Only SRs (with or without meta-analysis) were included as they provided reliable evidence.
Evidence Search and Selection
We conducted searches of systematic literature on PubMed, EMBASE, Epistemonikos, and The Cochrane Library Published before January 23, 2021. Medical subject headings (mesh terms), free-text terms, and synonymous terms available for NP and transcranial magnetic stimulation, such as “neuralgia,” “neurodynia,” “atypical neuralgia,” “nerve pain,” and “stump neuralgia,” were combined. Literature published in non-English languages was excluded. References of the relevant studies that met the inclusion criteria were added to potential additional reviews. The details of the search strategies are reported in Supplementary Material 1. EndNote (version X9) was applied to manage the search results. Duplicate SRs were removed, and two reviewers (YZa and XL) independently screened the titles and abstracts to exclude irrelevant studies. Full-text studies were obtained and reviewed to make a terminal decision. Any disagreements in the decision-making process were resolved through negotiation or by discussion with a third reviewer (YoZ).
Data Analysis
A data extraction table was designed to record the main characteristics and compare the methodological differences of the included SRs. Two authors (YZa and XL) assessed the methodological quality and extracted data independently. Any difference of opinions was discussed with the third author (YoZ). The original authors were contacted for missing information when necessary. Data were grouped into three categories:
(a) The Assessment Methodological Quality for Systematic Reviews (AMSTAR-2) was used to assess the methodological quality of SRs (Shea et al., 2017). AMSTAR-2 is a practical tool used to assess the quality of SRs that include randomized or non-randomized studies of healthcare interventions or both. It has 16 items, with an overall rating based on weaknesses in critical domains (items: 2, 4, 7, 9, 11, 13, and 15). In brief, the evaluation results of the SRs are generally divided into the following four categories: “High,” no critical weakness and no more than one non-critical weakness; “Moderate,” no critical weakness and more than one non-critical weakness; “Low,” one critical flaw with or without non-critical weaknesses; and “Critically low,” more than one critical flaw with or without non-critical weaknesses.
(b) The following characteristics of SRs were extracted: authors, years of publication, types of SR (with or without meta-analysis), objectives, dates of search, sample sizes, designs, and numbers of included studies.
(c) The PICO framework was used to extract data from primary studies included in the SRs. The four key components are populations, intervention, comparison, and outcomes. Details including population characteristics (e.g., NP related to spinal cord injury, post-stroke symptoms, and complex regional pain syndrome), interventions (e.g., target area, frequency, and intensity of transcranial magnetic stimulation), comparative measures (e.g., placebo and sham stimulation), and outcomes were extracted. The obtained data of PICO from primary studies in SRs were classified into different groups based on similar characteristics (population; interventions: frequency, session, and intensity; control group; and outcome scale).
For descriptive purposes, the effect of rTMS on NP reported by the SR authors was grouped into the following five categories on the basis of the previously reported criteria: “Potentially better,” the conclusions reported rTMS as more beneficial than the control group; “Mixed results,” the same primary study had different findings among different studies (e.g., some studies found no difference in rTMS compared with the control group in the same population, whereas others found potential benefits of transcranial magnetic stimulation over the control group); “Unclear,” insufficient evidence to draw definitive conclusions about the effectiveness of rTMS on pain; “No difference,” the conclusions provided evidence of no difference between intervention and control; and “Potentially worse,” the conclusions reported TMS as less beneficial than the control group. The same primary study may be included in multiple SRs. If the primary study has multiple consistent results, it would be added to the appropriate group, and multiple conflicting findings were included in the “Mixed Results” group (Miake-Lye et al., 2019).
Reporting Synthesized Findings
Each clinical question addressed in each included review was adapted into a PICO format that specified the types of participants, interventions (or comparison), and outcomes. Evidence mapping allows the reader to visualize any gaps in the literature base, with results presented in the form of tables and graphs.
(a) The basic characteristics, quality assessment of the included SRs, and characteristics of all integrated PICOs were described in tables.
(b) A heat map was used to present the quality of the included SRs.
