Transcutaneous Electrical Nerve Stimulation (TENS) and TENSlike devices: do they provide pain relief?

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2 Transcutaneous Electrical Nerve Stimulation (TENS) and TENSlike devices: do they provide pain relief? Mark I Johnson School of Health Sciences, Faculty of Health and Environment, Leeds Metropolitan University, UK The term transcutaneous electrical nerve stimulation (TENS) is synonymous with a standard TENS device. Increasingly, nonstandard TENS-like devices are being marketed to health care professionals for pain relief. These include: interferential current therapy, microcurrent electrical therapy, high-voltage pulsed (galvanic) currents, TENS-pens, transcranial electrical stimulation and Limoge currents, Codetron, transcutaneous spinal electroanalgesia, action potential simulation, and H-wave therapy. This review evaluates the effectiveness of TENS and TENS-like devices for pain relief, to inform health care professionals about device selection. The results from systematic reviews suggest that TENS is not effective for postoperative pain and labour pain, although volatile evaluation models may partly explain the findings. Evidence is inconclusive for chronic pain. Health care professionals should not dismiss the use of TENS for any condition until the issues in clinical trial design and review methodology have been resolved. There is limited experimental evidence available for most TENS-like devices. Claims by manufacturers about the specificity and extent of effects produced using TENS-like devices are overstated and could probably be achieved by using a standard TENS device or a microcurrent electrical therapy device. When making decisions about device selection, health care professionals should consider the physiological intention of currents and whether this can be achieved by using particular devices. Clinical trials that examine the relative effectiveness of TENS-like devices with a standard TENS device are desperately needed. Introduction Transcutaneous electrical nerve stimulation (TENS) is used by health care professionals throughout the world to provide pain relief for a wide range of conditions, including postoperative pain, labour pain and chronic pain. During Address for correspondence: Mark I Johnson, School of Health Sciences, Faculty of Health and Environment, Leeds Metropolitan University, Calverley Street, Leeds LS1 3HE, UK. M.Johnson@LMU.ac.uk TENS, electrical currents are generated by a stimulating device and delivered across the intact surface of the skin via conducting pads called electrodes (Figure 1). The popularity of TENS has grown because it is noninvasive, easy to administer and has few side-effects or drug interactions. There is no potential for toxicity or overdose and patients can administer TENS themselves at home and titrate the dosage of treatment as required. When compared with long-term drug therapy, TENS treatment is considerably cheaper. 1 3 Recently, systematic reviews have challenged Arnold / pr182ra

3 8 MI Johnson Figure 1 A standard TENS device. An electrical pulse generator delivers currents via conducting electrodes attached to the intact surface of the skin. Traditionally, carbon rubber electrodes smeared with conducting gel and attached to the skin using self-adhesive tape were used to deliver the electrical currents. Nowadays, selfadhesive electrodes are used (modified from Figure 17.1 in: Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science) claims that TENS is clinically effective. Bandolier, the journal for evidence-based health care that uses the findings of systematic reviews to provide clinical bottom lines states that: TENS is not effective in the relief of postoperative pain 4 ; TENS does not alleviate labour pain nor reduce the use of additional analgesics 5 ; There is a lack of evidence for the effectiveness of TENS [for chronic pain] at recommended treatment schedules. 6 Concerns about TENS effectiveness have not reduced the variety of TENS devices reaching the market, which seem to be fuelled in part by advances in electronic technology and the need to gain a competitive edge in the market-place. The aim of this article is to review critically the clinical effectiveness of TENS and TENS-like devices for pain relief in order to inform decisions about device selection. For the purpose of this article TENS-like devices include any stimulating device that delivers electrical currents across the intact surface of the skin and whose generic name differs from TENS. This will include interferential current therapy (IFT), microcurrent electrical therapy (MET), highvoltage pulsed (galvanic) currents (HVPC), TENS-pens (in particular, high-voltage TENSpens), transcranial electrical stimulation (TCES, in particular Limoge currents), Codetron, transcutaneous spinal electroanalgesia (TSE), action potential simulation (APS), and H-wave therapy (HWT). This list is not exhaustive. The review will not address the potential use of TENS-like devices for nonpainful conditions, although reference to these uses will be made where deemed appropriate.

4 Efficacy of TENS and TENS-like devices in pain relief 9 Defining TENS In broad terms TENS is anything that delivers electricity across the intact surface of the skin to activate underlying nerves. This would include the delivery of electric shocks by electrogenic fish, as was commonly used in early history, and the harnessed and controlled delivery of currents with specific characteristics as used in most modern-day TENS devices. A broad definition of TENS would not take account of the electrical characteristics of the currents (i.e. the output characteristics or technical specifications of the device). However, in health care the term TENS is commonly used to describe currents delivered by a standard TENS device (Figure 1). The standard TENS device Standard TENS devices are distinguished by their output characteristics. They usually deliver biphasic pulsed currents in a repetitive manner with a pulse duration between 50 µs and 1000 µs and pulse frequencies between 1 and 250 pulses per second (pps). 1,2,7 9 Pulses are usually delivered in a continuous pattern, although most modern-day devices have other patterns available such as burst and modulation (Table 1, Figure 2). The technical specifications and output characteristics of standard TENS devices vary between manufacturers, as they attempt to achieve uniqueness and a competitive edge in the marketplace. However, these variations are minor and probably have limited impact on the physiological effects produced by the devices. As TENS is a technique-based intervention, outcome will be dictated by the appropriateness of TENS procedures used to deliver currents as determined by the end-user. A number of factors need to be considered when determining a TENS procedure, including the characteristics of the electrical currents selected by the user (i.e. the output characteristics), the application procedure (i.e. electrode type and location) and the dosing regimen (Figure 3). The number of potential TENS procedures is vast, even with a simple TENS device, so it is important that the user has basic knowledge about the principles underpinning TENS techniques. Principles of TENS The purpose of TENS is to activate selectively different populations of nerve fibres in order to produce particular physiological outcomes. The common types of TENS described in the literature are 1,7,9 : Table 1 The technical specifications of a standard TENS device modified from Table 1 in: Johnson MI. A critical review of the analgesic effects of TENS-like devices. Phys Ther Rev 2001; 6: ) Weight Dimensions g cm (small device) cm (large device) Cost Pulse waveform (usually fixed) Monophasic Symmetrical biphasic Asymmetrical biphasic Pulse amplitude (usually adjustable) 1 50 ma into a 1 k Ω load Pulse duration (sometimes fixed, sometimes adjustable) µs Pulse frequency (usually adjustable) pps Pulse pattern Channels 1 or 2 Batteries PP3 (9V), rechargeable Additional features Timer Continuous and burst Some devices have random pulse frequency Some devices have modulated pulse amplitudes, frequencies and/or duration

