TY - JOUR
AU - Kumbhare,, Dinesh
AB - Abstract Background Myofascial pain is a prevalent chronic pain disorder, affecting a large proportion of the general population. Electric stimulation techniques such as transcutaneous electric stimulation (TENS) and electroacupuncture have been shown to be effective for managing chronic pain conditions including myofascial pain. The goal of this study was to review the literature on the effectiveness of electric stimulation techniques on myofascial pain. Methods A comprehensive systematic search of three databases—Medline, EMBASE, and Cochrane CENTRAL—was conducted using key words related to myofascial pain and trigger points, as well as various electric stimulation techniques. A total of 15 articles passed the inclusion and exclusion criteria for the study. Data were extracted from these studies and assessed qualitatively and quantitatively. Standardized mean differences (SMDs) were computed from pain intensity measures extracted from these studies. Subgroup analyses were performed to assess the effectiveness of treatment modality, number of treatment sessions, frequency of stimulation, location of treatment, and duration of treatment. Results The SMD for electric stimulation techniques on reported pain intensity was significant (P = 0.03), as was the SMD for the electroacupuncture subgroup (P = 0.02); the TENS subgroups’ effect was not significant (P = 0.17). The subgroup analyses tentatively suggest that frequency and number of treatments do not influence pain intensity, whereas the duration of treatment may have an effect. The qualitative results of this study revealed variability in the results among studies delivering TENS treatments. Conclusions Electric stimulation is effective at mitigating reported pain intensity at the location of the trigger point. Electroacupuncture presented with significant and larger effect sizes of improvement relative to TENS for reported pain intensity. Given that this review included a small number of studies, there is a need for additional research to confirm its findings. Additionally, studies assessing the parameters and physiological location of treatment are needed to inform the clinical use and recommendations of electric stimulation treatments. Myofascial Trigger Point, Electric Stimulation Techniques, PENS, TENS, Electroacupuncture Introduction Myofascial pain is a prevalent chronic musculoskeletal pain condition. The prevalence of this condition varies from 30% among patients reporting regional pain in general medicine clinics to as high as 85% and 93% of patients in pain clinics [1]. Myofascial pain is characterized by the presence of myofascial trigger points (MTrPs). Clinically, MTrPs present as stiff taut bands of skeletal muscle that are hypersensitive on palpation and are associated with decreased range of motion and pain among patients [2–4]. MTrPs are thought to develop as a consequence of poor posture, emotional stress, muscle overexertion, trauma, or repeated contractions [5–7]. The muscle as a consequence is thought to experience reduced blood perfusion, adenosine triphosphate supplies, and increased calcium levels, ultimately causing an “energy crisis” in the muscle and hindering effective muscle relaxation. The maintenance of the contracture is thought to occur through central and peripheral sensitization mechanisms [3,8]. The neurochemical and inflammatory environment at the MTrP sensitizes afferent neurons and leads to increased nociceptive input to the dorsal horn at the associated spinal segment. Persistent nociceptive input sensitizes neurons at the spinal cord, resulting in increased pain perception [7]. Ineffective nociceptive neurons are also activated during this sensitization process, amplifying pain perception [7]. Efferent neurons from the affected spinal cord segment contribute to the maintenance of the MTrP as well. Studies have found increased spontaneous electrical activity at the motor endplate region proximal to MTrPs. This activity results in increased acetylcholine release, which maintains the muscle contracture, contributing to cramping and ischemia at the region of the MTrP. MTrPs are found close to motor endplate innervation zones at the muscle belly, suggesting that abnormal peripheral and central spinal facilitation mechanisms are contributing to the presence of MTrPs [6, 9, 10]. Current therapeutic interventions for myofascial pain aim to disrupt the maintenance of a contracted state within the MTrP by inhibiting spinal facilitation mechanisms and increasing blood perfusion to the muscle region. Promising interventions for treating myofascial pain include a range of electrotherapeutic modalities such as transcutaneous electrical nerve stimulation (TENS), percutaneous electrical nerve stimulation (PENS), and electroacupuncture [11–13]. Conventional TENS delivers electric stimulation to the site of pain through the use of superficial electrodes. PENS and electroacupuncture treatments deliver electric stimulation through needles situated within a superficial layer of the skin. Electric stimulation interventions activate peripheral and central analgesia mechanisms. For instance, electric stimulation has been found to increase periaqueductal grey activity and the subsequent release of endorphins and endogenous opioids in people with myofascial pain [14]. Additional evidence suggests that electric stimulation also induces central serotonergic and noradrenergic analgesic activity [15]. Peripherally, electric stimulation induces the blockade of A delta fiber innervated receptors and enhancements in opioid and alpha noradrenergic activity to reduce pain at the site of stimulation. Electric stimulation also inhibits endplate activity associated with the presence of MTrPs [16]. Previous systematic reviews assessing the efficacy of electric stimulation on pain disorders such as rheumatoid arthritis, chronic widespread pain, and low back pain found reduced reports of pain among studied patients [17, 18]. However, none of the available systematic reviews provide an updated analysis on the impact of electric stimulation on myofascial pain alone. The aim of this study was to assess the effectiveness of electrical stimulation modalities on reported pain intensity and pressure pain threshold (PPT) in patients with myofascial pain. Results from this study should provide insight into the benefits and clinical utility of electrical stimulation techniques for myofascial pain management. Methods An information specialist conducted a comprehensive systematic search for this study. The search strategy included text words to describe the concepts “myofascial pain,” “trigger point,” and various electric stimulation techniques. The systematic search was conducted in Medline (including Medline ePub Ahead of Print, In-Process, and Other Non-Indexed Citations) and translated into EMBASE and the Cochrane CENTRAL Register of Controlled Trials. Searches in each database were conducted from the inception of the database until April 2018. Searches excluded animal studies and were limited to English language papers. The full Medline search can be reviewed in the Appendix. Study Types Randomized controlled trials (RCTs) investigating the effects of electric stimulation on myofascial pain were included in this review. Only English studies were included. Systematic reviews, crossover trials, and case control, cohort, and controlled studies were excluded. Inclusion Criteria for Participants A myofascial pain disorder and or the presence of trigger points Exclusion Criteria for Participants Arthrosis and inflammatory arthropathy Chronic widespread pain Radiculopathy or other neurological disorders Pediatric populations Animal populations Types of Interventions TENS Electro-acupuncture Electrostimulation therapy PENS Other electric stimulation techniques Comparison Group Placebo/sham stimulation control group Outcome Measures Visual analog scale (VAS) or other reported pain intensity measures for subjective pain Pain pressure threshold (PPT) Data Collection and Synthesis Two reviewers, SA and CH, independently assessed the resultant articles from the systematic search. The initial screen assessed study titles and abstracts for inclusion. Studies that did not pass the inclusion criteria were discarded, and those that fulfilled the criteria remained for a full-text screening. Conflicts were resolved by a separate reviewer, SS, or by consensus between the reviewers. Reviewers SA and CH screened the full-text articles and determined those to be included in the review. Data were extracted using standardized data extraction forms. If data were not reported numerically in the articles but within figures, the data were extracted from the figures using standardized measurement tools. Values were rounded to the nearest hundredth. If figures were not available for data extraction and post-treatment values were not reported, authors were emailed for their raw data twice. If they did not reply, their data were not included in the analysis. Sample size, control treatment, the distribution of males and females within the study sample, mean age of participants, number of treatment sessions and duration, type of intervention, electric stimulation parameters, region treated, a brief summary of the study results for both within- and between-group effects based on the studies’ reporting, and the mean and standard deviation of the post-treatment measures were extracted from the articles. