After searching the databases and removing duplicate items, 397 potentially relevant titles and abstracts were identified. After screening the title and abstracts, 362 articles were excluded. Thirty-five studies were selected for full-text review. Finally, six studies were included in this systematic review based on the inclusion and exclusion criteria. The most frequent reasons for excluding studies were unrelated titles during the initial review, studies without electrical stimulation application via a needle, conference papers, etc. The details of the excluded studies with justification for exclusion are presented in the “Appendix 2: Table 4”. The process of searching and screening is summarized in Fig. 1.
Characteristics of included studies
A summary of the methodological characteristics of the included studies and their results are presented in Table 1. Among the selected studies, five studies were in English [21,22,23,24,25], and one study was in Korean . Two studies used patient and assessor blinding [22, 24], and two studies only used assessor blinding [21, 23]. Two studies did not include any blinding [25, 26].
Among the included studies, 76 and 82 patients were allocated to IMES and control groups, respectively. Three studies had parallel RCT designs with IMES and control groups [23,24,25]. Two other studies featured three parallel RCT designs. One compared the effectiveness of low-level laser therapy, IMES, vs. a control group . Another included DN, IMES, and intramuscular stimulation (Gunn-IMS) groups . One study had four groups, including repetitive Transcranial Magnetic Stimulation (rTMS) + IMES, rTMS + sham-IMES, sham- rTMS + IMES, and sham- rTMS + sham-IMES. We considered sham- rTMS + IMES and sham- rTMS + sham-IMES as experimental and control group, respectively in this study . All studies recruited patients with chronic cervical MPS.
In three studies, sham-IMES groups were used as a control group [22,23,24]. In one study, participants in the control group received prescribed home-based exercises , while subjects in another two studies control group received DN [25, 26]. The number of treatment sessions varied from one to ten sessions between studies.
Risk of bias assessment of selected articles
Among six included studies, two had low risk of bias [22, 24] and three of them had moderate risk of bias [21, 23, 26]. The study by Brennan et al.  was the only study with high risk of bias due to inappropriate intention to treat analysis. Details of the study quality assessment are presented in Fig. 2. The details of the scoring of each item for the included studies are presented in the Additional file 1.
Wave properties and needle location in IMES group
In four studies, the upper trapezius muscle was treated [21, 23, 25, 26]. Medeiros et al. and Botelho et al. applied IMES to the cervical paraspinal muscles [22, 24]. Only three studies targeted trigger points [21, 23, 25]. The frequencies of the electrical stimulation ranged from 2 to 80 Hz. In two studies, the intensity was increased to the point of contraction [23, 26]. Sumen et al.  increased the intensity until the patient sensed the stimulus. Three studies did not report any details about the intensity [22, 24, 25]. Wave properties and IMES technical characteristics are summarized in Table 2.
Outcome measures and summary of results
The visual analog scale (VAS) was the most common pain outcome measure. Three studies evaluated ROM measurements [21, 23, 26]. Other outcome measurements included pain by numeric pain rating scale (NPRS), pain pressure thresholds (PPT), biomarkers such as BDNF, pain or functional ability questionnaires, the neck disability index (NDI) and the McGill pain questionnaire (MPQ), and analgesic drug intake (Table 1). Also, the Details of included studies outcome measurements in assessment times (means with standard deviations) are presented in the “Appendix 1: Table 3”.
Byeon et al.  compared the effectiveness of IMES and DN; they showed improvement in pain and cervical lateral flexion ROM in all groups, but there were no significant differences of all outcome measurements in all assessment times in both groups . Sumen et al.’s  results present statistically significant VAS decreases and PPT increases in the IMES group vs. the control group. Medeiros et al.  showed a significant difference in pain reduction between the IMES and control groups but no change in peripheral biomarkers parameters in the experimental and control groups. Hadizadeh et al.  showed that ROM was significantly higher in the IMES group than the control group one week after treatment. There were no significant differences in pain in all assessment times between both groups. Botelho et al.  showed a significant improvement in pain and analgesic drugs in the IMES group compared to the control group. Brennan et al.  compared the effectiveness of IMES and DN; they showed a significant improvement in pain and disability index in both groups and did not NDI or NPRS differ significantly between groups in any assessment times.