5 10 MI Johnson HIGH (250pps) FREQUENCY LOW (1pps) PATTERN CONTINUOUS BURST On Off I F D C B M FREQUENCY MODULATED HIGH SHORT LONG LOW RANDOM PULSES Figure 2 AMPLITUDE DURATION Figure 2 Common output characteristics on standard TENS devices (topographic view). Most devices allow the amplitude and frequency of electrical pulses to be controlled by the end-user. Pulse duration and pulse pattern options are also available on some standard TENS devices: (pps: pulses per second; I: intensity; F: frequency; D: duration; C: continuous; B: burst; M: modulation) (modified from Figure 17.1 in: Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science) Conventional TENS; Intense TENS; Acupuncture-like TENS (AL-TENS) (Table 2). These types of TENS have evolved from knowledge about the ability of various nerve fibres to activate different analgesic mechanisms in the body. Evidence from axonal stimulation studies in vitro suggests that excitability varies according to the characteristics of an externally applied electrical current. The different types of TENS attempt to describe the most efficient characteristics of current to activate endogenous analgesic mechanisms and they have been widely accepted in the health care profession. Unfortunately, the use of these banner terms oversimplifies TENS techniques and this has resulted in TENS literature that tends to focus on the output characteristics of TENS devices rather than the physiological intention of the currents. Evidence suggests that the theoretical relationship between output characteristics and nerve fibre activation may break down in practice owing to the nonhomogeneous nature of the tissue underlying the electrodes. 11,12 It is important to clarify the physiological intention of different types of TENS when delivered by a standard TENS device. Conventional TENS The purpose of conventional TENS is to activate selectively large diameter Aβ fibres without

6 Elecrical Application Dosage characteristics procedure regimen Pulse Pulse Pulse Electrode Prescribed Open frequency intensity pattern Number Location Treatment Treatment As much as you like pps 1 50 ma Continuous Pen (point) Site of pain Time frequency Before pain Paraesthesia Burst 2 electrodes Nerve bundle Seconds Per day During pain barely perfceptible Random single channel Acupuncture points Minutes Per week comfortable (weak) Modulated 3 electrodes Trigger point Hours Per month comfortable (strong) amplitude Limoge currents Dermatomal painful frequency 4 electrodes Myotomal painful duration dual channel Transcranial Muscle contraction > 4 electrodes Transpinal yes/no multichannel Other... tetanic/phasic 6 active electrodes Codetron Figure 3 Variables influencing the way in which TENS can be administered. Many possible combinations of TENS parameters can be chosen by the end-user Efficacy of TENS and TENS-like devices in pain relief 11

7 Table 2 TENS techniques that can be achieved using a standard TENS device. The output (electrical) characteristics identified for each TENS technique are based on the strength and duration of pulsed currents necessary to generate an action potential in different types of axon. 7 In clinical practice, patients and practitioners use the sensation produced by TENS to determine the appropriate stimulating characteristics (modified from Table 2 in: JOhnson MI. A critical review of the analgesic effects of TENS-like devices. Phys Ther Rev 2001; 6: ) Purpose of Theoretical optimum Sensory experience Electrode position Analgesic profile Duration of Mechanism of currents output characteristics during stimulation treatment analgesic action (desired outcome) 12 MI Johnson Conventional Selective High frequency/low Strong but Site of pain Rapid onset <30 min Continuously Segmental TENS activation of intensity: comfortable Dermatomal after switch-on when in pain nonnoxious cutaneous Amplitude = low electrical Rapid offset <30 mins afferents (e.g. Aß Duration = µs paraesthesia after switch-off fibres from Frequency = pps with minimal mechano receptors) Pattern = continuous muscle contraction AL-TENS Selective Low frequency/high Strong but Motor point/muscle Delayed onset >30 mins ~30 mins/session Extrasegmental activation of intensity: comfortable at site of pain after switch-on as muscle fatigue Segmental motor efferents to Amplitude = high muscle twitches Myotomal Delayed offset >1 h may occur generate a muscle Duration = µs after switch-off twitch and activity Frequency = 2 bps and in nonnoxious muscle 100 pps within burst afferents (i.e. GIII Pattern = burst fibres from ergoreceptors) Intense TENS Activation of noxious High frequency/high Highest tolerable Site of pain or main Rapid onset <30 min ~15 mins/session Peripheral activation of noxious intensity: level with nerve bundlel after switch-on as patients Extrasegmental pinprick cuntaneous Amplitude = high minimall proximal to delayed offset 1h experience Segmental afferents (i.e. Aß fibres Duration = 1000 µs muscle pain after switch-off discomfort from nocipeptors Frequency = 200 pps contractionl Pattern = continuous bps: burst per second; pps: pulses per second