(c) Graphical display was provided through bubble plots. The bubble plot displayed information in four dimensions: (1) bubble size (number of articles), the size of each bubble is proportional to the number of individual trials included in the SRs; (2) bubble color (research characteristics), bubbles labeled with different colors indicate different PICOs; (3) X-axis (effect of TMS on NP), the classification of authors’ conclusions represented on the X-axis (“potentially better,” “mixed results,” “unclear,” “no difference,” and “worse”); and (4) Y-axis (AMSTAR-2 evaluation results), four different colors were used to indicate study quality, with red indicating critically low, orange indicating low, yellow indicating moderate, and green indicating high quality.
Results
Selected Studies
This retrieval yielded 125 records. Another 11 articles were added from the references that met the inclusion criteria. After duplicates were removed, 97 articles remained for screening of the titles and abstracts. Subsequently, 44 articles were excluded after the screening. In the remaining 53 articles, 13 were excluded after full-text reviews. Finally, 38 articles met the eligibility criteria (Figure 2). The list of excluded studies along with exclusion rationales can be found in Supplementary Material 2.
Figure 2. PRISMA flow diagram detailing the selection process.
Methodological Quality of Systematic Reviews
According to the AMSTAR-2 criteria, 34 SRs scored “critically low,” 2 SR scored “low,” 1 SR scored “moderate,” and 1 SR scored “high” in terms of methodological quality (Figure 3). The most frequent drawbacks were as follows: no mention of the protocol in the systematic overview, no description of the rationale for the study designs included in the review, no report of excluded studies or reasons for exclusion, and no statement of funding for the included studies. The detailed assessment process is provided in Supplementary Material 3.
Figure 3. Methodological quality of the included systematic reviews.
Characteristics of the Included Systematic Reviews
Table 1 shows the characteristics of the included SRs. All SRs were published between 2009 and 2020. Among the 38 included SRs, 17 SRs conducted a meta-analysis. The number of included studies ranged from 3 to 131, and they were conducted between 2001 and 2020. Each SR included patients ranging from 97 to 15,776. Six SRs did not report or incompletely reported the designs of the included individual studies. All studies reported study designs, and a total of 678 randomized controlled trials (RCTs) accounted for 86% of the included studies in all SRs. Of all SRs, 18 SRs included only RCTs, 15 SRs included patients with NP with different underlying causes, and 25 were exclusively conducted on NP with specific etiologies or due to a single disease. Six SRs included pain after spinal cord injury (SCI), another 5 SRs included central post-stroke pain (P), 3 SRs included phantom limb pain (PLP), 2 SRs included migraine, 2 SRs included complex regional pain syndrome (CRPS), 2 SRs included headache, 1 SR included diabetic peripheral neuropathy (DPN), 1 SR included multiple sclerosis (MS), and 1 SR included orofacial pain (OFP). As for the intervention, 13 SRs only assessed rTMS, 12 SRs assessed other non-invasive stimulations, 3 SRs assessed neuromodulation techniques, 4 SRs assessed non-pharmacological interventions, 5 SRs assessed pharmacological and non-pharmacological management of NP, and 1 SR assessed non-invasive brain stimulation combined with exercise.
Table 1. Characteristics of included systematic reviews.
Characteristics of Population, Intervention, Comparison, and Outcomes From Systematic Reviews
After merging the duplicate primary studies included in the 38 SRs, 70 primary studies were integrated into 19 PICO groups based on the PICO characteristics. Studies that did not provide the mandatory parameter information were not included in the PICOs. Among the included SRs, populations with NP were from various diseases and etiologies, and the treatment protocols adopted various parameters, including frequency, sessions, and pulses. Sham stimulation or placebo was the most common intervention in the control group of rTMS on the M1. The primary outcome of the included studies was self-reported subjective nociception. VAS and NRS were the most commonly validated pain assessment scales. The details of the characteristics are enumerated in Supplementary Material 4.
As a result of unavoidable heterogeneity of the rTMS protocol among studies, classifying and categorizing all parameters can be difficult. Thus, the classification of PICO focused on interventions and comparison, as well as the population involved and outcome assessments, as presented in Table 2. We use Figure 4 to explain the connection between the bubble polt and Table 2. In terms of interventions, we classified them to frequency and session of rTMS, as they have been shown to influence the analgesic effects and are identified as clinically significant factors (Ahmed et al., 2012; O’Connell et al., 2018; Gatzinsky et al., 2020; Pacheco-Barrios et al., 2020). High and low frequency of rTMS can induce transient excitatory and inhibitory effects, respectively (Klein et al., 2015). Sessions of rTMS are considered an important factor in maintaining the effects. The characteristics of the interventions were categorized based on frequency (low or high frequency) and number of sessions (short: 1–5 sessions, medium: 5–10 sessions, and long: >10 sessions).