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses diagram (Figure 1) summarizes the results of the inclusion and exclusion process. A qualitative analysis was performed on the study data to assess statistical significance trends among studies delivering different types of electric stimulation, different frequencies, single or multiple sessions of treatment, and duration of treatment. Assessing the effect of stimulation intensity was not performed as not all studies reported quantitative data on the parameters they delivered. If a study did not report the stimulation parameters, they were included in the respective qualitative analysis. Figure 1 Open in new tabDownload slide Preferred Reporting Items for Systematic Reviews and Meta-Analyses diagram of article exclusion and inclusion. Figure 1 Open in new tabDownload slide Preferred Reporting Items for Systematic Reviews and Meta-Analyses diagram of article exclusion and inclusion. Risk of Bias Reviewers SA and SS assessed the risk of bias in the included studies independently, without blinding to authorship or journal. Risk of bias was determined using the parameters outlined in the Cochrane Handbook of Systematic Reviews [19]. The items assessed were selection, performance, blinding, measurement, attrition, and reporting. Six items were assessed and were given a rating of low risk of bias, unclear risk of bias, or high risk of bias. A low risk of bias was assigned if the article authors described their methodology toward mitigating the item of interest, an unclear risk of bias was assigned if the authors did not discuss the item, and a high risk of bias was assigned if the authors reported a limitation on the item of interest or if the reviewer saw reason for the bias to affect the results. For the “blinding of participants” item, although many studies employed a double-blinded procedure, the sensation associated with electric stimulation is an indicator of treatment. Therefore, studies that provided minimal stimulation for a restricted time period and/or stated efforts to simulate identical experiences between experimental groups were given a low rating on this item. Statistical Analysis Standardized mean differences (SMDs) with 95% confidence intervals (CIs) were computed using Revman v5.3 for studies reporting pain intensity. An SMD was computed as it normalizes variability among studies and allows for increased generalizability and comparability as different electric stimulation modalities were used, different parameters were employed, and various treatment locations were targeted in this review [20]. A quantitative analysis on studies reporting PPT outcomes was not performed as there were too few studies reporting adequate PPT data to perform a quantitative analysis that was not influenced by the violation of independence for multiple subtrials within studies. Cohen’s criteria were used to determine the effect size of SMDs: an SMD of 0.2 to 0.5 was considered small, an SMD of 0.5 to 0.8 was considered moderate, and an SMD above 0.8 was considered large [21]. SMDs below 0.2 were considered unsubstantial. A random-effects model (DerSimonian and Laird method) was used to pool all the studies’ results as different electric stimulation methods were employed in each study (Table 1). If a study reported multiple interventions or measurements at various body locations, these observations were entered separately into the analysis. This violates the assumption of independence, and therefore a separate analysis that included one subgroup from each study was conducted to determine the robustness of the quantitative analysis, as was done by Johnson & Martinson [18]. Publication bias using the Egger’s test for publication bias was planned; however, there were fewer than 10 studies in the analysis, giving the test low power [22]. Table 1 Summary of the included articles Author Sample Size Mean Age, y Number and Time Period of Treatment Sessions Electrical Stimulation Parameters (Pulse Width, Frequency, Amplitude) Intervention Region Treated Outcomes Aranha et al. [11] N = 60 60 F 27.33 8 sessions, 30 minutes 2–100 Hz, maximum intensity tolerated by participant No electrical stimulation on needling at acupoints No electrical stimulation, needle inserted 1 cm away from acupoints Electroacupuncture Sham acupuncture Upper trapezius muscle VAS Significant decrease in general pain intensity No significant decrease in general pain intensity Conti et al. [24] N = 15 12 F 3 M 34.6 At least 10 sessions (nights wearing electric stimulation device) Parameters were not reported No activity Active contingent electrical stimulation Inactive contingent electrical stimulation Anterior temporalis VAS Interaction between pretreatment and post-treatment measurements between groups was not significant PPT Interaction between baseline/post-treatment measurements between groups was not significant Ferrandiz et al. [25] N = 32 32 F 40.76 6 sessions, 2 minutes per trigger point for 20 minutes 300 microseconds, 8 Hz, 0.4–16 mA (amplitude calibrated until the participant reports a noticeable but tolerable sensation) No electrical stimulation delivered TENS Sham TENS Lower back: spina iliaca posterior superior VAS Interaction between group and time was not significant Ferreira et al. [26] N = 40 30 F 10 M 24.6 1 session, 50 minutes 100 microseconds, 4–100 Hz, high intensity but did not induce muscle contraction 40 seconds of stimulation, then the stimulation was tapered to nil TENS Placebo TENS Masseter and anterior temporalis muscles VAS Significant decrease in pain intensity No significant decrease in pain intensity No between-group differences observed PPT Significant increase in PPT at the masseter, anterior temporalis, and sternocleidomastoid muscles No significant PPT changes Significant between-group differences immediately following treatment at the anterior temporalis muscle, temporomandibular joint, and sternocleidomastoid muscle Gemmell & Hilland [27] N = 60 28 F 32 M 24.9 1 session, 3 minutes Pulse width not reported, 8–10 Hz, 0–45 mA Light pressure applied using machine TENS Sham TENS Trapezius muscle NRS Within-group significance not reported Significantly greater decrease in pain intensity on pressure at the trigger point in the TENS group relative to the placebo group PPT Within-group significance not reported No significant change in PPT between groups Gomes et al. [28] N = 20 20 F 22.50 10 sessions, 30 minutes 20 microseconds, 10 Hz, intensity not reported No stimulation High-voltage electrical stimulation Placebo Anterior temporalis, masseter VAS Significant reduction in pain intensity Insignificant reduction in pain intensity No significant between-group differences Graff-Radford et al. [29] N = 60 45 F 15 M 43.33 1 session, 10 minutes 250 microseconds, 2 Hz, 10–20 mA 250 microseconds, 100 Hz, <39 mA 50 microseconds, 100 Hz, <39 mA 15 milliseconds, 1,200–20,000 Hz, 1–4 mA No electric stimulation delivered TENS A TENS B TENS C TENS D Sham TENS Trapezius, paraspinalis, splenius capitis, rhomboid, temporalis, levator scapulae, masseter, sternocleidomastoid, deltoid, pectoralis major, infraspinatus VAS No significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity No significant reduction in pain intensity TENS B had a significantly greater pain intensity reduction relative to other groups TENS C and TENS D had similar pain intensity reductions, which were significantly higher than TENS A and the control group’s pain intensity TENS A and control did not differ in pain intensity PPT Main effect for baseline and post-treatment scores was significant Group main effect was not significant No significant between-groups differences when assessing pretreatment to post-treatment change scores Hsueh et al. [30] N = 60 35 F 25 M 44.4 1 session, 20 minutes Pulse width not reported, 60 Hz, intensity strong enough to be felt but not to induce muscle contraction Pulse width not reported, 10 Hz, high intensity to induce muscle contraction 0 mA Electric nerve