8 Efficacy of TENS and TENS-like devices in pain relief 13 Conventional TENS TENS electrodes TENS Currents Anode Cathode Proximal A Aβ -segmental Add C Muscle Figure 4 The purpose of conventional TENS is selectively to activate nonnoxious cutaneous afferents (Aβ) to Figure initiate 4segmental antinociceptive mechanisms. Arrows indicate selective activation of nerve fibre transmitting impulses towards the central nervous system (modified from Figure 17.1 in: Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science) concurrently activating small diameter Aδ and C (pain-related) fibres or muscle efferents (Figure 4). 1,2,7,9,13 Theoretically, high-frequency (~ pps), low-intensity (nonpainful) currents with a pulse duration between 100 µs and 200 µs would be most efficient in selectively activating Aβ fibres (Table 2). 7 In practice, Aβ afferent activity is recognized by the user reporting strong but comfortable nonpainful electrical paraesthesia beneath the electrodes Animal and human studies have demonstrated that TENS-induced Aβ activity inhibits ongoing transmission of nociceptive information in the spinal cord and that this produces segmental analgesia with a rapid onset and offset The main determinant of Aβ activity is sufficient current amplitude; users are easily trained to titrate amplitude so that it is strong enough to generate a nonnoxious paraesthesia (Aβ activity) without frank pain (representative of Aδ or C fibre activity). It is claimed that the magnitude of analgesia achieved during conventional TENS is dependent in part on the pulse frequency, but the findings of experiments in healthy people and patients are inconsistent It has been suggested that patient preferences for different TENS settings when delivering currents at a strong nonnoxious intensity may be for reasons of comfort rather than putative differences in analgesic profiles. 15,25 Acupuncture-like TENS The purpose of AL-TENS is to generate activity in small diameter muscle afferents (Aδ or Group III) arising from ergoreceptors that respond to muscle contraction.+>9,26 This is achieved indirectly by delivering currents at low frequencies (~1 10 Hz) at high but nonpainful intensities over motor points in order to activate

9 14 MI Johnson Aα efferents resulting in a forceful but nonpainful phasic muscle twitch 27,28 The subsequent volley of impulses from muscle afferents mediates an extrasegmental antinociceptive mechanism and the release of endogenous opioid peptides in a manner similar to that suggested for acupuncture (Figure 5). 7,29 32 Low frequency burst patterns of pulse delivery were incorporated in TENS devices because they were found to be more comfortable than low-frequency single pulses in producing muscle twitches (Table 2). 28 It should be remembered that currents delivered during AL-TENS will also activate Aβ during their passage through the skin, leading to segmental analgesia. AL-TENS has also been described as the delivery of TENS to acupuncture points without reference being made to the presence of muscle contractions. The use of the term in this way is not entirely appropriate. 33 Intense TENS The purpose of intense TENS is to activate small diameter Aδ cutaneous afferents by delivering TENS over peripheral nerves arising form the site of pain at an intensity that is just tolerable to the patient (Figure 6). 2,34 36 Currents are administered at high frequencies (up to 150 pps) to prevent phasic muscle twitches that would be too forceful for the patient to tolerate (Table 2). Cutaneous Aδ afferent activity has been shown to block transmission of nociceptive information in peripheral nerves and to activate extrasegmental antinociceptive mechanisms Intense TENS will also activate Aβ fibres, producing seg- AL-TENS Cathode Motor point TENS electrodes Anode Proximal TENS Currents Aß Aβ -segmental MUSCLE GI CONTRACTION MUSCLE GIII - extrasegmental Figure 5 Figure 5 The purpose of AL-TENS is selectively to activate large diameter motor efferents to elicit a nonpainful muscle twitch. This muscle twitch generates activity in ergoreceptors and small diameter muscle afferents to initiate extrasegmental antinociceptive mechanisms. In addition, Aβ afferents are also likely to become active. Arrows indicate direction of relevant impulse information (modified from Figure 17.1 in: Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidencebased practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science)

10 Efficacy of TENS and TENS-like devices in pain relief 15 Intense TENS TENS electrodes TENS Currents Anode Cathode Proximal A Aβ - segmental Ad - extrasegmental C Muscle Figure 6 The purpose of intense TENS is to activate noxious cutaneous afferents (Aδ) to initiate extrasegmental antinociceptive mechanisms and peripheral blockade of nociceptive impulses travelling in Aδ fibres. In Figure 6 addition, Aβ afferents are also likely to become active. Arrows indicate direction of relevant impulse information (modified from Figure 17.1 in: Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science) mental antinociceptive effects. As intense TENS acts in part as a counter-irritant, it can be delivered only for a short time, but it may prove useful postoperatively and for minor surgical procedures and such as wound dressing and suture removal. 13,41 In clinical practice in the UK, conventional TENS is most commonly used. AL-TENS and intense TENS are used only in specific situations. Despite a large published literature on TENS, there is a lack of good quality and systematic experimental work that has directly compared the clinical effectiveness and analgesic profiles of these types of TENS. The clinical effectiveness of TENS When assessing TENS effectiveness one needs to isolate the effects due to the currents from the effects associated with the act of giving the currents. Many early TENS trials lacked appropriate controls and therefore changes observed in trials could have been due to patients expectation that TENS would reduce pain. In addition, many early trials lacked randomization, leading to the overestimation of treatment effects. This was elegantly demonstrated by Carroll et al., who found that 17/19 controlled clinical trials that were not randomized reported that TENS was beneficial for postoperative pain, whereas 15/17 trials that were randomized reported that it was not. 42 Recently, a number of systematic reviews and meta-analyses on TENS have challenged the