Table 2. PICOs included in systematic reviews.
Figure 4. Interpretation of evidence mapping.
The key characteristics of PICOs are listed in Supplementary Material 5. A total of 19 PICOs were categorized based on stimulation target, frequency, and session (short, medium, and long). On the basis of the stimulation target, 17 PICOs used high-frequency rTMS (>1 Hz), and 2 used low-frequency rTMS (<1 Hz). In terms of the number of sessions, 1–5 sessions were considered short sessions, 6–10 were medium sessions, and more than 10 were regarded as long sessions. Twelve PICOs had short sessions, three had medium sessions, and four had long sessions. In addition, 12 PICOs used the same sessions of sham stimulation or placebo as a control to study the effectiveness of rTMS in patients with NP. Two PICOs studied the effects of different sessions of rTMS, while other PICOs involved different stimulation areas: M1 unilateral stimulation versus bilateral stimulation, rTMS compared with botulinum toxin injection, rTMS compared with transcranial direct current stimulation (tDCS), or rTMS combined with theta-burst stimulation. The PICOs were concentrated in the following characteristics: 20 Hz, short-term sessions versus sham stimulation (11 PICOs), and 10 Hz, short-term sessions versus sham stimulation (18 PICOs).
Specific Findings From Systematic Reviews in Evidence Mapping
The evidence map of rTMS for NP is presented in Figure 5. The bubble diagram is a visual display of data represented in Supplementary Material 5. We integrated similar intervention characteristics from primary studies into PICOs. In the bubble chart, different colors indicate varying PICOs. Each bubble was plotted in accordance with the conclusion of the effect of rTMS on NP (X-axis) and the quality of the related SRs (Y-axis), while the size of bubbles represented the number of primary studies included in PICOs. The evidence tables (Supplementary Material 5) provided details of the included SRs (Supplementary Material 4). Some primary studies may be included in multiple SRs. If SRs synthesized different conclusions for the same primary study, the same PICOs would appear in different classifications on the X-axis. If the same primary study was included by SRs of different quality, the same PICOs would appear in different classifications on the Y-axis. Evidence mapping showed that 5–20 Hz, high-frequency rTMS of M1 with short (1–5), medium (6–10), or long (>10) sessions usually lead to “potentially better” treatment effects compared with sham stimulation, although some had transient effects. By contrast, the synthesis results for the lower frequencies (1 and 0.5 Hz) showed either no difference or mixed effects. Thirteen PICOs included 52 primary studies rated as “potentially better,” and four of these PICOs involved 13 primary studies that were also included in a high-quality meta-analysis. In accordance with the AMSTAR-2 quality assessment, the interventions in these four PICOs were considered beneficial in most cases. Nine PICOs included 18 primary studies with different findings in different SRs and were rated as “mixed,” implying that the interventions in these eight PICOs had limited confidence in the effect estimates, and the true effect may be different from the study reports (Miake-Lye et al., 2019). One PICO conclusion was rated as “unclear” because its effect was not reported in the SR (Yang and Chang, 2020) with a critically low quality. Eight PICOs included 17 primary studies that concluded that rTMS showed no difference compared with controls. Of these, six studies showed a potentially better effect of rTMS in short-term follow-up but no difference during long-term follow up (Supplementary Material 4 and Supplementary Material 5). After studies that were ineffective during follow-up were excluded, 8 of 11 primary studies were also included by a high-quality meta-analysis. This finding indicated less effectiveness of these intervention protocols or inapplicability to a particular NP, and the treatment effects could be uncertain. Two PICOs included two primary studies that showed a “potentially worse” conclusion.
Figure 5. Evidence mapping of the rTMS on neuropathic pain (NP). short, 1–5 sessions, medium, 5–10 sessions, long, >10 sessions. rTMS, repetitive transcranial magnetic stimulation; tDCS, transcranial direct current stimulation; TBS, theta-burst stimulation; M1,motor cortex, S1, primary somatosensory cortex; SMA, supplementary motor cortex; PMC, premotor cortex; BTX-A, botulinum toxin type A.
