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Transcutaneous Electric Nerve Stimulation Reduces Acute Postoperative Pain and Analgesic Use After Open Inguinal Hernia Surgery: A Randomized, Double-Blind, Placebo-Controlled Trial

Open AccessPublished:December 10, 2020DOI:https://doi.org/10.1016/j.jpain.2020.11.006

      Highlights

      • TENS is safe and effective for acute postoperative pain treatment.
      • TENS reduces postoperative pain following open inguinal hernia repair.
      • TENS increases pressure pain threshold following open inguinal hernia repair.
      • TENS reduces analgesic intake following open inguinal hernia repair.

      Abstract

      In this randomized, double-blind, placebo-controlled trial, we evaluated the role of transcutaneous electrical nerve stimulation (TENS) in the multimodal treatment (nonopioid analgesics and kinesiotherapy) of postoperative pain following open inguinal hernia repair.
      In total, 80 males participants with elective primary unilateral hernia Lichtenstein repair were randomly allocated to receive TENS or a placebo-TENS procedure. The TENS group received local and segmental conventional TENS on the first and second postoperative days. In the placebo-TENS group, intensity was set at 0 to 0.5mA. Change of pain level at rest, when walking, when standing up from bed, pressure algometry parameters and additional analgesic use were the main outcomes.
      Reduction of VAS pain score and absolute and relative pain relief were observed in the TENS group following the procedures compared to the placebo-TENS group (P< .001). The pressure pain threshold and maximal tolerable pressure in the hernia side were equal before the TENS procedure in both groups (P= .84), but after the procedure, these were higher in TENS group (P< .001). Additional nonopioid analgesics requirements were lower in the TENS group on the first and second postoperative days (P< .001).
      TENS is a safe procedure that can reduce postoperative pain and analgesic use after open inguinal hernia repair.
      The study was registered in the database of clinicaltrials.gov (register number NCT03739060).

      Perspective

      This article presents TENS as a safe and effective nonpharmacologic intervention to reduce postoperative pain after open inguinal hernia repair. TENS could be used in daily practice as part of a multimodal postoperative pain treatment, especially for patients suffering from hyperalgesia.

      Graphical abstract

      Key Words

      Worldwide, more than 20 million patients undergo inguinal hernia repair annually.
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      Classifying post-herniorrhaphy pain syndromes following elective inguinal hernia repair.
      Acute postoperative pain can cause analgesic overdosing, associated side effects, prolonged recovery and return to work, high treatment costs, and a deterioration in quality of life.
      Many different strategies have been investigated to reduce postoperative pain following inguinal hernia repair. Different operative techniques, various types of meshes and fixation techniques, neurectomies, as well as different analgesics, local anesthetic blocks, and nonpharmacological interventions are used to reduce pain in the postoperative period. Multimodal access to analgesia, using a variety of analgesic medications and techniques, combined with nonpharmacological interventions, is recommended for postoperative pain treatment.
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      Management of postoperative pain: A clinical practice guideline from the american pain society, the american society of regional anesthesia and pain medicine, and the american society of anesthesiologists' committee on regional anesthesia, executive committee, and administrative council.
      Transcutaneous electrical nerve stimulation (TENS) is defined by the American Physical Therapy Association as the application of electrical stimulation to the skin for pain control.
      American Physical Therapy Association
      Section on Clinical Electrophysiology. Electrotherapeutic terminology in physical therapy.
      It is a noninvasive technique used to manage acute and chronic pain conditions of nociceptive and neuropathic origin.
      • Johnson MI
      Transcutaneous Electrical Nerve Stimulation (TENS) Research to Support Clinical Practice.
      TENS equipment is inexpensive, and the application technique is easy to learn.
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      Direct and indirect benefits reported by users of transcutaneous electrical nerve stimulation for chronic musculoskeletal pain: Qualitative exploration using patient interviews.
      TENS has low risks of harm, including toxicity or other side effects. The postoperative analgesic benefits of TENS, along with secondary beneficial effects, is supported by previous studies following various surgeries: thoracotomies,
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      major and minor visceral surgery,
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      Adjunct high frequency transcutaneous electric stimulation (TENS) for postoperative pain management during weaning from epidural analgesia following colon surgery: Results from a controlled pilot study.
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      Effectiveness of transcutaneous electrical nerve stimulation on postoperative pain with movement.
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      liposuction,
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      gynecological,
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      High and low frequency TENS reduce postoperative pain intensity after laparoscopic tubal ligation: A randomized controlled trial.
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      High-frequency TENS in post-episiotomy pain relief in primiparous puerpere: A randomized, controlled trial.
      and urological,
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      as well as orthopedic
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      Transcutaneous electrical nerve stimulation for the control of pain during rehabilitation after total knee arthroplasty: A randomized, blinded, placebo-controlled trial.
      and spine surgery.
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      Postoperative and preincisional electrical nerve stimulation TENS reduce postoperative opioid requirement after major spinal surgery.
      However, the data on the role of TENS in pain relief following inguinal hernia repair are controversial. The aim of the present trial was to evaluate the role of TENS in the multimodal treatment of the postoperative pain following open inguinal hernia repair.