11 16 MI Johnson Table 3 A summary of systematic reviews and meta-analysis on the clinical effectiveness of TENS.(modified from Table 17.4 in: Johnson M. Trancutaneous electrical nerve stimulation (TENS). In Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: ; with permission from Elsevier Science) Condition Acute Pain Postoperative pain Labour Pain Chronic pain Existing reviews Reeve et al., : SR Mixed conditions (dysmenorrheoa, dental, cervical, orofacial) TENS > control in 7/14 RCTs Reviewers conclusion: evidence inconclusive poor RCT methodology Reeve et al., : SR TENS > control in 12/20 RCTs Reviewers conclusion: evidence inconclusive poor RCT methodology Carroll et al., : SR TENS > control in 2/17 RCTs Reviewers conclusion: evidence of no effect Bjordal et al., in press. 48 MA TENS > sham for reducing analgesic consumption (MWD = 35.5%) Reviewers conclusion: evidence of effect analgesic sparing Reeve et al., : SR TENS > control in 3/9 RCTs Reviewers conclusion: evidence inconclusive poor RCT methodology Carroll et al., : SR TENS > control in 3/8 RCTs Reviewers conclusion: evidence of no effect Carroll et al., : update of Carroll et al., 51 : SR TENS > control in 3/10 RCTs Reviewers conclusion: evidence of no effect Reeve et al., : SR Mixed conditions (low back, pancreatitis, arthritis, angina) TENS > control in 9/20 RCTs Reviewers conclusion: evidence inconclusive poor RCT methodology McQuay and Moore, : SR Mixed conditions (low back, pancreatitis, osteoarthritis, dysmenorrhoea) TENS > control in 10/24 RCTs Reviewers conclusion: evidence inconclusive inadequate TENS doses Carroll et al., : SR Mixed conditions (19 RCTs, 652 patients) TENS > control in 10/15 RCTs Reviewers conclusion: evidence inconclusive inadequate TENS doses Gadsby and Flowerdew, ; Flowerdew and Gadsby, : MA Low back pain (6 RCTs) TENS > sham for pain relief (OR = 2.11) Reviewers Conclusion: TENS effective poor RCT methodology Milne et al., ; Brosseau et al., : MA Low back pain (5 RCTs, 421 patients) TENS = sham for pain relief (SMD = 0.207) Reviewers conclusion: evidence of no effect

12 Efficacy of TENS and TENS-like devices in pain relief 17 Price and Pandyan, : MA Post-stroke shoulder pain (4 RCTs, 170 patients); any surface ES ES = sham/no treatment control for pain relief (WMD = 0.13) ES > sham/no treatment control for range of movement (WMD = 9.17) Reviewers conclusion: evidence inconclusive Osiri et al., : MA Knee osteoarthritis (7 RCTs, 294 patients) TENS > sham for pain relief (SMD = 0.448, although only 2/7 RCTs +ve) Reviewers conclusion: evidence of effect pain relief Proctor et al., : MA Primary dysmenorrhoea (8 RCTs, 213 patients) HF TENS > sham for pain relief (OR = 7.2) LF TENS = sham for pain relief (OR = 1.3) Reviewers conclusion: evidence of effect pain relief for HF TENS only SR: systematic review; RCT: randomized-controlled trial; MWD: mean weighted difference; MA: meta-analysis; OR: odds ratio; SMD, standardized mean difference; ES, electrical stimulation; WMD, weighted mean difference (= MWD); HF, high-frequency; LF, low-frequency belief that its effects are clinically meaningful and/or a result of the electrical currents themselves (Table 3). TENS and postoperative pain Early reports suggested that TENS reduced postoperative pain and opioid consumption However, a health technology assessment by Reeve et al. 46 reported that TENS was demonstrated to be of benefit in only 12/20 randomized controlled trials (RCTs). A systematic review by Carroll et al. 42 reported that TENS did not produce significant benefit when compared with placebo in 15/17 RCTs. Both reviews used pain relief as the primary outcome measure, although patients in some of the trials had access to additional analgesic drugs so that those in sham and active TENS groups could titrate analgesic consumption to achieve similar levels of pain relief. There were also minor inconsistencies in judgements of trial outcome between the reviewers because of the difficulty of dichotomizing multiple outcome measures in RCTs. TENS is known to be less effective for severe pains, like those associated with thoracic surgical procedures, and detecting reductions in mild pain (i.e. against a small pre-tens baseline) requires large sample sizes to achieve statistical power. 47 Some RCTs used sample sizes with insufficient statistical power to detect potential differences between groups. Recently, my colleagues and I have performed a meta-analysis of 21 RCTs that accounts for some of these issues. 48 We found that the mean reduction in analgesic consumption after TENS was 26.5% (range 6% to +51%) better than placebo. It is important that a subgroup analysis of 11 trials (964 patients) that met our criteria for optimal TENS dosage (i.e. a strong, subnoxious electrical stimulation) reported a mean weighted reduction in analgesic consumption of 35.5% (range 14 51%) better than placebo. In the trials without explicit confirmation of optimal TENS dosage, the mean weighted analgesic consumption was 4.1% (range 10% to +29%) in favour of active TENS. The difference in favour of adequate stimulation was highly significant (p = ). This suggests that adequate TENS technique is necessary in order to achieve an effect. TENS and labour pain Augustinsson et al. pioneered the use of TENS in labour pain by delivering currents to areas of the spinal cord that correspond to the input of nociceptive afferents associated with the first and second stages of labour (e.g. T10 L1 and S2 S4 respectively). 49 Early reports of TENS success resulted in the design of specialized obstetric TENS devices with dual channel output and boost controls for contraction pain. Despite extensive use of TENS, systematic reviews conclude that TENS provides little, if any, pain relief