stimulation Electric muscle stimulation Placebo stimulation Upper trapezius VAS Within-group changes not reported Improvements in pain intensity were significantly higher in the ENS group relative to the placebo and EMS groups PPT Within-group changes not reported Improvement in PPT was significantly higher in the ENS group relative to the placebo and EMS groups Kim et al. [31] N = 20 9 F 11 M 26.22 1 session, 5 minutes 200 microseconds, 100 Hz, intensity set to a strong but comfortable sensation No electrical stimulation TENS Sham TENS Infraspinatus and upper trapezius muscles PPT No statistically significant differences were found for time or time by treatment interaction Significantly higher PPT in the TENS group relative to the sham TENS group in the ipsilateral infraspinatus muscle only Müller et al. [23] N = 24 24 F 27.33 8 sessions, 30 minutes 500–700 microseconds, 2–100 Hz, intensity set to maximum painless stimuli with muscle contraction observed No electrical stimulation, needles inserted at the same acupoints as the electro-acupuncture group No electrical stimulation, needles inserted 1 cm away from acupoints Electro-acupuncture Sham acupuncture Upper trapezius muscles VAS Significant decrease in pain intensity in the right and left trapezius, as well as general pain No significant change in pain intensity No significant differences between groups Rai et al. [32] N = 60 (excluding US intervention patients) 42 F 18 M 32.3 18 sessions or until muscle pain was below 10 on a 0–100 scale, treatment continued if patient’s symptoms were not relieved or until pain was rated below 10 No reported methods No reported methods TENS Control Not reported VAS Within-group changes not reported No reported statistical differences in pain intensity between the TENS and placebo groups Rodríguez-Fernández et al. [33] N = 76 31 F 45 M 23 1 session, 10 minutes 200 microseconds, 2–100 Hz, intensity until able to induce a contraction No intensity application TENS Sham TENS Upper trapezius muscle PPT Interaction between group and time was significant Significantly greater increase in referred PPT in the TENS group compared with the sham TENS Sahin et al. [13] N = 75 40 F 35 M 31.64 10 sessions, 30 minutes 40 microseconds, 100 Hz, low amplitude 250 microseconds, 4Hz, high amplitude 40 microseconds, 2–100 Hz, high amplitude Stimulation until the patient felt it, then the current was interrupted and patient said that they were used to the stimulus Conventional TENS Acupuncture like TENS Burst TENS Sham TENS Cervical muscles VAS No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant differences in pain intensity between groups Schabrun et al. [34] N = 23 8 15 F 8 M 23.15 1 session, 10 minutes Pulse width not reported, 15–Hz, intensity increased until strong tingling and comfortable sensation perceived 2 seconds of stimulation, then machine turned off InterX neurostimualtion (superficial stimulation) Sham control Neck and shoulder muscles VAS Significant reduction in pain intensity Significant reduction in pain intensity No significant differences in pain intensity between groups PPT No significant reduction in PPT No significant reduction in PPT No significant differences in PPT between groups Smania et al. [35] N = 36 (excluding the repetitive magnetic stimulation group) 25 F 11 M 40.6 10 sessions, 20 minutes 250 microseconds, 100 Hz, <39 mA (below muscular contraction threshold) In active ultrasound treatment TENS Placebo Superior trapezius muscle VAS Significant decrease in pain intensity No significant change in pain intensity Significant difference between groups in VAS change between pretreatment and post-treatment PPT No significant change in PPT No significant change in PPT No significant between-group PPT change differences Author Sample Size Mean Age, y Number and Time Period of Treatment Sessions Electrical Stimulation Parameters (Pulse Width, Frequency, Amplitude) Intervention Region Treated Outcomes Aranha et al. [11] N = 60 60 F 27.33 8 sessions, 30 minutes 2–100 Hz, maximum intensity tolerated by participant No electrical stimulation on needling at acupoints No electrical stimulation, needle inserted 1 cm away from acupoints Electroacupuncture Sham acupuncture Upper trapezius muscle VAS Significant decrease in general pain intensity No significant decrease in general pain intensity Conti et al. [24] N = 15 12 F 3 M 34.6 At least 10 sessions (nights wearing electric stimulation device) Parameters were not reported No activity Active contingent electrical stimulation Inactive contingent electrical stimulation Anterior temporalis VAS Interaction between pretreatment and post-treatment measurements between groups was not significant PPT Interaction between baseline/post-treatment measurements between groups was not significant Ferrandiz et al. [25] N = 32 32 F 40.76 6 sessions, 2 minutes per trigger point for 20 minutes 300 microseconds, 8 Hz, 0.4–16 mA (amplitude calibrated until the participant reports a noticeable but tolerable sensation) No electrical stimulation delivered TENS Sham TENS Lower back: spina iliaca posterior superior VAS Interaction between group and time was not significant Ferreira et al. [26] N = 40 30 F 10 M 24.6 1 session, 50 minutes 100 microseconds, 4–100 Hz, high intensity but did not induce muscle contraction 40 seconds of stimulation, then the stimulation was tapered to nil TENS Placebo TENS Masseter and anterior temporalis muscles VAS Significant decrease in pain intensity No significant decrease in pain intensity No between-group differences observed PPT Significant increase in PPT at the masseter, anterior temporalis, and sternocleidomastoid muscles No significant PPT changes Significant between-group differences immediately following treatment at the anterior temporalis muscle, temporomandibular joint, and sternocleidomastoid muscle Gemmell & Hilland [27] N = 60 28 F 32 M 24.9 1 session, 3 minutes Pulse width not reported, 8–10 Hz, 0–45 mA Light pressure applied using machine TENS Sham TENS Trapezius muscle NRS Within-group significance not reported Significantly greater decrease in pain intensity on pressure at the trigger point in the TENS group relative to the placebo group PPT Within-group significance not reported No significant change in PPT between groups Gomes et al. [28] N = 20 20 F 22.50 10 sessions, 30 minutes 20 microseconds, 10 Hz, intensity not reported No stimulation High-voltage electrical stimulation Placebo Anterior temporalis, masseter VAS Significant reduction in pain intensity Insignificant reduction in pain intensity No significant between-group differences Graff-Radford et al. [29] N = 60 45 F 15 M 43.33 1 session, 10 minutes 250 microseconds, 2 Hz, 10–20 mA 250 microseconds, 100 Hz, <39 mA 50 microseconds, 100 Hz, <39 mA 15 milliseconds, 1,200–20,000 Hz, 1–4 mA No electric stimulation delivered TENS A TENS B TENS C TENS D Sham TENS Trapezius, paraspinalis, splenius capitis, rhomboid, temporalis, levator scapulae, masseter, sternocleidomastoid, deltoid, pectoralis major, infraspinatus VAS No significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity No significant reduction in pain intensity TENS B had a significantly greater pain intensity reduction relative to other groups TENS C and TENS D had similar pain intensity reductions, which were significantly higher than TENS A and the control group’s pain intensity TENS A and control did not differ in pain intensity PPT Main effect for baseline and post-treatment scores was significant Group main effect was not significant No significant between-groups differences when assessing pretreatment to post-treatment change scores Hsueh et al. [30] N = 60 35 F 25 M 44.4 1 session, 20 minutes Pulse width not reported, 60 Hz, intensity strong enough to be felt but not to induce muscle contraction Pulse width not reported, 10 Hz, high intensity to induce muscle contraction 0 mA Electric nerve stimulation Electric muscle stimulation Placebo stimulation Upper trapezius VAS Within-group changes not reported Improvements in pain intensity were significantly higher in the ENS group relative to the placebo and EMS groups PPT Within-group changes not reported Improvement in PPT was significantly higher in the ENS group relative to the placebo and EMS groups Kim et al. [31] N = 20 9 F 11 M 26.22 1 session, 5 minutes 200 microseconds, 100 Hz, intensity set to a strong but comfortable sensation No electrical stimulation TENS Sham TENS Infraspinatus and upper trapezius muscles PPT No statistically significant differences were found for time or time by treatment interaction Significantly higher PPT in the TENS group relative to the sham TENS group in the ipsilateral infraspinatus muscle only Müller et al. [23] N = 24 24 F 27.33 8 