      Methods

      Study Design

      Eighty male patients with unilateral inguinal hernia treated by elective surgery (Lichtenstein technique) were recruited to a prospective, randomized, double-blind, placebo controlled clinical trial at Kaunas clinical hospital of Lithuanian university of health sciences, Lithuania, from August 2018 to November 2019 (Fig 1). The study evaluated the role of TENS as a component of multimodal postoperative pain control.
      The study was approved by the regional ethics committee (protocol number BE-2-44; 2018-06-05) and registered in the database of clinicaltrials.gov (register number NCT03739060).
      Eligible patients were invited by the principal investigator to participate in the study and signed an informed consent form. Demographic and clinical data of the participants were collected preoperatively in a pre-designed questionnaire. The summarized study design is presented in Table 1.
      Table 1Summarized Study Design
      OP DayPostoperative Day 1
      ActionAssessment of eligibility to trial

      Surgery

      Pharmacological analgesia
      6 a.m.

      NSAIDS

      8 a.m. Physiotherapist
      RandomizationTENS

      or

      placebo-TENS

      1st procedure
      2h intervalTENS

      or

      placebo-TENS

      2nd procedure
      8 p.m.

      NSAIDS
      OutcomePreoperative and operative dataPatient

      mobilization
      Group allocationPreintervention

      VAS

      Algometer parameters
      Postintervention

      VAS

      Algometer parameters
      Preintervention

      VAS

      Algometer parameters
      Postintervention

      VAS

      Algometer parameters
      Postoperative Day 2
      6 AM NSAIDS

      8 AM

      Physiotherapist
      TENS

      or

      placebo-TENS

      3rd procedure
      2h intervalTENS

      Or

      placebo-TENS

      4nd procedure
      8p.m

      NSAIDS
      Additional analgesics consumption day IPatient

      mobilization
      Preintervention

      VAS

      Algometer parameters
      Postintervention

      VAS

      Algometer parameters
      Preintervention

      VAS

      Algometer parameters
      Postintervention

      VAS

      Algometer parameters
      Additional analgesics consumption day II

      Inclusion and Exclusion Criteria

      The inclusion criteria were: primary hernia; no TENS procedures in the past; American Society of Anesthesiologists physical status I to III; no cognitive, language, hearing or visual disorders; no movement disorders. The exclusion criteria were: surgery complications; nerves in inguinal region were not possible to identify or nerves were damaged; known allergy to a patch glue; chronic use of nonsteroidal anti-inflammatory drugs or opioids; neuropathic diseases; general contraindications for TENS procedures (such as skin disorders in site of electrodes application or pacemaker); non Lichtenstein modification for hernia repair.

      Sample Size Calculation

      Interim analysis for sample size calculation from the first 30 randomized participants was conducted by a statistician to keep the investigator blind throughout the study period.
      Due to observation that a quarter of participants had no pain at rest after surgery, the pain ratings when walking were selected for the study sample size calculation. A 2-group design, α = 0.05, and power = 0.80, a sample size of 64 (32 participants per group) was targeted and 20% additional participants were added, ensuring the number of analyzed cases, a total 80 participants were recruited.

      Surgery

      The hernia repair was performed under spinal anesthesia in all cases using the Lichtenstein technique
      • Reinpold W
      • Chen D
      Evidence-based lichtenstein technique.
      by different experienced surgeons. In brief, the skin incision was made parallel and superior to the inguinal ligament considering to hernia size from 3 to 7 cm. in length. In all cases, n. ilioinguinalis, n. iliohipogastricus and genital branch of n. genitofemoralis were identified and preserved during preparation of hernia sack. Additional, the rupture of fascia transversalis was sutured in a tension free manner using running 1-0 polydioxanone suture. For alloplasty, a polypropylen mesh was used and sutured with running and single 1-0 polypropilen sutures. The external m. oblique abdominis fascia was closed using 2-0 running or single sutures according to decision of the surgeon. Skin was sutured with continuous intradermal 4-0 polydioxanone suture. No additional local anesthesia was used. All participants received intravenous Paracetamol 10 mg/mL 100 mL infusion at the end of the surgery.