13 18 MI Johnson Table 4 Common characteristics of generic categories TENS-like devices (taken in part from Table 3 in: Johnson MI. A critical review of the analygesic effects of TENS-like devices. Phys Ther Rev 2001; 6: ) Typical Standard TENS IFT MET HVPC characteristics Delivery system 1 channel Quadripolar = 2 1 channel 1 channel (2 electrodes) channels (4 (2 electrodes) (2 electrodes) electrodes) Single point Bipolar = 1 channel pen electrode (2electrodes) Suction electrodes sometimes used Pulse generator Hand-held Desktop and Desktop and Desktop and hand-held hand-held hand-held Recommended Site of pain Site of pain Site of pain Site of pain electrode position Either side of Either side of wound wound Acupuncture/ Motor point for trigger points muscle transcranial stimulation Recommended Self-administration Under Under supervision Under treatment regimen as required supervision supervision of therapist supervision of Continuous of therapist and self-admistration therapist Intermittent stimulation whenever Intermittent Intermittent stimulation Intemittent stimulation in pain stimulation (e.g. (e.g. ~20 60 min (e.g. ~20 60 min ~30 mins during for 1 3 times a day) for 1 3 times visit to clinic) a day) Waveform Monophasic Amplitude Modified Twin peak symmetrical biphasic modulated square direct monophasic Asymmetrical biphasic interference current with spiked pulse wave generated monophasic or by 2 out-of-phase sinusoidal currents biphasic pulse changing polarity at regular intervals Amplitude ma 1 60mA µa 1 2 A intensity Non-noxious Non-noxious No paraesthesia Paraesthesia paraesthesia paraesthesia (i.e. below sensory detection threshold) Pulse rate Adjustable Adjustable Adjustable Adjustable pps Hz for pps pps amplitude-modulated wave Carrier wave Hz Pulse duration Fixed and/or Carrier waves = Fixed/adjustable <100 µs adjustable unknown adjustable µs Range unknown Pulse pattern Continuous Amplitude Continuous Continuous Burst modulated Burst Modulated frequency, wave can be Modulated amplitude and pulse continuous or amplitude duration modulated in frequency using sweeps and swing patterns

14 Efficacy of TENS and TENS-like devices in pain relief 19 during labour. 46,50,51 Carroll et al. 50 reported that 10/10 RCTs showed that pain relief scores produced by TENS were no greater than sham TENS or a no treatment control. However, the self-report of pain relief may have been compromised by access to additional analgesics in some of the RCTs. The finding that analgesic intervention may be less likely with TENS, as reported in the original systematic review, was not confirmed in the updated review when data from an additional study was added. These findings seem to conflict with clinical experience where midwives and patients report satisfaction with TENS effects. 52 It is possible that pain relief ratings were influenced by fluctuating emotional and physical conditions during labour because one trial found that significantly more women and midwives favoured active rather than sham TENS when recorded under double-blind conditions at the end of childbirth. 53 After childbirth, women are more likely to be relaxed and perhaps better able to reflect on the effects of the intervention. The systematic reviews also included RCTs that used unconventional TENS devices. 54,55 These studies used Limoge currents, which are administered transcranially and clearly differ from conventional obstetric TENS (see section on TCES). It is interesting that both studies reported that Limoge currents produced analgesic sparing effects when compared with sham or no-treatment control. TENS and chronic pain A large number of clinical trials suggest that TENS is useful for chronic pain. Three systematic reviews have examined TENS effectiveness on mixed populations of chronic pain patients. Reeve et al. reported that TENS was more effective than sham (n = 7) or no treatment (n = 2) in 9/20 RCTs. 46 McQuay and Moore stated that TENS was better than sham TENS, placebo pills, or inappropriate electrode placements in 10/24 RCTs. 56 Carroll et al. reported that TENS provided better pain relief than sham or no treatment in 10/15 RCTs. All reviewers concluded that the evidence for TENS in chronic pain was inconclusive. 57 Reviews on specific populations of chronic pain patients are also inconclusive. A meta-analysis of any form of surface electrical stimulation on 170 patients with post-stroke shoulder pain found no significant change in pain incidence (odds ratio = 0.64) or pain intensity (standardized mean difference (SMD) = 0.13) after electrical stimulation compared with control. 58,59 However, electrical stimulation improved the pain-free range of passive humeral lateral rotation (weighted mean difference (WMD) = 9.17) and reduced the severity of glenohumeral subluxation (SMD = 1.13). For low back pain the findings of reviews have been contradictory. A meta-analysis on 321 patients reported no statistically significant differences between active and sham TENS for pain relief. 60,61 In contrast, a metaanalysis on 288 patients reported that TENS reduced pain and improved the range of motion. 62,63 The overall odds ratio for pain relief against placebo was only 2.11, although an odds ratio of 7.22 was reported in favour of AL-TENS. However, RCTs on AL-TENS did not state that TENS generated muscle contractions, which is considered to be a prerequisite for AL-TENS. A meta-analysis of the effect of TENS on pain associated with primary dysmenorrhoea reported that high-frequency but not low-frequency TENS was more effective for pain relief than sham. 64 A meta-analysis of 294 patients with knee osteoarthritis reported that TENS produced significantly better pain relief and reductions in knee stiffness than placebo. 65 All reviewers conclude that the low methodological quality of TENS trials has contributed to the uncertainty in the clinical evidence for effective use in chronic pain. Underdosing of TENS has been recognized as a problem and some trials measure outcome after a single TENS intervention or following a course of intermittent TENS treatments. 57 This differs from clinical practice, where long-term users of TENS administer it over long periods of time because the effects of TENS appear to be maximal when the device is switched on. 15 Nevertheless, the uncertainty about the clinical effectiveness of standard TENS devices for pain relief has questioned it as a viable treatment option. Attempts to improve efficacy by searching for optimal stimulator settings have largely been unsuccessful. As a result, health care professionals are turning to commercially available TENS-like devices with novel technical specifications that have emerged from

15 Table 5 Common characteristics of some other TENS-like devices (taken in part from Table 3 in: Johnson MI. A critical review of the analgesic effects of TENS-like devices. Phys Ther Rev 2001; 6: ) Typical TENS-pen TCES using Stimulators to TSE APS HWT characteristics (Pain Gone) Limoge currents reduce TENS tolerance (Codetron) 20 MI Johnson Delivery system Single point 1 channel (3 Currents delivered 1 channel (2 1 channel (2 1 channel (2 pen electrode electrodes randomly to 1 electrodes) electrodes) electrodes) of 6 active electrodes Pulse generator Hand-held Desktop/hand-held Desktop Codetron Hand-held Desktop APS Desktop and pen device Limoge device device TSE device device hand-held using HWT devices piezoelectric elements Recommended Site of pain 2 positive Acupuncture Spinal cord at Site of pain Site of pain electrode Acupuncture electrodes in points T1 and T12 position points retromastoid or C3 and C5 region and 1 negative electrode between eyebrows Recommended Intermittent Intermittent Intermittent Intermittent Continuous treatment individual stimulation stimulation stimulation stimulation stimulation regimen shocks Stimulation (e.g. ~30 min (e.g. ~20 min for either 8 or whenever repeated periods appear 3 times a day) at a time) 16 minutes at in pain when ever to be long a time pain returns (e.g. start 60 min before medication and continue throughout the time of the pharmacological action of the drug)