Discussion
Principal Findings of Evidence Mapping
This evidence map included 40 SRs, and the majority of the primary studies included were RCTs, which is the best study design to assess the effectiveness of interventions (Sylvester et al., 2017). Evidence mapping provided a broad overview of the available evidence of rTMS on NP, showing the focus and counting contributions of available studies by categorizing and generalizing them to help interpret the published SRs. (1) Research gaps: The included SRs covered most types of NP, including SCI, CPSP, CRPS, PLP, DNP, and headache; however, this left an evidence gap of the rTMS for some specific types of NP, such as postherpetic pain, radiculopathy pain, trigeminal neuralgia, post-traumatic brain injury pain, and cancer-related NP. In addition, the control groups were mostly given sham stimulation. Open questions about the effectiveness of rTMS associated with other therapies (such as pharmacotherapy, neurorehabilitation, and psychotherapy) are recommended. Future SRs are needed to analyze immediate, short-term, and long-term effects, which may help clarify the sessions of rTMS. Stimulation parameters, namely, frequency and intensity variable time, are also the direction for further research. (2) Summarization and classification of evidence: Evidence mapping showed that 5–20 Hz, high-frequency rTMS of M1 with short (1–5), medium (6–10), or long (>10) sessions usually lead to a “potentially better” conclusion compared with sham stimulation, suggesting that these interventions are beneficial in most cases. By contrast, the synthesis results for the low frequencies (1 and 0.5 Hz) showed no difference or were mixed, meaning these intervention protocols may be less effective or inappropriate for some specific NPs. (3) The impact of the quality of the SRs on the strength of evidence: Some PICOs from high-quality SRs drew a potentially better conclusion, suggesting that these interventions were beneficial in most cases. Similarly, some PICOs from high-quality SRs did not show any difference in the conclusion, indicating that these interventions may be less effective or inappropriate for some specific NPs. This evidence map is not intended to replace any clinical protocol or guidelines, nor is it intended to provide a standardized protocol. Therefore, the clinical diagnosis of each patient, the existing alternatives, cost-effectiveness, available resources, and other factors must be carefully considered before offering any recommendation (Nussbaumer-Streit et al., 2018).
Comparison of Results of Systematic Reviews
By comparing the results of the SRs, the same PICOs obtained from SRs were presented with different conclusions in the evidence map. The possible reasons were as follows: (1) some studies that reported different sessions had varied effects of rTMS. For example, a good outcome could be found in the short term, but not in the long term, that led to a mixed conclusion. Future SRs should focus on follow-up and explore the long-term effectiveness of the intervention. (2) Some SRs conducted meta-analysis but some did not. Qualitative studies may arrive at conclusions different from those in quantitative studies. For example, a primary study by Onesti et al. (2013) and meta-analysis of Zeng et al. (2020) had a “no difference” conclusion, while the review of Yang and Chang (2020), which only mentioned pain relief, showed a “potentially better” conclusion. The final conclusion in the evidence map of Onesti et al.’s study was “mixed.” (3) Populations with different diseases that cause NP could cause heterogeneity. Previous studies have indicated that patients with orofacial pain have a better analgesic response than those with CPSP, SCI, or BPL (Lefaucheur et al., 2004). For example, a primary study on patients with CRPS (Picarelli et al., 2010) by Chang et al. (2020) and another one involving chronic pain by O’Connell et al. (2018) did not reach the same conclusion. This gap could also be a caveat to provide more reliable and reproducible data. Future studies must consider including more homogeneous groups of participants or stratifying patients in accordance with clinical characteristics and underlying pathogenesis.
If different SRs with varied conclusions included some primary studies that were overlapping and some unique studies, future investigations could synthesize studies that were included in these reviews and find new outcomes including all potential evidence. From the “mixed” results, future investigations could focus on comparing different stimulation protocols (doses, sessions, variable time, and intervals among sessions) of rTMS on NP.