      Postoperative Pharmacological Analgesia and Kinesiotherapy

      On the operative day, at the conclusion of spinal analgesia, all participants received pharmacological analgesia with intramuscular (IM) injections of Ketoprofenum 100 mg/2 mL. Pethidin 50 mg/1 mL IM was used in cases of insufficient analgesia or pain in rest was more than 6 cm of 10 cm VAS.
      On the first and second postoperative days all participants received Ketoprofenum 100 mg/2 mL IM at 6:00 AMand 8:00 PM (Table 1). They were also able to ask for additional doses of Ketorolac 30 mg/1 mL IM at any time if needed to control pain. Pethidin 50 mg/1 mL IM was used in cases of insufficient analgesia or when pain at rest was more than 6 cm of 10 cm VAS. All participants were visited by the physiotherapist every morning in the postoperative period before TENS procedure. The participants were instructed in walking exercises beyond the pain threshold and dynamic low-burden kinesic exercises.

      Randomization and Blinding

      On the first postoperative day participants were randomized to receive active TENS procedures (TENS group) or inactive TENS procedures (placebo-TENS group) according to a computer-generated random assignment list. The first investigator was responsible for outcome measurements and data collection, and the second investigator performed the randomization procedure. All measurements were assessed by the first investigator and only the second investigator was aware that the participants received active TENS or placebo-TENS therapy. Group affiliation was revealed at the end of the study.
      The participants were informed that during the TENS procedure they may feel strong, mild tickling, various other sensations, or nothing at all. They were told that just one parameter of TENS is tested and sensationally it is impossible to distinguish whether placebo or real TENS was used. The second investigator asked the participants not to tell about their experiences during TENS procedure to the principal investigator. The second investigator regularly asked the participants of placebo-TENS group if they feel good, if any adverse sensations or pain appeared and from time to time imitated adjustment of the TENS device. To reduce investigator and participant related bias, all participating patients were accommodated separately from each other and the procedures were performed in a different time. After last procedure, the principal investigator and participant were asked to guess the group of allocation to evaluate blinding.

      Intervention: TENS and Placebo-TENS Procedure

      The conventional TENS treatment was applied on the first and second postoperative days, twice a day for 30 minutes with a break of 2 hours between (Table 1). Segmental and local stimulation was used. Two adhesive electrodes (10 × 5 cm) were placed on the skin at the inguinal region in both sides of the surgical incision and two adhesive electrodes (5 × 5 cm) were placed on the skin parallel to spine 2 cm from the adjacent spinal processes of the 12th thoracic to 3rd lumbar vertebrae at the surgery side, according to segmental innervation of inguinal region (Fig 2). The subjects were placed in comfortable supine position.
      Figure 2
      Figure 2Schematic representation of electrode placement in front (local) and back (segmental).
      The TENS device (Endomed 682, Enraf-Nonius, Delft, the Netherlands), generating a biphasic square, was set at a constant stimulation frequency of 100 Hz and a pulse duration of 200 μs. In the active TENS group, the intensity of each channel was increased until the participant was able to feel a maximal tolerable tingling sensation without causing discomfort, pain, or muscle contraction. The intensity was constantly increased during application of TENS to avoid accommodation effects. In the placebo-TENS group, intensity was set at 0 to 0.5 mA, and participants did not feel any sensations.

      Outcome Measurements

      The primary outcome was the change in pain intensity after each TENS procedure. Pain was measured using the visual analog scale (VAS) and algometer. A laminated cardboard ruler with a 100 mm length scale issued by the Lithuanian Pain Society was used to measure pain intensity. On one side it has 2 marks at both ends of the scale: “no pain” and “the most intense pain.” The participants used this side and moved the cursor of the ruler showing pain intensity. The other side of the scale has the usual 100 mm ruler markings and the investigator can easily assess pain intensity indicated by the participant to the nearest millimeter. Before the TENS procedure, participants were asked to show pain intensity while laying down in supine position (in rest), when standing up from the bed, and when walking.
      After the TENS procedure, participants were asked to evaluate whether the procedure was effective and pain was assessed same way in same activities (in rest, when standing up from the bed, and when walking).
      Pressure pain was measured with an algometer in both inguinal regions. The measurement point was 2 cm above the operative wound on the line, running upright from middle of inguinal ligament and was marked using permanent marker. The point on the opposite side was marked symmetrically (Fig 3). The hand-held electronic pressure algometer Pain Test FDX25 (Wagner instruments, Greenwich) with a linear response to force application between 0 and 100 Newton (N) was used. The stimulation transducer of the algometer has a circular rubber tip of 1 cm2, which was applied perpendicularly to the skin, slowly increasing pressure at a rate of 5 N/s. Participants interrupted the stimulation by quick verbal signal, saying “yes” when the pressure reached a measurable pain level. The assessments included pressure pain threshold (PPT), maximal tolerable pressure threshold (MTPT), and interval of pressure pain tolerance (IPPT). For assessment of PPT, patients were instructed to report when the pressure sensation changed into the pain sensation. The pressure was then stopped and that value in newton's (N) was recorded. Assessing MTPT, patient signaled when the tolerable pain changed into suffering pain. IPPT is the calculable parameter (IPPT = MTPT-PPT). IPPT shows the interval at which the participant feels mild pain during pressure. If IPPT is 0, pain is acute and sudden.
      Figure 3
      Figure 3Schematic representation of algometry point. Red points represent algometry points in surgery and contralateral sides. Red line in the left inguinal region represents surgical incision.
      The algometer parameters of all participants were assessed by the same examiner. The algometer maintained its peak value, so the examiner was blinded to see the score while applying pressure.
      The secondary outcome was the need of additional medication intake for pharmacological analgesia on the first and second postoperative days.