16 Waveform Single Positive pulse Square wave Differentiated/ Monophasic Biphasic wave Monophasic (high intensity with DC rectangular square pulse with spiked pulse short duration) current of with exponential followed by opposite exponential decay negative pulse polarity decay and DC (low intensity following offset of 5 V long duration) each pulse delivered in trains (bursts) Amplitude + Low ampere ~2 ma/30 V Milliampere Low ampere Microampere Milliampere high voltage <60 m\a high voltage No <10 ma low (e.g. 6 µa/ Non-noxious No paraesthesia output to 15,000 V) paraesthesia paraesthesia (i.e. below reduce Non-noxious to pinprick (i.e. below sensory heating to mild pain sensory detection Non-noxious noxious pinprick detection threshold) paraesthesia sensation threshold) Pulse rate Ad hoc: 125 khz Presets Presets Fixed at Adjustable or depends on interrupted include include ~150 pps presets rate of button with an 1, 2, 4, 200 pps 600 between 2 and press intermittent 10,000 pps 60 pps current of 83 Hz Pulse duration Unknown but Positive phase Fixed preset Fixed preset Fixed preset Fixed preset at at fixed preset of pulse between between 1.5 between µs 1.7 µs, µs and 4 µs and 6600 µs negative phase of pulse 6.3 µs Pulse pattern Ad hoc: Trains of high- Continuous Continuous Continuous Continuous depends on frequency Burst pattern of pulses button press interrupted by low frequency pulses Efficacy of TENS and TENS-like devices in pain relief 21

17 Table 6 TENS-like device identity. Randomized controlled clinical trials that compare the analgesic effects of TENS-like devices with a standard TENS are needed for all categories (taken in part from Table 4 in: Johnson MI. A critical review of the analgesic effects of TENS-like devices. Phys Ther Rev 2001; 6: ) 22 MI Johnson Standard IFT MET HVPC TENS-pen TCES Stimulators to TSE APS HWT TENS using Limoge reduce TENS currents tolerance (Codetron) Main Pain relief Pain relief Assist Muscle Pain relief Reduce Pain relief Pain relief Pain relief Pain relief indications Muscle tissue stimulatio analgesic/ stimulation healing Assist anaesthetic Anti- Pain relief tissue intake inflammatory healing Pain relief Claims for Currents Currents Currents Currents Currents Currents Currents Currents Currents Currents uniqueness selectively excite deep- mimic pass easily activate directly reduce by-pass skin mimic more activate seated current of through body s pain influence nervous and action comfortable different tissue injury to skin to relief brain system directly potentials than TENS nerve accelerate activate system function habituation excite Relationship fibres to healing motor and mimic central to H-reflex initiate nerves electro- nervous unclear pain selectively acupuncture system modulatory tissue to mechanisms reduce central sensitization Principle of Proven Unclear Unclear Unclear Unclear Unclear Evidence Interesting Unclear Unclear action for Similar to may be due Similar to Could be Evidence supports but Similar to Similar to pain relief standard to tissue standard noninvasive suggests reduction unproven MET standard TENS? healing; TENS? acupuncture that central in TENS Different evidence nervous habituation analgesic conflicting system but needs effects activity to be resulting

18 from and neuro shown that deep hormonal this and levels translates superficial change into fibre clinically activation meaningful need to be reductions shown in the incidence of TENS tolerance Evidence Effects > Effects >? effects = Unknown Unknown Unknown Effects >? effects = Unknown? effects > for placebo placebo placebo placebo placebo Effects = placebo analgesic Analgesic Effects = standard Effects = effects in profiles conventional TENS and standard healthy types of TENS IFT TENS people TENS needed Evidence Large Little Little Very little No available Some Little Little Evidence Very little amount of evidence evidence evidence evidence evidence evidence evidence available evidence evidence available available, available available available available but available available and trials with on pain but? effects > Effects = inappropriate Evidence but have conflicting outcomes conflicting placebo placebo in interpretation inconclusive conflicting design outcomes Evidence findings? effects > one of findings and flaws? effects > inconclusive? effects > conventional unpublished? effects = dependent? effects = placebo in placebo in TENS study placebo on placebo initial trials sparing Evidence Effects > Evidence condition Evidence Evidence analgesics inconclusive conventional inconclusive treated inconclusive inconclusive for labour TENS in? effects > pain pilot study placebo if Evidence Evidence administered inconclusive inconclusive correctly Evidence inconclusive Efficacy of TENS and TENS-like devices in pain relief 23

