From inception to March 2022, four Chinese electronic databases (China National Knowledge Infrastructure, Wanfang Database, Chongqing VIP, and Chinese Biomedical Literature Database) and four English electronic databases (PubMed, Cochrane Library, EMBASE, Web of Science) were systematically searched. Key phrases included “tinnitus,” “acupuncture,” “systematic review,” or “meta-analysis.” We conducted an updated search in September 2022 in order to prevent missing the most recent studies and provide the most comprehensive evidence. Table 1 represents the search strategy for PubMed. More search strategies are available in Appendix 1.

The title and abstract of the articles were read by two independent reviewers (XP-X and ZF-L) and validated, screened, and extracted according to the inclusion criteria. Any disagreements were reached through discussion and negotiation or resolved through consensus and consultation with an experienced, authoritative third reviewer (H-X). The included studies extracted detailed features: author(s), year, publication language, number of included studies, sample size, treatment intervention, control intervention, quality assessment tool, relative effect (95% CI), P value and principal conclusions.

We extracted the studies included in each included review, using an Excel spreadsheet, to explore overlap in the literature and assess overlap. Graphical Representation of Overlap for OVErviews (GROOVE) (13) is an easy-to-use tool, where the matrices of evidence and the calculation of the corrected covered area (CCA) are probably one of the most exhaustive methods of measuring overlap. Using statistics, it calculates the coverage area CCA and provides an interpretation of the overall overlap assessment, which is slight if the CCA is < 5%, moderate if it is ≥5% and < 10%, high if it is ≥10% and < 15%, and very high if CCA is ≥15% (13).

Two independent reviewers (XP-X and ZF-L) used the Assessing the Methodological Quality of Systematic Reviews 2 (AMSTAR-2) (14), the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (12) checklist, and the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) (15), to assess the methodological, reporting, and evidence quality of the included studies, respectively. Any disagreements were reached through discussion and negotiation or resolved through consensus and consultation with an experienced, authoritative third reviewer (H-X).

The AMSTAR-2 is an effective tool to evaluate the methodological quality of SRs/MAs, mainly used to critically assess the systematic review of randomized controlled clinical trials. The AMSTAR-2 includes 16 projects, each of which is a standardized problem, and the results are rated as “yes,” “partially yes” and “no”.

The PRISMA checklist provides reporting specifications for SRs/MAs and references for the standardized writing and reporting of SRs/MAs. Standardized reporting can reduce bias between research and published results and increase article transparency. The PRISMA checklist includes 27 projects, each of which is a standardized problem, and the results are rated as “yes,” “partially yes” and “no”.

The GRADE system is an internationally unified literature evaluation system for grading the evidence quality and the recommendations' strengths. The GRADE system divides the factors that may affect the quality of evidence from RCTs into five downgrading factors (risk of bias, inconsistency, indirectness, imprecision, publication bias) and three upgrading factors (a large effect, plausible confounding would change the effect, and dose–response gradient). The GRADE system details the factors that affect the quality of evidence and gives a standard for the level of evidence: high, moderate, low, and very low.

According to the established search strategy, we conducted a preliminary search and searched 188 articles. A total of 71 duplicate articles were excluded, and after reading the title and abstract, 33 articles were excluded not related to SRs/MAs, 28 articles not related to tinnitus, and 25 articles not related to acupuncture were excluded. Six articles that were read in full were excluded for the following reasons: graduation dissertation, protocol, comparison of acupuncture methods, lack of further data, and another language article. A total of 14 systematic reviews (16–29) finally met the requirements and were included. The process and results of the literature screening are shown in Figure 1.

The 14 studies were published over a significant period, spanning from 2000 to 2020. Of which nine were published in Chinese and five were published in English. The number of RCTs included in SRs/MAs was significant, ranging from 5 to 25 eligible trials and 186 to 2,654 sample sizes. There were also slight differences in the intervention measures in the treatment group, among which 12 articles were mainly acupuncture in the treatment group, and two articles were mainly electroacupuncture; the intervention measures in the control group were similar, primarily using sham acupuncture and conventional treatment. To assess the quality of the original articles, the Jadad scale was used for six articles and the Cochrane risk of bias criterion was used for eight articles. The main characteristics of the study are detailed in Table 2.