      Statistical Analyses

      SPSS version 22 software (SPSS, Inc., Chicago, IL) was used for statistical analysis. Basic characteristics of the groups were normally distributed and Students t-test was used for intergroup analysis. Categorical variables were analyzed using Pearson's chi-square test. Mean of means of multiple time points of each participant were analyzed in between group analysis. The Mann-Whitney test for independent samples was used to compare the pain intensity scores, algometer parameters, and means of analgesic intake between placebo-TENS and TENS groups. Results for all tests were considered significant at P< .05. Continuous variables are represented as mean ± Standard Deviation (SD), or mean ± Standard Errors of Mean (SEM), categorical variables are represented as percentages. The chi-square test was used to compare the blinding of the groups against an expected result of 50:50, as random chance blinding. No adjustments were made for multiplicity.

      Results

      A total of 156 patients were assessed for eligibility for the trial and 76 patients were excluded for various reasons. Eighty participants were enrolled in the trial (TENS [n = 40], and placebo-TENS [n = 40] group). No participant was lost during postoperative follow-up period. The flow chart of the study is shown in Fig 1. The TENS and placebo-TENS groups matched well for basic characteristics (P > .05; Table 2).
      Table 2Basic Characteristics of the Trial Participants
      CharacteristicsPlacebo -TENS groupTENS GroupP Value
      Age, years (mean ± SD)61.08 ± 12.5161.77 ± 10.84.793
      BMI, kg (mean ± SD)26.10 ± 2.9926.75 ± 3.78.398
      ASA physical status (%)
       I32.522.5.184
       II57.552.5
       III10.025.0
      Inguinal hernia side (%)
       Right57.565.0.491
       Left42.535.0
      Inguinal hernia type (%)
       Lateral70.067.5.809
       Medial30.032.5
      Inguinal hernia size EHS
      Hernia size according to EHS classification.27
      (%)
       15.02.5.836
       247.547.5
       347.550.0
      Surgery duration, min. (mean ± SD)75.25 ± 18.9576.38 ± 18.40.788
      Basline VAS pain scores, cm (mean ± SD):
       Pain in rest, cm1.34 ± 0.751.57 ± 1.12.766
       Pain when walking, cm2.93 ± 1.392.93 ± 1.42.934
       Pain when standing up, cm4.06 ± 1.824.29 ± 2.03.754
      Basline algometry parameters surgery side (mean ± SD):
       PPT, N/cm212.56 ± 6,1211.91 ± 5.37.615
       MTPT, N/cm218.96 ± 9,9819.05 ± 10.19.968
       IPPT, N/cm26.40 ± 6,787.15 ± 7.61.642
      low asterisk Hernia size according to EHS classification.
      • Miserez M
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      • Mandala V
      • Palot JP
      • Schumpelick V
      • Simmermacher RK
      • Stoppa R
      • Flament JB
      The european hernia society groin hernia classification: Simple and easy to remember.
      No postoperative surgical complications or TENS related side effects were observed.