sessions, 30 minutes 500–700 microseconds, 2–100 Hz, intensity set to maximum painless stimuli with muscle contraction observed No electrical stimulation, needles inserted at the same acupoints as the electro-acupuncture group No electrical stimulation, needles inserted 1 cm away from acupoints Electro-acupuncture Sham acupuncture Upper trapezius muscles VAS Significant decrease in pain intensity in the right and left trapezius, as well as general pain No significant change in pain intensity No significant differences between groups Rai et al. [32] N = 60 (excluding US intervention patients) 42 F 18 M 32.3 18 sessions or until muscle pain was below 10 on a 0–100 scale, treatment continued if patient’s symptoms were not relieved or until pain was rated below 10 No reported methods No reported methods TENS Control Not reported VAS Within-group changes not reported No reported statistical differences in pain intensity between the TENS and placebo groups Rodríguez-Fernández et al. [33] N = 76 31 F 45 M 23 1 session, 10 minutes 200 microseconds, 2–100 Hz, intensity until able to induce a contraction No intensity application TENS Sham TENS Upper trapezius muscle PPT Interaction between group and time was significant Significantly greater increase in referred PPT in the TENS group compared with the sham TENS Sahin et al. [13] N = 75 40 F 35 M 31.64 10 sessions, 30 minutes 40 microseconds, 100 Hz, low amplitude 250 microseconds, 4Hz, high amplitude 40 microseconds, 2–100 Hz, high amplitude Stimulation until the patient felt it, then the current was interrupted and patient said that they were used to the stimulus Conventional TENS Acupuncture like TENS Burst TENS Sham TENS Cervical muscles VAS No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant differences in pain intensity between groups Schabrun et al. [34] N = 23 8 15 F 8 M 23.15 1 session, 10 minutes Pulse width not reported, 15–Hz, intensity increased until strong tingling and comfortable sensation perceived 2 seconds of stimulation, then machine turned off InterX neurostimualtion (superficial stimulation) Sham control Neck and shoulder muscles VAS Significant reduction in pain intensity Significant reduction in pain intensity No significant differences in pain intensity between groups PPT No significant reduction in PPT No significant reduction in PPT No significant differences in PPT between groups Smania et al. [35] N = 36 (excluding the repetitive magnetic stimulation group) 25 F 11 M 40.6 10 sessions, 20 minutes 250 microseconds, 100 Hz, <39 mA (below muscular contraction threshold) In active ultrasound treatment TENS Placebo Superior trapezius muscle VAS Significant decrease in pain intensity No significant change in pain intensity Significant difference between groups in VAS change between pretreatment and post-treatment PPT No significant change in PPT No significant change in PPT No significant between-group PPT change differences NRS=numerical rating scale; PPT=pain pressure threshold; TENS = transcutaneous electric stimulation; US = ultrasound; VAS=visual analog scale. Open in new tab Table 1 Summary of the included articles Author Sample Size Mean Age, y Number and Time Period of Treatment Sessions Electrical Stimulation Parameters (Pulse Width, Frequency, Amplitude) Intervention Region Treated Outcomes Aranha et al. [11] N = 60 60 F 27.33 8 sessions, 30 minutes 2–100 Hz, maximum intensity tolerated by participant No electrical stimulation on needling at acupoints No electrical stimulation, needle inserted 1 cm away from acupoints Electroacupuncture Sham acupuncture Upper trapezius muscle VAS Significant decrease in general pain intensity No significant decrease in general pain intensity Conti et al. [24] N = 15 12 F 3 M 34.6 At least 10 sessions (nights wearing electric stimulation device) Parameters were not reported No activity Active contingent electrical stimulation Inactive contingent electrical stimulation Anterior temporalis VAS Interaction between pretreatment and post-treatment measurements between groups was not significant PPT Interaction between baseline/post-treatment measurements between groups was not significant Ferrandiz et al. [25] N = 32 32 F 40.76 6 sessions, 2 minutes per trigger point for 20 minutes 300 microseconds, 8 Hz, 0.4–16 mA (amplitude calibrated until the participant reports a noticeable but tolerable sensation) No electrical stimulation delivered TENS Sham TENS Lower back: spina iliaca posterior superior VAS Interaction between group and time was not significant Ferreira et al. [26] N = 40 30 F 10 M 24.6 1 session, 50 minutes 100 microseconds, 4–100 Hz, high intensity but did not induce muscle contraction 40 seconds of stimulation, then the stimulation was tapered to nil TENS Placebo TENS Masseter and anterior temporalis muscles VAS Significant decrease in pain intensity No significant decrease in pain intensity No between-group differences observed PPT Significant increase in PPT at the masseter, anterior temporalis, and sternocleidomastoid muscles No significant PPT changes Significant between-group differences immediately following treatment at the anterior temporalis muscle, temporomandibular joint, and sternocleidomastoid muscle Gemmell & Hilland [27] N = 60 28 F 32 M 24.9 1 session, 3 minutes Pulse width not reported, 8–10 Hz, 0–45 mA Light pressure applied using machine TENS Sham TENS Trapezius muscle NRS Within-group significance not reported Significantly greater decrease in pain intensity on pressure at the trigger point in the TENS group relative to the placebo group PPT Within-group significance not reported No significant change in PPT between groups Gomes et al. [28] N = 20 20 F 22.50 10 sessions, 30 minutes 20 microseconds, 10 Hz, intensity not reported No stimulation High-voltage electrical stimulation Placebo Anterior temporalis, masseter VAS Significant reduction in pain intensity Insignificant reduction in pain intensity No significant between-group differences Graff-Radford et al. [29] N = 60 45 F 15 M 43.33 1 session, 10 minutes 250 microseconds, 2 Hz, 10–20 mA 250 microseconds, 100 Hz, <39 mA 50 microseconds, 100 Hz, <39 mA 15 milliseconds, 1,200–20,000 Hz, 1–4 mA No electric stimulation delivered TENS A TENS B TENS C TENS D Sham TENS Trapezius, paraspinalis, splenius capitis, rhomboid, temporalis, levator scapulae, masseter, sternocleidomastoid, deltoid, pectoralis major, infraspinatus VAS No significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity No significant reduction in pain intensity TENS B had a significantly greater pain intensity reduction relative to other groups TENS C and TENS D had similar pain intensity reductions, which were significantly higher than TENS A and the control group’s pain intensity TENS A and control did not differ in pain intensity PPT Main effect for baseline and post-treatment scores was significant Group main effect was not significant No significant between-groups differences when assessing pretreatment to post-treatment change scores Hsueh et al. [30] N = 60 35 F 25 M 44.4 1 session, 20 minutes Pulse width not reported, 60 Hz, intensity strong enough to be felt but not to induce muscle contraction Pulse width not reported, 10 Hz, high intensity to induce muscle contraction 0 mA Electric nerve stimulation Electric muscle stimulation Placebo stimulation Upper trapezius VAS Within-group changes not reported Improvements in pain intensity were significantly higher in the ENS group relative to the placebo and EMS groups PPT Within-group changes not reported Improvement in PPT was significantly higher in the ENS group relative to the placebo and EMS groups Kim et al. [31] N = 20 9 F 11 M 26.22 1 session, 5 minutes 200 microseconds, 100 Hz, intensity set to a strong but comfortable sensation No electrical stimulation TENS Sham TENS Infraspinatus and upper trapezius muscles PPT No statistically significant differences were found for time or time by treatment interaction Significantly higher PPT in the TENS group relative to the sham TENS group in the ipsilateral infraspinatus muscle only Müller et al. [23] N = 24 24 F 27.33 8 sessions, 30 minutes 500–700 microseconds, 2–100 Hz, intensity set to maximum painless stimuli with muscle contraction observed No electrical stimulation, needles inserted at the same acupoints as the electro-acupuncture group No electrical stimulation, needles inserted 1 cm away from acupoints Electro-acupuncture Sham acupuncture Upper trapezius muscles VAS Significant decrease in pain intensity in the right and left trapezius, as well as general pain No significant change in pain intensity No significant differences between groups Rai et al. [32] N = 60 (excluding US intervention patients) 42 F 18 M 32.3 18 sessions or until muscle pain was