Overall Completeness and Validity of Evidence Mapping
We evaluated the quality of SRs and the credibility of the results of SRs with a new version of AMSTAR. Compared with the previous version, AMSTAR-2 was adapted to include SRs on the basis of RCTs or non-randomized intervention studies or both and provide more refined and rigorous evaluation item criteria. Assessment in this field suggested room for improving SRs’ quality. Future SRs should place more emphasis on the following domains to improve the quality of studies and the validity of the results: reporting an explicit statement about the description of the methodology prior to conducting the review; any significant deviations from the protocol should be justified; explaining the selection of the study designs for inclusion in the review; providing a list of excluded studies and justifying the exclusions; indicating the sources of funding or support for the individual studies included in the SRs; and interpreting or discussing the effect of the risk of bias in individual studies on the total effect.
Strengths and Limitations of This Evidence Mapping
This evidence map may be the first on rTMS for NP. Evidence mapping is a relatively new tool for the synthesis of available evidence, so we explain the methodology and results in more detail compared with published evidence maps and provide all the data of our study process to facilitate the reader’s understanding and use. To avoid selection and data extraction bias, we constructed a rich search string for retrieving from four different databases. In addition, the reference lists of the selected studies were manually scanned for the detection of additional relevant studies, minimizing the risk of missing relevant studies. Study selection and data extraction were made via a double confirmation. Two authors conducted the process of selecting and extracting separately from one another, and any disagreements were then discussed with a third researcher until a final agreement was reached. Furthermore, results were mapped using various graphs, such as bubble plots, heat maps, and tables, which helped improve traceability between the extracted data and the conclusion.
Certain limitations in this evidence map should be considered. First, the map only chose studies published in English, which limited the scope of evidence mapping. Second, given that only SRs were included as a source of evidence, some studies, such as newly published or studies included in these SRs, could have been missed. Third, the methodologies of some SRs had limitations. Furthermore, several different types of studies in SRs comparing therapeutic interventions for NP were included. Although most trials were RCTs, some case reports and observational, open-label, and cohort studies were also available. Finally, evidence mapping synthesized the SRs as a unit rather than individual studies, which could lead to some primary studies being repeated.
Conclusion
Neuropathic pain is a complex and refractory group of diseases. Evidence mapping showed that rTMS, as a compliant and safety neuromodulation treatment, is promising for the treatment of NP. Evidence mapping could encourage clinicians and professionals involved in related areas, such as pain, neurology, psychiatry, and anesthesiology, to pay more attention to non-pharmacological treatments on patients with NPs, especially those with drug resistance. Evidence mapping is a useful and reliable method to identify the currently available evidence on therapeutic interventions and pinpoint gaps to suggest future research. In the future, when designing treatment protocols, rehabilitation practitioners are recommended to consider the duration and sessions of rTMS. More research efforts are needed to highlight the optimal stimulation protocols and standardize all parameters to fill evidence gaps, and more homogeneous groups of participants should be considered. Meanwhile, as the methodological quality of most included SRs scored “critically low,” further efforts are needed to improve the methodological quality and reporting process of SRs.
Data Availability Statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.
Author Contributions
YZa, YoZ, and YiZ were designed the study. YZa and XL were collected the screening studies, extraction data and charted under the guidance of YoZ. YZa and YoZ analyzed the data and drafted the manuscript. YiZ reviewed the results. SG, YY, JG, and YiZ revised the manuscript for important intellectual content. All authors approved the final version of the manuscript.
Funding
This work was supported by National Natural Science Foundation of China Project (No. 81860875).
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s Note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary Material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fnhum.2021.743846/full#supplementary-material
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Keywords: neuropathic pain, non-pharmacological, repetitive transcranial magnetic stimulation, evidence mapping, evidence synthesis, motor cortex
Citation: Zang Y, Zhang Y, Lai X, Yang Y, Guo J, Gu S and Zhu Y (2022) Evidence Mapping Based on Systematic Reviews of Repetitive Transcranial Magnetic Stimulation on the Motor Cortex for Neuropathic Pain. Front. Hum. Neurosci. 15:743846. doi: 10.3389/fnhum.2021.743846
Received: 02 August 2021; Accepted: 15 November 2021;
Published: 16 February 2022.
Edited by:
Kai Wang, Anhui Medical University, ChinaReviewed by:
Joaquim Pereira Brazil-Neto, Unieuro, BrazilGong-Jun Ji, Anhui Medical University, China
Copyright © 2022 Zang, Zhang, Lai, Yang, Guo, Gu and Zhu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yi Zhu, emh1eWkxMDEwQDE2My5jb20=
†These authors have contributed equally to this work