The current study is the first systematic review evaluating IMES’s effectiveness in patients with MPS to the best of our knowledge. Six studies with a total of 158 subjects were included in this review. Pain, the most common outcome measurement, was assessed by the VAS and NPRS or the MPQ. The effectiveness of IMES was compared with sham IMES, DN, or no intervention. The number of sessions varied from 1 to 10 sessions. The duration of IMES ranged from 10 to 20 min. The study by Hadizadeh et al.  was the only study with a single-session intervention. Three articles reported following the patients from 1 to 6 weeks [21, 23, 25, 26].
In general, studies with a low risk of bias showed a significant improvement in the variables of pain, disability and analgesic use in the IMES group compared to the control group [22, 24]. Also, in studies with moderate risk of bias (Some concerns), reduced pain and improved range of motion have been reported. However, in some cases, there was no significant difference with the control group [21, 23, 26]. In a study with a high risk of bias, no significant difference was reported between the IMES group and the control group in the variables of pain and disability .
Initially, we aimed to determine what factors would impact the effectiveness of IMES on MPS, such as the frequency of the applied currents, the duration, the exact location of active and reference needles or electrodes, among others, but the limited number of studies and the heterogenicity among studies did not allow for this kind of analysis. The study by Hadizadeh et al. was the only study demonstrating that one session of IMES could effectively reduce pain and increase ROM not immediately but after a one-week follow-up. It can be due to inflammatory processes after needle insertion, which may present as muscle soreness . How many IMES sessions would be sufficient for clinical improvement cannot be deduced from the current research and requires further study.
There are some mechanisms explaining trigger points. One explanation is offered by the integrated hypothesis, which maintains that trigger points result from repetitive low-intensity trauma, leading to sarcoplasmic retinaculum injury, increased calcium concentration, and permanent contraction in the area. This would result in hypoxia and cell damage in the region [28,29,30]. It seems that surface, motor excitable electrical stimulation can increase the blood flow; therefore, it can decrease regional hypoxia. Commonly, IMES produces muscle contractions. This method can insert electrical stimulation to the depth of muscle with lower resistance against the current. Therefore, IMES seems to be more effective in managing regional hypoxia in TrP zone compared to superficial ES and the use of DN alone [15, 31]. Besides, most studies used low-frequency current; low frequencies may cause the release of endorphins and enkephalins, leading to a reduction in pain .
Our study has several limitations that should be mentioned. First, we included only primary RCT studies in this systematic review, which reduced the number of studies, limiting the ability to generalize the results of this study. Second limitation of this study is that, because the characteristics of the applied electrical stimulation like intensity, pulse duration, frequency, time, and etc. are not fully mentioned in all studies, it is not possible to make recommendations regarding the appropriate parameters. Third, we included RCTs with various type of interventions due to limitation in original studies. Fourth, the small number of included studies and clinical heterogeneity of included studies such as different fallow up point times, different sessions number, different control groups, and outcome measurements did not allow us to pool data and do a meta-analysis on the results. Further research is recommended to do a meta-analysis on this topic after further randomized controlled trials. Fifth, all of the included studies had a small sample size that can impact the result of the ROB2 tool. Therefore, the results of quality assessment in this study should be accepted with this limitation.
Further studies are needed to overcome these limitations. First, more RCT studies with larger sample sizes are needed to compare this intervention with other routine interventions. Second, studies are needed to investigate the placebo effects of this intervention. Studies with objective variables (like TrP size or stiffness found by radiologic methods) are also needed to evaluate this intervention’s effectiveness. Also, future studies should include more detailed parameters of the interventions.
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