below 10 on a 0–100 scale, treatment continued if patient’s symptoms were not relieved or until pain was rated below 10 No reported methods No reported methods TENS Control Not reported VAS Within-group changes not reported No reported statistical differences in pain intensity between the TENS and placebo groups Rodríguez-Fernández et al. [33] N = 76 31 F 45 M 23 1 session, 10 minutes 200 microseconds, 2–100 Hz, intensity until able to induce a contraction No intensity application TENS Sham TENS Upper trapezius muscle PPT Interaction between group and time was significant Significantly greater increase in referred PPT in the TENS group compared with the sham TENS Sahin et al. [13] N = 75 40 F 35 M 31.64 10 sessions, 30 minutes 40 microseconds, 100 Hz, low amplitude 250 microseconds, 4Hz, high amplitude 40 microseconds, 2–100 Hz, high amplitude Stimulation until the patient felt it, then the current was interrupted and patient said that they were used to the stimulus Conventional TENS Acupuncture like TENS Burst TENS Sham TENS Cervical muscles VAS No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant differences in pain intensity between groups Schabrun et al. [34] N = 23 8 15 F 8 M 23.15 1 session, 10 minutes Pulse width not reported, 15–Hz, intensity increased until strong tingling and comfortable sensation perceived 2 seconds of stimulation, then machine turned off InterX neurostimualtion (superficial stimulation) Sham control Neck and shoulder muscles VAS Significant reduction in pain intensity Significant reduction in pain intensity No significant differences in pain intensity between groups PPT No significant reduction in PPT No significant reduction in PPT No significant differences in PPT between groups Smania et al. [35] N = 36 (excluding the repetitive magnetic stimulation group) 25 F 11 M 40.6 10 sessions, 20 minutes 250 microseconds, 100 Hz, <39 mA (below muscular contraction threshold) In active ultrasound treatment TENS Placebo Superior trapezius muscle VAS Significant decrease in pain intensity No significant change in pain intensity Significant difference between groups in VAS change between pretreatment and post-treatment PPT No significant change in PPT No significant change in PPT No significant between-group PPT change differences Author Sample Size Mean Age, y Number and Time Period of Treatment Sessions Electrical Stimulation Parameters (Pulse Width, Frequency, Amplitude) Intervention Region Treated Outcomes Aranha et al. [11] N = 60 60 F 27.33 8 sessions, 30 minutes 2–100 Hz, maximum intensity tolerated by participant No electrical stimulation on needling at acupoints No electrical stimulation, needle inserted 1 cm away from acupoints Electroacupuncture Sham acupuncture Upper trapezius muscle VAS Significant decrease in general pain intensity No significant decrease in general pain intensity Conti et al. [24] N = 15 12 F 3 M 34.6 At least 10 sessions (nights wearing electric stimulation device) Parameters were not reported No activity Active contingent electrical stimulation Inactive contingent electrical stimulation Anterior temporalis VAS Interaction between pretreatment and post-treatment measurements between groups was not significant PPT Interaction between baseline/post-treatment measurements between groups was not significant Ferrandiz et al. [25] N = 32 32 F 40.76 6 sessions, 2 minutes per trigger point for 20 minutes 300 microseconds, 8 Hz, 0.4–16 mA (amplitude calibrated until the participant reports a noticeable but tolerable sensation) No electrical stimulation delivered TENS Sham TENS Lower back: spina iliaca posterior superior VAS Interaction between group and time was not significant Ferreira et al. [26] N = 40 30 F 10 M 24.6 1 session, 50 minutes 100 microseconds, 4–100 Hz, high intensity but did not induce muscle contraction 40 seconds of stimulation, then the stimulation was tapered to nil TENS Placebo TENS Masseter and anterior temporalis muscles VAS Significant decrease in pain intensity No significant decrease in pain intensity No between-group differences observed PPT Significant increase in PPT at the masseter, anterior temporalis, and sternocleidomastoid muscles No significant PPT changes Significant between-group differences immediately following treatment at the anterior temporalis muscle, temporomandibular joint, and sternocleidomastoid muscle Gemmell & Hilland [27] N = 60 28 F 32 M 24.9 1 session, 3 minutes Pulse width not reported, 8–10 Hz, 0–45 mA Light pressure applied using machine TENS Sham TENS Trapezius muscle NRS Within-group significance not reported Significantly greater decrease in pain intensity on pressure at the trigger point in the TENS group relative to the placebo group PPT Within-group significance not reported No significant change in PPT between groups Gomes et al. [28] N = 20 20 F 22.50 10 sessions, 30 minutes 20 microseconds, 10 Hz, intensity not reported No stimulation High-voltage electrical stimulation Placebo Anterior temporalis, masseter VAS Significant reduction in pain intensity Insignificant reduction in pain intensity No significant between-group differences Graff-Radford et al. [29] N = 60 45 F 15 M 43.33 1 session, 10 minutes 250 microseconds, 2 Hz, 10–20 mA 250 microseconds, 100 Hz, <39 mA 50 microseconds, 100 Hz, <39 mA 15 milliseconds, 1,200–20,000 Hz, 1–4 mA No electric stimulation delivered TENS A TENS B TENS C TENS D Sham TENS Trapezius, paraspinalis, splenius capitis, rhomboid, temporalis, levator scapulae, masseter, sternocleidomastoid, deltoid, pectoralis major, infraspinatus VAS No significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity Significant reduction in pain intensity No significant reduction in pain intensity TENS B had a significantly greater pain intensity reduction relative to other groups TENS C and TENS D had similar pain intensity reductions, which were significantly higher than TENS A and the control group’s pain intensity TENS A and control did not differ in pain intensity PPT Main effect for baseline and post-treatment scores was significant Group main effect was not significant No significant between-groups differences when assessing pretreatment to post-treatment change scores Hsueh et al. [30] N = 60 35 F 25 M 44.4 1 session, 20 minutes Pulse width not reported, 60 Hz, intensity strong enough to be felt but not to induce muscle contraction Pulse width not reported, 10 Hz, high intensity to induce muscle contraction 0 mA Electric nerve stimulation Electric muscle stimulation Placebo stimulation Upper trapezius VAS Within-group changes not reported Improvements in pain intensity were significantly higher in the ENS group relative to the placebo and EMS groups PPT Within-group changes not reported Improvement in PPT was significantly higher in the ENS group relative to the placebo and EMS groups Kim et al. [31] N = 20 9 F 11 M 26.22 1 session, 5 minutes 200 microseconds, 100 Hz, intensity set to a strong but comfortable sensation No electrical stimulation TENS Sham TENS Infraspinatus and upper trapezius muscles PPT No statistically significant differences were found for time or time by treatment interaction Significantly higher PPT in the TENS group relative to the sham TENS group in the ipsilateral infraspinatus muscle only Müller et al. [23] N = 24 24 F 27.33 8 sessions, 30 minutes 500–700 microseconds, 2–100 Hz, intensity set to maximum painless stimuli with muscle contraction observed No electrical stimulation, needles inserted at the same acupoints as the electro-acupuncture group No electrical stimulation, needles inserted 1 cm away from acupoints Electro-acupuncture Sham acupuncture Upper trapezius muscles VAS Significant decrease in pain intensity in the right and left trapezius, as well as general pain No significant change in pain intensity No significant differences between groups Rai et al. [32] N = 60 (excluding US intervention patients) 42 F 18 M 32.3 18 sessions or until muscle pain was below 10 on a 0–100 scale, treatment continued if patient’s symptoms were not relieved or until pain was rated below 10 No reported methods No reported methods TENS Control Not reported VAS Within-group changes not reported No reported statistical differences in pain intensity between the TENS and placebo groups Rodríguez-Fernández et al. [33] N = 76 31 F 45 M 23 1 session, 10 minutes 200 microseconds, 2–100 Hz, intensity until able to induce a contraction No intensity application TENS Sham TENS Upper trapezius muscle PPT Interaction between group and time was significant Significantly greater increase in referred PPT in the TENS group compared with the sham TENS Sahin et al. [13] N = 75 40 F 35 M 31.64 10 sessions, 30 minutes 40 microseconds, 100 Hz, low amplitude 250 microseconds, 4Hz, high