The AMSTAR-2 was used to assess the methodological quality of included studies. Of the 14 included studies, only one was moderate, and the rest were very low. Only one article (27) met the criteria for item 2 affecting literature quality, which was conducted according to the reporting guidelines and standards set out in the PRISMA checklist and was registered in the PROSPERO database. None of the included studies met the key item 7 “yes” criteria. The evaluation details of other projects are shown in Table 3.

For the 27 projects on the PRISMA checklist, none of the included studies met all projects; however, most projects were also fully reported, with more than half of them achieving a 100% completion rate. Regarding specific details, Q5 (protocol and registration) was the most difficult to succeed in; only one study completed the review protocol and provided registration information and number. The Q8 (search) completion rate was low at 21.4%. Most studies only give a simple search strategy rather than a detailed complete search strategy that can be easily reproduced. Other projects whose completion rate did not reach 100% include Q9, Q15, Q16, Q19, Q22, Q23, Q25, and Q27. More details are presented in Table 4.

The GRADE system was used to evaluate 54 items related to outcomes in 14 SRs/MAs. The review found that overall, the quality of the evidence from most studies was not satisfactory. No high-quality evidence was found; 17 moderate-quality pieces of evidence, 25 low-quality pieces of evidence, and 12 very low-quality pieces of evidence. Risk of bias is the most common downgrading factor across all projects, which may be related to the acupuncturist's difficulty in turning a blind eye to the treatment assignments, followed by publication bias, imprecision, and inconsistency, with no items downgraded for Indirectness. More details are presented in Table 5.

Graphical Representation of Overlap for OVErviews not only computes the overall CCA but also provides a new graphical representation of the overlap between each pair of possible SRs/MAs. We observed moderate overlap between the included reviews. There were 91 total nodes between reviews, of which 49 were slight overlaps, 21 were moderate overlaps, 6 were high overlaps, and 15 were very high overlaps. More details are presented in Figure 2.

Eleven studies (16–25, 27) reported positive results, and three studies (26, 28, 29) reported negative results, with positive results indicating that the acupuncture group was superior to the control group and negative results indicating that no significant difference between the acupuncture group and the control group.

All positive results reported the effectiveness of acupuncture for tinnitus, with nine studies (16–23, 25) comparing the effects of acupuncture with other treatments and two studies (24, 25) comparing the effects of electroacupuncture with other treatments. Nine studies (16–23, 25) compared the effectiveness of acupuncture and medication for tinnitus and showed that acupuncture was superior to medication. Four studies (16, 19, 20, 22) compared the efficacy of acupuncture plus medication with alone or acupuncture alone, showing that acupuncture plus medication was superior to medication or acupuncture. However, another study (21) showed that the combined effect of acupuncture plus medication was not significantly different from the overall efficacy. No conclusion was reached that acupuncture plus medication was better than medication. One study (25) reported that an analysis of pooled data seemed to suggest that acupuncture had some superiority over conventional therapy, as there was a significant difference in outcomes between the two groups. However, in this study (25), no significant differences were found when comparing the results of manual or electroacupuncture with conventional therapy and manual and electroacupuncture with medication. In another study (23), electroacupuncture was found to be more effective than traditional acupuncture. One study (27) reported that acupuncture had no significant effect on the primary outcome of VAS scores compared to controls; however, acupuncture had a relatively positive effect on the secondary outcomes of THI and TSI scores, which were significantly different from controls.

All negative (26, 28, 29) results reported no significant advantage of acupuncture compared to controls. Two studies (28, 29) reported that acupuncture showed no significant improvement in the efficacy of tinnitus compared to sham and placebo acupuncture. Another study showed no convincing evidence that electroacupuncture was beneficial for the treatment of tinnitus due to the small sample size and low methodological quality.

In recent years, the number of SRs/MAs for acupuncture treatment of tinnitus has been increasing, due to differences in evaluation systems, resulting in inconsistent quality and unsatisfactory results. To further summarize the quality of evidence, reporting, and methodological quality of SRs/MAs for acupuncture for tinnitus, we conducted a detailed and comprehensive summary of the 14 studies.