19 24 MI Johnson other areas of health care. At present, information about the effectiveness of TENS-like devices is limited and decisions about device selection are being based on unreliable sources such as manufacturers material or hearsay from colleagues. An assessment of the merits of some of the commercially available TENS-like devices is needed. Defining and categorizing TENS-like devices For the purpose of this review TENS-like devices are defined as any stimulating device that delivers electrical currents across the intact surface of the skin and whose generic name differs from TENS. Potential TENS-like devices were identified from a cursory search of published literature, coupled with discussions with colleagues in the field. This was followed by searches of MEDLINE ( ) using device names identified from the cursory search and the key words transcutaneous electrical nerve stimulation and electrical stimulation therapy. All potential links to related articles were followed, as were manual searches of items given in reference lists. Information from manufacturers and the Internet was used wherever possible to help to determine technical specifications, recommended treatment procedures, and establish claims of effectiveness. The search identified a variety of TENS-like devices, although categorization according to formal criteria was impossible. Some devices could be differentiated according to output characteristics (e.g. IFT, MET and HVPC; Table 4). Some devices could be differentiated according to the procedures used to deliver the currents (e.g. TENS-pens and TCES). Some could be differentiated according to novel principles of action (e.g. stimulators trying to overcome TENS tolerance, TSE, APS and HWT; Table 5). There was much overlap between these divisions. For example, MET could be administered transcranially (e.g. as a type of TCES) or using a TENSpen. Consequently, each TENS-like device is discussed separately for reasons of convenience (Table 6). Interferential current therapy IFT was developed in the 1950s and has remained for the most part within the discipline of physiotherapy. 66 Surveys have shown that IFT is used throughout the world 67,68 and there appeared to be more published information on IFT than any of the other TENS-like device in this review. Three textbooks on IFT were found that described the clinical use of the modality based on the personal experience of the authors There was an absence of good-quality experimental work to support the claims made in the textbooks. 67,72,73 IFT is most commonly used for pain relief 74 although advocates claim that it will also reduce inflammation, assist tissue repair (including bone fractures) and re-educate muscle (especially for incontinence) ,75,76 IFT devices are more expensive than standard TENS devices and tend to reside in physiotherapy clinics because they are relatively large. Recently, some battery-operated hand-held IFT devices have appeared on the market. The purpose of IFT appears to be to deliver currents to deep-seated structures. IFT stimulators are designed to generate an amplitude-modulated interference wave, sometimes called the IFT beat. This wave is created by two out-ofphase currents that collide with each other to generate an interference wave with a frequency usually between 1 Hz and 200 Hz (Figure 7). The two out-of-phase currents are delivered at frequencies between 2 Hz and 4 khz because such high-frequency short cycle duration waves will overcome skin impedance and penetrate deep body structures ,77 79 Advocates argue that these high-frequency kilohertz currents act as weak stimuli for nervous tissue, so a low frequency amplitude-modulated wave is created in order to excite neurones. Traditionally, the interference wave was created within the tissue by delivering two outof-phase currents across the skin via four electrodes (termed quadripolar IFT), although nowadays the interference wave is often premodulated within the IFT device and delivered via two electrodes (bipolar IFT). IFT devices have an array of settings. The amplitude-modulated wave can be set at frequencies between 1 F

20 Efficacy of TENS and TENS-like devices in pain relief 25 A B B A Current Amplitude Channel A 4000Hz Channel B 4100Hz Time AMF 100Hz Amplitude-Modulated Wave within deep seated tissue Figure 7 Principles used to generate an amplitude-modulated interference wave within deep tissue. Dark shaded electrodes attached anterior and lighter shaded electrodes attached posterior (A: electrodes for channel A; B: electrodes for channel B) (adapted from Figure 1 in: Johnson MI, Tabasam G. A double blind placebo controlled investigation into the analgesic effecs of interferential currents (IFC) and transcutaneous electrical nerve stimulation (TENS) on cold induced pain in healthy subjects. Physiother Theory Pract 1999; 15: ) Figure 7 Hz and 200 Hz and can fluctuate between upper and lower preset boundaries (termed the sweep) over a set time duration (termed the swing pattern). For example, a 6 Λ 6 swing pattern delivers amplitude-modulated frequencies between preset lower and upper frequencies over a 6-second time period. A 6 Λ 6 swing pattern delivers amplitude-modulated waves at the lower frequency for 6 s and then at its upper frequency for 6 s. When used for pain relief, IFT is delivered to generate a strong but comfortable electrical paraesthesia at the site of the pain. This approach is comparable with conventional TENS and is likely to generate Aβ activity and segmental analgesia. Similarities in administration procedures for IFT and conventional TENS have led some commentators to challenge assumed differences in analgesic profiles between the two modalities. 66,73,80 Studies using healthy people have found that IFT elevates an experimentally induced cold pain threshold and reduces experimentally induced ischaemic pain when compared with sham, although there were no differences in IFT effects when compared with conventional TENS. 19,81,82 It is claimed that different amplitude-modulated wave frequencies selectively activate different populations of nerve fibres to generate specific physiological outcomes. For example, Savage 70 claims that frequencies of Hz are in the analgesic range and the sedative range, and that frequencies of Hz activate autonomic nerves. No evidence could be found to support such claims. Furthermore, the

21 26 MI Johnson physiological rationale for including frequency sweeps and swing patterns in IFT device design is obscure. Systematic investigations into the effects of different frequencies and swing patterns of IFT on experimentally induced pain in healthy people has found that analgesia was not affected by frequency or swing pattern when IFT was administered at a strong but comfortable intensity without concurrent muscle contraction The majority of clinical reports on IFT effects are anecdotal and lack appropriate controls. Taylor et al. reported no significant differences between the effects of sham and active IFT on pain and dysfunction in 40 patients suffering from jaw pain. 86 Quirk et al. found no additional benefits from IFT when compared with exercise in 38 patients suffering pain and dysfunction associated osteoarthrosis of the knee. 87 An RCT on 152 patients by Werners et al. 87 reported that there were no significant differences in the magnitude of pain relief achieved using IFT when compared with motorized lumbar traction with massage management for low back pain. The lack of IFT effects in these controlled studies may be due to underdosing of IFT because it is believed that patients experience fatigue if stimulation lasts more than min. Observations of patients using TENS at strong but comfortable intensities in a similar manner to that described for IFT suggest that this is not the case. The acceptance of short duration IFT treatment may be due to constraints of the clinical rota because most IFT treatment sessions take place in the clinical setting under the supervision of a therapist. Furthermore, applying IFT may not be the most appropriate approach; Hurley et al. 88 have shown that IFT delivered over the spinal nerve produced greater reductions in functional disability when compared with IFT administered directly over the painful area. Explanations of how IFT produces pain relief are at best vague and tend to focus on pain gates and endorphins. The justification for using an amplitude-modulated interference wave to stimulate neural tissue rather than a biphasic pulsed current as generated by standard TENS devices seems to be entirely speculative. Demmink 11 has reported that IFT modulation patterns could be reproduced in water but not in biological tissue, where current distribution was unpredictable. Palmer et al. 90 found no differences in psychophysiological outcomes when IFT was administered in both the presence and the absence of the amplitude-modulated wave. Thus, it is possible that any pain relieving effects of IFT are due to the higher frequency current (i.e. 2 4 khz) rather than to the amplitude-modulated wave. Microcurrent electrical therapy MET uses currents that are 1/1000th of an ampere smaller than those delivered by standard TENS devices (milliamperes). Advocates claim that MET devices can be used to accelerate tissue healing and relieve pain, especially pain related to sporting injuries MET comes under a range of guises, including microcurrent electrical nerve stimulation, microamperage stimulation, low-intensity direct current, and pulsed lowintensity direct current. Definitions of MET are varied, although the American Physical Therapy Association has defined it as a low-intensity direct current that delivers monophasic or biphasic pulsed microamperage currents across the intact surface of the skin. 8 Often, MET devices utilize adjustable pulse frequencies between 0.5 pps and 150 pps with periodic reversals in polarity. MET can be delivered using probe electrodes (sometimes in the form of a pen) or pad electrodes, which are applied to acupuncture points, trigger points or over the site of pain. Some MET devices have a point finder to detect areas of the skin with low resistance, which are believed to correspond to acupuncture points. 95 MET can also be administered on ear lobes and transcranially, where it is claimed that it will relieve migraine, headache, insomnia and stress. 96 MET developed from the claim that tissue health is maintained by a direct current electrical system in the human body and that a shift in this normal current flow occurs when tissue is damaged This direct current shift was described as the current of injury, with a magnitude in the microampere range. Advocates claim that MET simulates this current of injury to assist tissue growth and healing, and that milliampere currents delivered by standard TENS devices are detrimental to this process of repair. 100,101 They also claim that MET provides