amplitude 40 microseconds, 2–100 Hz, high amplitude Stimulation until the patient felt it, then the current was interrupted and patient said that they were used to the stimulus Conventional TENS Acupuncture like TENS Burst TENS Sham TENS Cervical muscles VAS No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant decrease in pain intensity No significant differences in pain intensity between groups Schabrun et al. [34] N = 23 8 15 F 8 M 23.15 1 session, 10 minutes Pulse width not reported, 15–Hz, intensity increased until strong tingling and comfortable sensation perceived 2 seconds of stimulation, then machine turned off InterX neurostimualtion (superficial stimulation) Sham control Neck and shoulder muscles VAS Significant reduction in pain intensity Significant reduction in pain intensity No significant differences in pain intensity between groups PPT No significant reduction in PPT No significant reduction in PPT No significant differences in PPT between groups Smania et al. [35] N = 36 (excluding the repetitive magnetic stimulation group) 25 F 11 M 40.6 10 sessions, 20 minutes 250 microseconds, 100 Hz, <39 mA (below muscular contraction threshold) In active ultrasound treatment TENS Placebo Superior trapezius muscle VAS Significant decrease in pain intensity No significant change in pain intensity Significant difference between groups in VAS change between pretreatment and post-treatment PPT No significant change in PPT No significant change in PPT No significant between-group PPT change differences NRS=numerical rating scale; PPT=pain pressure threshold; TENS = transcutaneous electric stimulation; US = ultrasound; VAS=visual analog scale. Open in new tab The chi2 test was used to assess heterogeneity with an alpha of 0.05. The I2 test for heterogeneity was also used to indicate the extent of heterogeneity. Heterogeneity cutoffs defined by the Cochrane Handbook for Conducting Systematic Reviews [19] were used. An I2 value of <25% indicated low heterogeneity, 25–50% indicated moderate heterogeneity, and >50% indicated high heterogeneity. Reported pain intensity scores, which were measured immediately after the final treatment administration, were extracted. Studies that only reported mean differences rather than post-treatment values were qualitatively reported but not included in the SMD computations. Post-hoc subgroup analyses were computed on reported pain intensity data to assess the effect of number of sessions, degree of frequency, and location of treatment performed. Results After removal of duplicates, 377 studies were available for screening. A full screen of the articles identified 15 eligible articles. Seven articles had a low risk of bias on four or more of the assessed items (Figure 2). Four articles had a low risk of bias on three of the assessed items, and six articles had a low risk of bias on two or fewer items. All studies controlled for confounding factors such as alternate disorders that present with pain and treatments that can mask or exacerbate the experimental treatment effect. Figure 2 Open in new tabDownload slide Risk of bias summary (+ = low risk of bias, − = high risk of bias, ? = unclear risk of bias). Figure 2 Open in new tabDownload slide Risk of bias summary (+ = low risk of bias, − = high risk of bias, ? = unclear risk of bias). Figure 3 Open in new tabDownload slide Standardized mean differences for visual analog scale scores. Figure 3 Open in new tabDownload slide Standardized mean differences for visual analog scale scores. Qualitative Analysis Electroacupuncture Two studies assessed the effects of electroacupuncture on myofascial pain [11, 23]. These studies employed a sham acupuncture intervention as a control. Sham acupuncture consisted of needle insertions 1 cm away from the acupoints without electric stimulation. These studies delivered alternating frequencies of electric stimulation between 2 Hz and 100 Hz at a high intensity reaching motor threshold. The pulse widths were set to 500 to 700 microseconds in one study and were not reported in the other. Eight treatment sessions were delivered in these studies. Both studies found significant within-group decreases in pain intensity after the electroacupuncture treatment. TENS Thirteen studies investigated the effects of TENS on myofascial pain [13, 23–35]. All studies had sham TENS as a control. Further details about the studies can be found in Table 1. Seven of the 13 studies administered only one treatment session, which was comprised of 11 experimental trials [26, 27, 29–31, 33, 34]. The remaining studies administered six to 12 sessions of TENS treatments and consisted of 10 trials. Five of the 11 trials delivering a single session of TENS found significant within-group improvements in reported pain intensity [26, 29, 34], and five trials found significant between-group differences [26, 29, 30]. Six of 11 trials found significant within-group PPT improvements [26, 29–31, 33], and four of 11 trials found significant between-group improvements in the experimental group [26, 30, 31, 33]. Graff-Radford et al. [29] reported a significant main effect of time when assessing the effect of TENS on PPT. Among the studies implementing multiple sessions, four of 10 trials found within-group improvements in reported pain intensity [11, 23, 28, 35]. A majority of TENS studies employed high-frequency stimulation (>50 Hz), inclusive of both high-frequency stimulation and alternating high- and low-frequency stimulation [13, 15, 26, 29–31, 33–35]. These studies included 11 trials, six of which found significant within-group reductions in reported pain intensity, and five reported significant between-group improvements in the experimental group relative to the sham or placebo group. Five trials found significant within-group changes in PPT scores following high-frequency stimulation, and three trials reported significant between-group differences favoring the experimental group. Kim et al. [31] found higher PPT scores overall in the TENS group; however, only PPT measurements in the ipsilateral infraspinatus muscle were significantly higher in the TENS group. Graff-Radford et al. [29] reported a significant pretreatment post-treatment PPT effect for all trials. Among the studies that employed low-frequency stimulation, two trials of six reported significant within-group decreases in reported pain intensity in the TENS group [13, 25, 27–31]. Gemmell and Hilland [27] reported a significant between-group improvement in the experimental group. Two studies did not report the stimulation parameters [24, 32]. Eight trials within the TENS studies delivered treatment with a duration of less than 15 minutes [27, 29, 31, 33, 34]. Four trials found a significant within-group reduction in pain intensity following treatment, and four found a significant between-group improvement in pain intensity in the TENS group relative to the control or other subtrials in the article. Six trials found a significant within-group improvement in PPT, four of these trials found a significant main effect of time, and two trials found significant between-group improvements in PPT in the experimental group. Twelve trials delivered treatment with a duration of 15 minutes or longer [13, 23–26, 28, 30, 35]. Five trials found within-group improvements in pain intensity among the experimental group, and two trials found between-group differences favoring the experimental group. One trial found a within-group improvement in PPT in the TENS group, and two trials found a between-group improvement in PPT favoring the experimental group among studies delivering longer-duration treatments. The reader should note that not all studies reported both pain intensity and PPT. Information about which articles assessed pain intensity, PPT, or both is outlined in Table 2. Table 2 Sensitivity and subgroup analysis results Subgroup VAS Studies in Each Subgroup Multiple sessions –0.36 (–0.65 to –0.06)* Aranha et al. [11], Conti et al. [24], Müller et al. [23], Sahin et al. [13] without electroacupuncture –0.24 (–0.59 to 0.11) Single session –0.10 (–0.41 to 0.22) Ferreira et al. [26], Graff-Radford et al. [29], Schabrun et al. [34] <15-minute session duration –0.01 (–0.37 to 0.35) Graff-Radford et al. [29], Schabrun et al. [34] >15-minute session duration –0.36 (–0.62 to –0.09)**** Conti et al. [24], Ferreira et al. [26], Sahin et al. [13], Aranha et al. [11], Müller et al. [23] without electroacupuncture studies –0.27 (–0.58 to 0.03) High frequency –0.28 (–0.52 to –0.03)** Aranha et al. [11], Ferreira et al. [26], Graff-Radford et al. (TENS B, C, D) [29], Müller et al. [23], Sahin et al. (conventional and burst-type TENS) [13], Schabrun et al. [34] without electroacupuncture studies –0.19 (–0.46 