      Effect of TENS to Postoperative Pain Relief

      No differences in the postoperative pain VAS scores between groups were found across all pain properties evaluated before first TENS or placebo-TENS procedure.
      There was a significant reduction of VAS pain score in the TENS group following the procedure (P< .001). However, the mean VAS pain scores were higher before the next procedure during the same day compared to the mean VAS pain scores after the first procedure (Fig 4). Detailed data are shown in Supplement Table 1. The absolute and relative pain relief was significantly higher in the TENS group comparing to the placebo-TENS group for pain in rest (P< .01), when walking (P< .01), and when standing up from the bed (P< .01; Table 3).
      Figure 4
      Figure 4Pain change during different physical activities. Lines represent the changes of postoperative pain measured at rest (a), when walking (b) and when standing up from the bed (c). Symbols represent means ± SEM of VAS measurements. Points at procedure numbers correspond measurements before TENS or placebo-TENS procedure, points between the procedures numbers correspond measurements after procedure. Mann-Whitney test, *, ⁎⁎, , †† P < .05. Detailed data are presented in the supplement .
      Table 3An Absolute (Mean ± SD) and Relative (%) Pain Relief in Placebo-TENS and TENS Groups, Measured by VAS (cm)
      Pain PropertiesVASPlacebo-TENSTENSP Value
      Pain in restmean ± SD0.08 ± 0.130.55 ± 0.29<.001
      %5.19 ± 7.8543.14 ± 22.93<.001
      Pain when walkingmean ± SD0.17 ± 0.250.74 ± 0.40<.001
      %9.49 ± 15.1843.98 ± 17.23<.001
      Pain when standing up from bedmean ± SD0.19 ± 0.300.98 ± 0.56<.001
      %7.42 ± 14.6133.32 ± 14.43<.001
      Pain relief in TENS group was about 40% from baseline.
      NOTE. Bold values represent significant differences.
      Estimated PPT and MTPT were similar in both groups as measured by algometer before procedure, but significantly increased after the active TENS procedure when measured on the surgical side (Fig 5). However, there were no differences in PPT or MTPT on the contralateral side before or after the procedures in the TENS and placebo-TENS groups. The IPPT rates were also similar before and after procedures on the surgery and contralateral side (Table 4).
      Figure 5
      Figure 5Change in pressure pain parameters. Lines represent PPT (a) and MTPT (b) in placebo-TENS and TENS groups at surgery and contralateral side expressed in means ± SEM. Points at procedure numbers correspond measurements before TENS or placebo-TENS procedure, points between the procedures numbers correspond measurements after procedure. Mann-Whitney test, *, ⁎⁎, , †† P < .05.
      Table 4Algometry Parameters (N/cm2) Before and After TENS Procedure. Data Are Expressed as Mean ± SD
      ParameterMeasurementSideMeasurement TimePlacebo-TENSTENSP Value
      PPTSurgeryBefore14.41 ± 6.8013.90 ± 5.28.840
      After13.54 ± 7.2717.36 ± 6.64.006
      ContralateralBefore27.04 ± 10.5627.80 ± 7.77.271
      After25.96 ± 10.4328.51 ± 8.64.070
      MTPTSurgeryBefore19.73 ± 9.7620.92 ± 8.64.322
      After19.30 ± 10.4124.92 ± 10.50.008
      ContralateralBefore38.52 ± 10.3638.52 ± 10.36.090
      After35.42 ± 17.1738.00 ± 10.47.053
      IPPTSurgeryBefore5.32 ± 5.887.03± 5.55.242
      After5.76 ± 4.647.56 ± 5.95.109
      ContralateralBefore9.81 ± 9.0610.75 ± 4.96.086
      After9.46 ± 9.259.52 ± 4.41.187
      NOTE. Bold values represent significant differences.

      The Need of Additional Analgesics During the Early Postoperative Days

      There was no need for opioid or semiopioid analgesics on the first and second postoperative days in both groups. The mean additional Ketorolac 30 mg/1 mL IM dose intakes were higher in the placebo-TENS group compared with the TENS group on postoperative day 1 (P= .03), postoperative day 2 (P< .001), and in total (P< .001; Table 5).
      Table 5Additional Postoperative Analgesic Intake
      Placebo-TENSTENSP Value
      Day 12.62 ± 0.912.05 ± 0.88.003
      Day 21.85 ± 1.110.93 ± 0.92<.001
      Total4.46 ± 1.822.98 ± 1.48<.001
      Additional NSAID's doses are expressed as mean ± SD. One dose is equal to one intramuscular injection of Ketorolac 30 mg/1 mL.
      NOTE. Bold values represent significant differences.

      Blinding Results

      Assessor blinding was confirmed as the assessor only guessed the correct group in (26/40) 65.0% of the cases (P = .055) for the active TENS group and (22/40) 55.0% of the cases (P = .487) for the placebo-TENS group. Patients correctly identified the treatment in (31/40) 77.5% of the cases in the active TENS group (P < .001) and (23/40) 57.5% in the placebo-TENS group (P = .342).