It was found that among the 14 included SRs/MAs, 11 (78.6%) studies reported that acupuncture was effective in the treatment of tinnitus, and 3 (21.4%) studies reported that no firm conclusions were reached on the effectiveness of acupuncture in the treatment of tinnitus. All of the studies published in Chinese showed that acupuncture treatment for tinnitus was positive and effective, while three-fifths of the English papers showed that the efficacy of acupuncture treatment was inconclusive. The majority of SRs/MAs published in Chinese were mainly included in clinical trials published in Chinese, and only three SRs/MAs published in Chinese included a small number of clinical trials published in English, which may be the reason why all SRs/MAs published in Chinese had positive results. The GROOVE tool was used to extract and analyze all original RCTs included in SRs/MAs for acupuncture for tinnitus. Overall, the overlap between SRs/MAs that we included was moderate. According to the overlapping distribution area in Figure 2, most of the overlaps > 5% occurred between studies published in the same language. This finding suggests that studies published in the same language may have similar results, which could explain the difference in results between studies published in English and Chinese.

Vickers et al. (30) found that in some specific countries and regions (such as China, Japan, Korea, and Russia/Soviet Union), the results of published papers in the field of acupuncture may show a trend of positive results. It is well known that acupuncture originated in China 3,000 years ago, and was gradually introduced to Japan, Korea, and Southeast Asian countries; it was not until the 18th century that acupuncture research emerged in Europe and the United States (31). From a cultural and historical perspective, East Asian countries are more familiar with alternative and complementary medicine (32). Therefore, as the results of the overview found, the selection of acupoints, courses of treatment, manipulation, and acupuncturist proficiency in clinical trials published in Chinese may be more appropriate. However, this result must be treated with caution due to the low quality of the included studies, many of which suffer from methodological shortcomings, low quality of evidence, and the risk of bias. It is worth noting that although nearly 80% of the studies showed the efficacy of acupuncture in the treatment of tinnitus, the authors of the SRs/MAs seemed reluctant to draw definitive conclusions on this result because of the small sample sizes and methodological shortcomings of the included clinical trials. Meanwhile, the results of this review show that the methodological quality and evidence quality of almost all reviews are unsatisfactory, and based on the current evidence we, cannot draw firm conclusions about the efficacy of acupuncture in the treatment of tinnitus. There is a need for more rigorous, standardized, and comprehensive SRs/MAs to provide strong evidence for a convincing conclusion.

Currently, no medication has been approved by the Food and Drug Administration (FDA) or the European Medicines Agency (EMA) for the treatment of tinnitus (33). Although there is no cure for primary tinnitus, a wide range of treatments have been used and studied in an attempt to provide relief from symptoms. Unfortunately, although there are many treatments, evidence-based medicine still does not provide the most authoritative evidence for the treatment of tinnitus (12). Based on psychological intervention measures, especially those based on cognitive behavior therapy (CBT) intervention measures are often considered to be the most effective treatment of tinnitus; however, this approach is designed to reduce the pain associated with tinnitus, not to reduce tinnitus itself (34). In clinical practice, many patients may be reluctant to choose CBT or reject it outright due to practical considerations, and acupuncture is the only treatment that has clinical evidence of the improved quality of life in patients with tinnitus (28). The mechanism of acupuncture in the treatment of tinnitus is complex, and it has been reported that acupuncture has been evaluated for the possibility of affecting cochlear blood flow, which has been considered one of the causes of tinnitus (8). However, in many clinical trials, acupuncture points are not around the ear, but far away from the ear, such as the extremities or chest and back, which has a limited impact on cochlear blood flow. Therefore, in the process of clinical research, the selection of a rigorous and reliable randomization method, standard and sufficient sample size, and appropriate control group are the necessary conditions for clinical trials of acupuncture treatment of tinnitus, among which the careful selection of appropriate acupuncture placebo control group is one of the most important links. Acupuncture as a manual physical therapy method inevitably involves contact with patients, which requires balancing the background factors of acupuncture and control groups with the details of sham acupuncture in control groups to better measure the power of our treatment regimen and increase the internal and external validity of the findings (35).