to 0.08) Low frequency –0.13 (–0.63 to 0.38) Graff-Radford et al. (TENS A) [29, Sahin et al. (electroacupuncture) [13] Cervical and shoulder muscles –0.23 (–0.46 to 0.00)*** Aranha et al. [11], Graff-Radford et al. [29], Müller et al. [23], Sahin et al. [13], Schabrun et al. [34] without electroacupuncture studies –0.14 (–0.40 to 0.12) Fascial muscles –0.25 (–0.78 to 0.28) Conti et al. [24], Ferreira et al. [26] Subgroup VAS Studies in Each Subgroup Multiple sessions –0.36 (–0.65 to –0.06)* Aranha et al. [11], Conti et al. [24], Müller et al. [23], Sahin et al. [13] without electroacupuncture –0.24 (–0.59 to 0.11) Single session –0.10 (–0.41 to 0.22) Ferreira et al. [26], Graff-Radford et al. [29], Schabrun et al. [34] <15-minute session duration –0.01 (–0.37 to 0.35) Graff-Radford et al. [29], Schabrun et al. [34] >15-minute session duration –0.36 (–0.62 to –0.09)**** Conti et al. [24], Ferreira et al. [26], Sahin et al. [13], Aranha et al. [11], Müller et al. [23] without electroacupuncture studies –0.27 (–0.58 to 0.03) High frequency –0.28 (–0.52 to –0.03)** Aranha et al. [11], Ferreira et al. [26], Graff-Radford et al. (TENS B, C, D) [29], Müller et al. [23], Sahin et al. (conventional and burst-type TENS) [13], Schabrun et al. [34] without electroacupuncture studies –0.19 (–0.46 to 0.08) Low frequency –0.13 (–0.63 to 0.38) Graff-Radford et al. (TENS A) [29, Sahin et al. (electroacupuncture) [13] Cervical and shoulder muscles –0.23 (–0.46 to 0.00)*** Aranha et al. [11], Graff-Radford et al. [29], Müller et al. [23], Sahin et al. [13], Schabrun et al. [34] without electroacupuncture studies –0.14 (–0.40 to 0.12) Fascial muscles –0.25 (–0.78 to 0.28) Conti et al. [24], Ferreira et al. [26] TENS = transcutaneous electric stimulation; VAS=visual analog scale. * P = 0.02; **P = 0.03; ***P = 0.05; ****P < 0.01. Open in new tab Table 2 Sensitivity and subgroup analysis results Subgroup VAS Studies in Each Subgroup Multiple sessions –0.36 (–0.65 to –0.06)* Aranha et al. [11], Conti et al. [24], Müller et al. [23], Sahin et al. [13] without electroacupuncture –0.24 (–0.59 to 0.11) Single session –0.10 (–0.41 to 0.22) Ferreira et al. [26], Graff-Radford et al. [29], Schabrun et al. [34] <15-minute session duration –0.01 (–0.37 to 0.35) Graff-Radford et al. [29], Schabrun et al. [34] >15-minute session duration –0.36 (–0.62 to –0.09)**** Conti et al. [24], Ferreira et al. [26], Sahin et al. [13], Aranha et al. [11], Müller et al. [23] without electroacupuncture studies –0.27 (–0.58 to 0.03) High frequency –0.28 (–0.52 to –0.03)** Aranha et al. [11], Ferreira et al. [26], Graff-Radford et al. (TENS B, C, D) [29], Müller et al. [23], Sahin et al. (conventional and burst-type TENS) [13], Schabrun et al. [34] without electroacupuncture studies –0.19 (–0.46 to 0.08) Low frequency –0.13 (–0.63 to 0.38) Graff-Radford et al. (TENS A) [29, Sahin et al. (electroacupuncture) [13] Cervical and shoulder muscles –0.23 (–0.46 to 0.00)*** Aranha et al. [11], Graff-Radford et al. [29], Müller et al. [23], Sahin et al. [13], Schabrun et al. [34] without electroacupuncture studies –0.14 (–0.40 to 0.12) Fascial muscles –0.25 (–0.78 to 0.28) Conti et al. [24], Ferreira et al. [26] Subgroup VAS Studies in Each Subgroup Multiple sessions –0.36 (–0.65 to –0.06)* Aranha et al. [11], Conti et al. [24], Müller et al. [23], Sahin et al. [13] without electroacupuncture –0.24 (–0.59 to 0.11) Single session –0.10 (–0.41 to 0.22) Ferreira et al. [26], Graff-Radford et al. [29], Schabrun et al. [34] <15-minute session duration –0.01 (–0.37 to 0.35) Graff-Radford et al. [29], Schabrun et al. [34] >15-minute session duration –0.36 (–0.62 to –0.09)**** Conti et al. [24], Ferreira et al. [26], Sahin et al. [13], Aranha et al. [11], Müller et al. [23] without electroacupuncture studies –0.27 (–0.58 to 0.03) High frequency –0.28 (–0.52 to –0.03)** Aranha et al. [11], Ferreira et al. [26], Graff-Radford et al. (TENS B, C, D) [29], Müller et al. [23], Sahin et al. (conventional and burst-type TENS) [13], Schabrun et al. [34] without electroacupuncture studies –0.19 (–0.46 to 0.08) Low frequency –0.13 (–0.63 to 0.38) Graff-Radford et al. (TENS A) [29, Sahin et al. (electroacupuncture) [13] Cervical and shoulder muscles –0.23 (–0.46 to 0.00)*** Aranha et al. [11], Graff-Radford et al. [29], Müller et al. [23], Sahin et al. [13], Schabrun et al. [34] without electroacupuncture studies –0.14 (–0.40 to 0.12) Fascial muscles –0.25 (–0.78 to 0.28) Conti et al. [24], Ferreira et al. [26] TENS = transcutaneous electric stimulation; VAS=visual analog scale. * P = 0.02; **P = 0.03; ***P = 0.05; ****P < 0.01. Open in new tab Quantitative Analysis SMDs were computed post-treatment on pain intensity scores from the studies included for analysis. Seven studies reported post-treatment values for pain intensity using VAS [11, 13, 23, 24, 26, 29, 34] (Figure 3). The remainder of the studies reported pretreatment and post-treatment mean change values, did not report control values, had incompatible metrics in their figures for extraction, normalized their data, or reported data in nonparametric values; therefore, these studies were not included in the computations [25, 27, 28, 30, 32, 35]. There were 170 participants receiving an experimental intervention and 171 participants receiving a control intervention in the quantitative analysis [11, 13, 23, 24, 26, 29, 34]. Seventy-four of the control participants were repeated from multiple trials within two studies [13, 29]. The analysis revealed an overall SMD of –0.23 (95% CI = –0.45 to –0.02) favoring the experimental treatment; this effect was significant (P = 0.03). When one subtrial from each study was included, by eliminating high-frequency articles and then low-frequency articles, as was done by Johnson and Martinson [18] to maintain diversity among the observations, to address the violation of independence, the overall effect remained statistically significant (P = 0.04). The heterogeneity among the studies was insignificant (chi2 [df = 11] = 5.79, P = 0.89). The I2 statistic was 0%, indicating low heterogeneity between the studies. The SMD for the TENS articles was –0.16 (95% CI = –0.39 to 0.07), which was not significant (P = 0.17) but favored the experimental treatment. The heterogeneity among the studies was insignificant (chi2 [df = 9] = 3.37, P = 0.95). The I2 statistic was 0%, indicating low heterogeneity between the studies. The SMD for the electroacupuncture articles was –0.64 (95% CI = –1.18 to –0.09), which was significant (P = 0.02) and favored the experimental treatment. The heterogeneity among the studies was insignificant (chi2 [df = 1] = 0.00, P = 0.95). The I2 statistic was 0%, indicating low heterogeneity between the studies. SMDs for each study can be found in Figure 2. Discussion Electric stimulation for pain management is a method employed within various health care settings for the treatment of myofascial pain. This review includes studies that performed electric stimulation through superficial electrodes and others that delivered electric stimulation through acupuncture needles in the form of electroacupuncture. The findings from this study indicate the following conclusions: The quantitative results suggest that electric stimulation techniques are effective for reducing reported pain intensity among individuals affected by myofascial pain. Electroacupuncture treatment offers greater pain relief relative to TENS treatments and influences the overall treatment effect of electric stimulation on pain intensity. Variability in treatment effect sizes and statistically significant findings were observed among studies delivering TENS treatments on reported pain intensity and PPT. Electroacupuncture Compared With TENS for Myofascial Pain Electroacupuncture is an extension of acupuncture, and electric stimulation is delivered through acupuncture needles situated at acupuncture points [36]. Our findings suggest that electroacupuncture offers greater degrees of improvement in comparison with TENS for decreasing reported pain intensity among individuals with myofascial pain. The effect sizes of electroacupuncture treatment were moderate and significant (P = 0.01), whereas TENS studies had small or negligible effect sizes. Given that there were only two studies included in the analysis that assessed the effects of electroacupuncture treatments, our conclusions are limited. However, the mechanism of electroacupuncture seems to target the pathophysiology of the myofascial trigger point and lends support to the superiority of electroacupuncture as an electric stimulation treatment for this disorder. Evidence comparing superficial electric stimulation with electric stimulation delivered through needles suggests that