      Discussion

      The results of our prospective, randomized, double-blind, placebo-controlled trial demonstrated that TENS reduces postoperative pain following open unilateral inguinal hernia repair surgery when added to multimodal postoperative pain treatment schemes. In particular, postoperative pain was reduced during physical activity, when the participants walked and stood up from bed in the TENS group, compared to placebo-TENS group. The role of TENS in pain relief at rest was difficult to evaluate because all participants received NSAID's twice a day during the postoperative period and pain at rest was mild. Furthermore, we demonstrated that the use of TENS in the postoperative period reduces the need for additional NSAID by up to 2 doses during the first 2 postoperative days.
      Earlier studies presented mixed results concerning the role of TENS in postoperative pain treatment. Gilbert et al did not observe benefits of TENS for pain relief following inguinal hernia repair compared to a placebo group.
      • Gilbert JM
      • Gledhill T
      • Law N
      • George C
      Controlled trial of transcutaneous electrical nerve stimulation (TENS) for postoperative pain relief following inguinal herniorrhaphy.
      In this study, unlike the present trial, they used semiopioid analgesics for postoperative pain control. Semiopioid analgesics have strong analgesic effects, consequently statistical and clinical effects in mild postoperative pain are difficult to identify. There is a relative lack of methodological information for this study, such as what kind of pain and when it was measured, the duration of the TENS procedure, and how the accommodation effect was avoided, making it difficult to compare this study with our trial.
      Another study conducted by de Santana et al demonstrated the benefit of TENS in pain relief for inguinal hernia patients in the first 24 hours after surgery.
      • DeSantana JM
      • Santana-Filho VJ
      • Guerra DR
      • Sluka KA
      • Gurgel RQ
      • da Silva Jr, WM
      Hypoalgesic effect of the transcutaneous electrical nerve stimulation following inguinal herniorrhaphy: A randomized, controlled trial.
      In contrast to this study, we did not observe such strong and long-lasting analgesic effects in the TENS group. In our study, two hours following the first and third TENS procedure (before second and fourth procedure), VAS pain and algometer readings approached baseline. Reasons for this may be due to the algometry measurements themselves, especially MTPT, which triggered nociceptive pain mechanisms. With every measurement, participants showed less patience, and this is seen in the algometry curves of the contralateral side in both groups and the surgery side in the placebo-TENS group. These findings lead to the indirect conclusion that algometry measurements should be taken with caution during the postoperative period, due to additional pain induction. We suggest measuring only PPT for postoperative pain investigations to induce less measurement associated pain. We included algometry measurements to increase the objectiveness and validation of pain change assessment.
      Our trial revealed that the role of TENS in the absolute and relative reduction of pain when standing up from bed and walking was greater than that at rest. It supports previous findings that TENS reduces movement, but not resting postoperative pain and the hypothesis that TENS works through reducing hyperalgesia.
      • Rakel BA
      • Zimmerman MB
      • Geasland K
      • Embree J
      • Clark CR
      • Noiseux NO
      • Callaghan JJ
      • Herr K
      • Walsh D
      • Sluka KA
      Transcutaneous electrical nerve stimulation for the control of pain during rehabilitation after total knee arthroplasty: A randomized, blinded, placebo-controlled trial.
      Some studies suggest that TENS is effective in inducing antihyperalgesia even when the electrodes were placed on the opposite side to the pain irritant,
      • Ainsworth L
      • Budelier K
      • Clinesmith M
      • Fiedler A
      • Landstrom R
      • Leeper BJ
      • Moeller L
      • Mutch S
      • O'Dell K
      • Ross J
      • Radhakrishnan R
      • Sluka KA
      Transcutaneous electrical nerve stimulation (TENS) reduces chronic hyperalgesia induced by muscle inflammation.
      ,
      • Sabino GS
      • Santos CM
      • Francischi JN
      • de Resende MA
      Release of endogenous opioids following transcutaneous electric nerve stimulation in an experimental model of acute inflammatory pain.
      ,
      • Somers DL
      • Clemente FR
      Transcutaneous electrical nerve stimulation for the management of neuropathic pain: The effects of frequency and electrode position on prevention of allodynia in a rat model of complex regional pain syndrome type II.
      because the inhibitory descending system are bilateral, diffuse, and connect both sides of the spinal cord.
      • Robinson D
      • Calejesan AA
      • Zhuo M
      Long-lasting changes in rostral ventral medulla neuronal activity after inflammation.
      In the current study, we did not observe pressure pain relief in contralateral side, but in contrast to other studies there was no hyperalgesia in contralateral side, and pressure algometric measurements could be evaluated just as in healthy subjects.