electroacupuncture is more effective for other pain conditions as well. Mechanism of Electric Stimulation, Electric Stimulation Parameters, and Myofascial Pain This review is comprised primarily of studies that assess the effects of TENS on myofascial pain. Among the methodological variations within these studies are the pulse width, frequency, and intensity parameters set. The qualitative results from this study suggest that high-frequency stimulation is more effective than low-frequency electric stimulation. However, given the small number of studies administering low-frequency electric stimulation, it is difficult to make any definitive conclusions. Peripherally, high- and low-frequency electric stimulation promotes analgesia by modulating adrenergic and opioidergic receptors, which are thought to be involved in the pathophysiology of the trigger point—norepinephrine activates alpha 1 adrenergic receptors, which depress muscle length feedback control mechanisms and may be involved in the development of the trigger point [15, 37, 38]. High-intensity electric stimulation that stimulates muscle contraction activates nociceptive afferent fibers and enhances muscle circulation, reducing muscle spasm, eliminating muscle pain, and increasing muscle strength [15, 30]. Myofascial trigger points are characterized by regions of localized ischemia and stiffness and elicit local twitch responses [3, 39, 40]. Assessing the effect of intensity on outcomes in patients with myofascial pain was not feasible in this study as intensity parameters were not consistently reported quantitatively or systematically; however, the effects of high-intensity stimulation seem promising for attenuating symptoms associated with myofascial pain. Hsueh et al. [30] compared pain intensity and PPT following one session of high-intensity/low-frequency and low-intensity/high-frequency electric stimulation in patients with myofascial pain. They found low-intensity/high-frequency stimulation to be more effective for mitigating pain intensity and PPT. Given that changes in muscle properties such as contractility and tone are influenced by longer-term treatment, as is observed in exercise training, it is likely that multiple sessions of high-intensity stimulation are needed before observing improvement in myofascial pain, warranting further empirical investigation. Lin et al. [41] have reported significantly reduced morphine requirements postsurgery among lower abdominal surgical patients receiving low- and high-frequency TENS, with larger reductions among patients receiving high-frequency TENS. Others have assessed the effectiveness of alternating low- and high-frequency TENS on opioid usage following surgery and have found the effects of this parameter to be superior to either high or low frequency alone; the alternating frequency groups utilized significantly lower doses of opioids postsurgery relative to the high- and low-frequency groups [42] Recommendations for Researchers and Clinicians Based on the qualitative evidence available from our systematic review, electroacupuncture is a level III recommendation [43]. It targets nerve regions associated with the dysfunction observed in myofascial pain and can permeate deeper tissue, allowing for analgesic effects at the muscle [44]. We recommend the use of electroacupuncture for treating myofascial pain; our recommendation is limited by the sparsity of studies assessing the effectiveness of electroacupuncture, lending support to the need for additional studies. It is unclear whether low- or high-frequency and -intensity stimulation is more effective for myofascial pain (level V) [43]. Clinical studies employing TENS at varying frequencies suggest that mid to high frequencies of TENS, between 20 Hz and 80 Hz, are most effective at reducing pain intensity relative to high frequencies such as 100 Hz or low frequencies below 10 Hz in both healthy and clinical samples [45, 46]. TENS treatments alternating between 15 Hz and 30 Hz were shown to induce significant reductions in reported pain intensity among low back pain participants following treatment, whereas low- and high-frequency stimulation did not [46]. This variability in findings may be attributed to differences in pathophysiological effects from each type of TENS intensity. The physiology underlying myofascial pain suggests that alternation between low- and high-frequency stimulation may provide the most benefits. Patients with myofascial pain present with nervous system sensitization—in the spine and peripheral nerves—and abnormal motor endplate activity, which benefits from high-frequency stimulation, as well as muscle stiffness and poor muscle tone, which benefit from high-intensity stimulation [30]. Patients with myofascial pain where the central nervous system has been notably sensitized, as demonstrated by symptoms such as enlarged regions of secondary pain and pain referral, would likely benefit from longer treatment periods of high-frequency stimulation. It decreases spinal glutamate levels and aberrant nerve activity [15, 24, 47]. Additionally, each of low- and high-frequency stimulation influences overlapping but distinct supraspinal endogenous mechanisms, which together may offer greater benefits for patients with chronic pain. Alternating frequencies can also overcome the desensitization to electric stimulation–induced analgesia [15]. Limitations There were several limitations in this study. First, the search was limited to English articles, which has the potential to create bias and reduce the observations available for study. Our inclusion criteria allowed any clinical population that presents with myofascial pain to be included in the review. This has the possibility of allowing heterogeneity within samples across studies as the criteria for myofascial Pain Syndrome (MPS) and diagnosing trigger points have changed over time, and they also differ between facial muscles and trigger points in the remainder of the body [3, 48–51]. However, given that MPS has consistently been characterized by localized muscle pain in taut bands or stiff muscles in the literature, our results are unlikely to be significantly affected by the variation in criteria. Conclusions This study is the first systematic review to assess the effects of electric stimulation techniques on myofascial pain. Our review demonstrated that electric stimulation techniques (level III) [43] are effective overall and offer a greater degree of pain relief relative to TENS patients. We also found high-frequency and low-frequency stimulation to be quantitatively similar (level V) [43]. The qualitative results of this review tentatively suggest that high-frequency stimulation may be more effective. Finally, the results also provide preliminary evidence suggesting that treatment sessions with durations longer than 15 minutes are more effective than short-duration sessions. Future studies should assess the effects of electric stimulation at various anatomical regions implicated in the pathophysiology of the trigger point. Different electric stimulation techniques should be assessed within these regions to determine which region is most effective for treatment application and to elucidate a mechanism of action for these interventions—that is, TENS, PENS, electroacupuncture. 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Google Scholar Crossref Search ADS PubMed WorldCat Appendix Search Strategy: Medline 1 exp Myofascial Pain Syndromes/ 2 exp Temporomandibular Joint Disorders/ 3 'myofascial pain syndrome*'.tw,kw. 4 synalg*.tw,kw. 5 (myofascia* adj6 pain* adj6 syndrome*).tw,kw. 6 (myofascia* and ('trigger point*' or trigger-point*)).tw,kw. 7 (pain* adj6 myofascia*).tw,kw. 8 or/1-7 9 exp Transcutaneous Electric Nerve Stimulation/ 10 TENS.tw,kw. 11 TNS.tw,kw. 12 ENS.tw,kw. 13 TES.tw,kw. 14 transcutaneous electric* nerve stimulation*.tw,kw. 15 transcutaneous nerve stimulation*.tw,kw. 16 electro-stimulation therap*.tw,kw. 17 electrostimulation therap*.tw,kw. 18 electric* nerve stimulation*.tw,kw. 19 electric* nerve therap*.tw,kw. 20 electroanalgesi*.tw,kw. 21 electro-analgesi*.tw,kw. 22 (transcutaneous electric* adj4 stimulat*).tw,kw. 23 or/9-22 24 8 and 23 25 24 not (exp animals/ not exp humans/) 26 limit 25 to English language © 2019 American Academy of Pain Medicine. All rights reserved. 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TI - The Effect of Electric Stimulation Techniques on Pain and Tenderness at the Myofascial Trigger Point: A Systematic Review
JF - Pain Medicine
DO - 10.1093/pm/pny278
DA - 2019-09-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-effect-of-electric-stimulation-techniques-on-pain-and-tenderness-YqsFhYYYi0
SP - 1774
VL - 20
IS - 9
DP - DeepDyve
ER -