      The present trial used combined local and segmental stimulation to obtain a stronger and long-lasting analgesic effect. However, no differences between dual-site stimulation and local or segmental stimulation alone have been reported in healthy humans.
      • Claydon LS
      • Chesterton LS
      • Barlas P
      • Sim J
      Alternating-frequency TENS effects on experimental pain in healthy human participants: A randomized placebo-controlled trial.
      However, this technique has been successfully used for postoperative pain treatment following other surgeries.
      • Bjersa K
      • Andersson T
      High frequency TENS as a complement for pain relief in postoperative transition from epidural to general analgesia after pancreatic resection.
      ,
      • Bjersa K
      • Jildenstaal P
      • Jakobsson J
      • Egardt M
      • Fagevik Olsen M
      Adjunct high frequency transcutaneous electric stimulation (TENS) for postoperative pain management during weaning from epidural analgesia following colon surgery: Results from a controlled pilot study.
      We used high frequency TENS due to reports of it being superior to low frequency stimulation for pain relief in healthy humans.
      • Chen CC
      • Johnson MI
      An investigation into the hypoalgesic effects of high- and low-frequency transcutaneous electrical nerve stimulation (TENS) on experimentally-induced blunt pressure pain in healthy human participants.
      It has been previously established that stimulation intensity is a critical factor in efficacy.
      • Moran F
      • Leonard T
      • Hawthorne S
      • Hughes CM
      • McCrum-Gardner E
      • Johnson MI
      • Rakel BA
      • Sluka KA
      • Walsh DM
      Hypoalgesia in response to transcutaneous electrical nerve stimulation (TENS) depends on stimulation intensity.
      ,
      • Vance CG
      • Dailey DL
      • Rakel BA
      • Sluka KA
      Using TENS for pain control: The state of the evidence.
      In our study, TENS intensity was increased, and strong but comfortable tingling sensation was ensured throughout the procedure, as is recommended.
      • Vance CG
      • Dailey DL
      • Rakel BA
      • Sluka KA
      Using TENS for pain control: The state of the evidence.
      High frequency TENS at intensity inducing a strong but comfortable tingling sensation and duration of a minimum 30 minutes was successfully used for pain relief after other surgeries, which include abdominal wall incisions.
      • Desantana JM
      • Sluka KA
      • Lauretti GR
      High and low frequency TENS reduce postoperative pain intensity after laparoscopic tubal ligation: A randomized controlled trial.
      ,
      • Galli TT
      • Chiavegato LD
      • Liebano RE
      Effects of TENS in living kidney donors submitted to open nephrectomy: A randomized placebo-controlled trial.
      ,
      • Kayman-Kose S
      • Arioz DT
      • Toktas H
      • Koken G
      • Kanat-Pektas M
      • Kose M
      • Yilmazer M
      Transcutaneous electrical nerve stimulation (TENS) for pain control after vaginal delivery and cesarean section.
      ,
      • Silva MB
      • de Melo PR
      • de Oliveira NM
      • Crema E
      • Fernandes LF
      Analgesic effect of transcutaneous electrical nerve stimulation after laparoscopic cholecystectomy.
      ,
      • Tokuda M
      • Tabira K
      • Masuda T
      • Nishiwada T
      • Shomoto K
      Effect of modulated-frequency and modulated-intensity transcutaneous electrical nerve stimulation after abdominal surgery: A randomized controlled trial.
      In this study, we compared placebo-TENS and active TENS procedures. Placebo treatment itself can have a powerful effect on pain scores,
      • Colloca L
      • Klinger R
      • Flor H
      • Bingel U
      Placebo analgesia: Psychological and neurobiological mechanisms.
      and small changes in pain could be due to a double placebo effect induced by strong tingling sensation during stimulation. Therefore, in the trial we evaluated not only the relief of pain, but also identified a reduced need for additional analgesics. This does not definitively negate the possibility of placebo effects, but it is possible that the placebo effect attenuated over time and the real effect of TENS was seen in analgesics intake.
      In the trial conducted by De Santana et al, an 80% reduction in analgesics intake was observed in the active TENS compared to the placebo-TENS group in first 24 hours after surgery.
      • DeSantana JM
      • Santana-Filho VJ
      • Guerra DR
      • Sluka KA
      • Gurgel RQ
      • da Silva Jr, WM
      Hypoalgesic effect of the transcutaneous electrical nerve stimulation following inguinal herniorrhaphy: A randomized, controlled trial.
      The Meta-analysis of Bjordal et al concluded that trials where adequate TENS parameters were used showed a 36% reduction in analgesic intake compared with those with inadequate.
      • Bjordal JM
      • Johnson MI
      • Ljunggreen AE
      Transcutaneous electrical nerve stimulation (TENS) can reduce postoperative analgesic consumption. A meta-analysis with assessment of optimal treatment parameters for postoperative pain.
      In our study, we documented reductions of mean NSAID intake by 21.7% and 49.7% on first and second postoperative day, respectively. Our findings are compatible with previous studies and demonstrate that TENS cannot completely replace analgesics in postoperative pain treatment, but can significantly reduce the dose.
      Accommodation effects can occur during TENS procedures and make it difficult to interpret the role of TENS in postoperative pain relief. In our study, an accommodation effect was prevented using non-continuous TENS, as TENS over short time intervals, rather than continually, may be advantageous by minimizing accommodation or habituation to TENS.
      • Fiorelli A
      • Morgillo F
      • Milione R
      • Pace MC
      • Passavanti MB
      • Laperuta P
      • Aurilio C
      • Santini M
      Control of post-thoracotomy pain by transcutaneous electrical nerve stimulation: Effect on serum cytokine levels, visual analogue scale, pulmonary function and medication.
      As with most of the studies mentioned above, we did not observe any TENS related side effects.
      Our study had several limitations:
      • a)
        Blinding was the main challenge for the present trial. Two investigators, computer generated allocation, protection from intercommunication of the patients and TENS naïve patients, knowing that they cannot distinguish between placebo-TENS and real TENS, were the main methodological practices used to reduce investigator and participant related bias. We did not use a sham TENS device because a second investigator was involved and this is more likely to be necessary for self-applying TENS investigations.
        • Rakel B
        • Cooper N
        • Adams HJ
        • Messer BR
        • Frey Law LA
        • Dannen DR
        • Miller CA
        • Polehna AC
        • Ruggle RC
        • Vance CG
        • Walsh DM
        • Sluka KA
        A new transient sham TENS device allows for investigator blinding while delivering a true placebo treatment.
        The blinding of the investigator and placebo-TENS group participants was successful, but participants in the active TENS group correctly identified the treatment in 77.5% of the cases which was different than chance, suggesting inadequate blinding Strong intensity of stimulation, which is necessary to produce an analgesic effect, made it difficult to blind the subject to the active unit, and this issue is present in many TENS investigations.
      • b)
        The influence of psychometrics information for efficacy of TENS was not analyzed. However, it could influence the TENS outcomes.
      • c)
        The study sample was limited to males who underwent Lichtenstein modification of hernia repair. It is known that inguinal hernia occurs predominantly in males and Lichtenstein modification is the predominant open technique. Inclusion of women and patients who underwent other open surgical technics could complicate the homogeneity of groups. However, sex dependent efficacy of TENS has not been reported in previous studies.
      • d)
        Participants in our study had the opportunity to ask for additional Ketorolac injections even between TENS/placebo-TENS procedures and it could leave a doubt, that TENS effect had influence to the pain ratings. We combined analgesic use by giving standard analgesics in the morning and evening in addition to controlled analgesia during the day. Giving Ketoprofen at 6 AM (approximately 3 hours before TENS/placeboTENS procedure) we minimized the use of analgesics during the period of TENS/placebo-TENS application. The Placebo- TENS group used more analgesics comparing to active TENS group, so this can only reaffirm that this reduction is due to TENS, but analgesic intake during period of TENS procedures was not analyzed. Furthermore, we used only Ketoprofen and Ketorolac, and there is a possibility that outcomes could vary using others NSAID's.
      The strengths of our study include the use of homogenous participant groups, with no differences in the: timing of basic pharmacological analgesia, TENS procedures and assessments, or the low-burden kinesics exercises in the early morning which were helpful to ensure consistent mobilization during the TENS intervention. In addition, all procedures and measurements were performed by the same individuals.
      In conclusion, our trial revealed that TENS is a safe procedure that can reduce postoperative pain following open inguinal hernia repair. TENS can contribute to the reduction of postoperative analgesics consumption. We suggest that TENS could be used in daily practice as part of a multimodal postoperative pain treatment plan following open inguinal hernia repair, especially for patients suffering from hyperalgesia.

      Acknowledgments

      The authors thank Dr. Diana Miloniene, Rasa Buinauskiene, Vilma Jurgeleviciene, Daiva Sadauskiene and all the staff in the General Surgery and Rehabilitation departments of Kaunas Clinical Hospital who have voluntarily given their valued time and effort for supporting this study. Special thanks to Violeta Simatoniene for valuable support during the statistical analysis. The authors sincerely thank all the participants for their cooperation.

      Appendix. Supplementary data

      • Supplement table 1. Differences in VAS (cm) before and after TENS intervention. * significantly different at P < .05. ** significantly different at P < .05. *** significantly different at P < .05. † significantly different at P < .05. †† significantly different at p < 0.05.

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