22 Efficacy of TENS and TENS-like devices in pain relief 27 pain relief, although it is unclear whether this is a primary effect through direct action on the antinociceptive system or a secondary effect from tissue healing. Users do not perceive MET currents, so it seem likely that the putative mechanism of action differs from conventional TENS. A relatively large body of published research was found for MET that could be divided into effects on pain and on tissue healing. MET and pain relief Investigations into the effects of MET on experimentally induced pain in healthy prople have produced contradictory results. Weber et al. reported no significant differences between massage, upper body ergometry, MET and a no treatment control on delayed-onset muscle soreness induced by high-intensity exercise in 40 healthy volunteers. 102 In contrast, Lambert et al. reported that MET reduced the severity of delayed-onset muscle soreness in 30 healthy men under double-blind, placebo-controlled conditions. 103 A study using cold-induced pain found no significant differences between active and placebo MET on experimentally-induced pain threshold and pain intensity rating in 36 healthy volunteers using single-blind methodology. 104 evidence available from clinical trials is also inconclusive. Clinical trials on MET often lack methodological rigor. For example, a report of a double-blind placebo-controlled trial claimed that MET significantly reduced chronic back pain in 40 patients, yet details about the statistical analysis were omitted from the report. 105 MET was administered for two 6-second periods to 16 points on the back, three times per week for two weeks. No physiological rationale was given for such a prescriptive treatment regimen. Similarly, MET was given to the affected hands of 36 patients with carpal tunnel syndrome for three treatments per week for 4 5 weeks in combination with low-level laser acupuncture and other alternative therapies. 106 Although it was claimed that this treatment approach was successful in relieving pain, it was not possible to determine the exact contribution of MET. Clinical trials have also found that MET effects are comparable to TENS in patients with migraine and chronic headaches, 107 but less effective than a laser for improving mobility and pain in patients with degenerative joint disease. 108 MET and tissue healing It is possible that putative pain relief may be a byproduct of the accelerated healing process. Initial reports of experiments in vitro suggested that MET accelerates the healing of damaged tissue, possibly through increased protein synthesis 100,101,113 or through antimicrobal effects However, two well-controlled animal studies found that MET did not accelerate the healing of experimentally induced wounds in rats and Yucatan mini pigs. 117,118 The editor of one journal concluded that the time has come to weigh the evidence and to face the accumulation of data from these and other reports indicating that this modality [MET] does not assist in wound healing when used in the manner described. 119 Clinical evidence is also inconclusive. Encouraging reports of MET accelerating the healing of ulcers and wounds are often undermined by the lack of appropriate control groups. 109, Carley and Wainapel 123 administered MET for two hours twice a day for six weeks and found that it accelerated the healing of ulcers in 30 patients when compared with conventional wound dressings. However, the absence of a placebo control group meant that observed effects may have been due to the act of giving MET rather than the electrical currents generated by MET. Randomized double-blind shamcontrolled multicentre studies on the effects of electrical stimulation on ulcers and wounds do exist, although it is not certain whether the types of electrical stimulation used were strictly MET. Mulder 124 found that pulsed electrical stimulation decreased wound size by 56% when compared with a 33% reduction with sham on 59 patients with open wounds of pressure, vascular and surgical origin. A similar study on 47 patients with chronic dermal ulcers found differences in wound size and healing rate in favour of electrical stimulation. 125 Pulsed cathodal electrical stimulation was delivered twice daily at a pulse frequency of 128 pps, although the peak amplitude of 29.2 ma was higher than that seen for MET. A metaanalysis of 15 trials on a variety of forms of electrical stimulation reported that the healing rate was 22% per week compared with 9% for controls. 126 Unfortunately, findings on the relative effectiveness of the different types of electrical stimulation devices used in the studies were inconclusive.

23 28 MI Johnson Rebox devices also deliver microampere currents and, as a consequence, could be classed as MET. They were developed in the 1970s and use current trains of unipolar rectangular pulses via a charged probe electrode using microampere amplitudes (1 300 µa), pulse frequencies between 200 Hz and 5000 Hz, and a pulse duration of µs. 127,128 Available evidence about the pain relieving effects of Rebox is conflicting. Johannsen et al. 129 reported that Rebox improved both pain and function in patients with chronic lateral epicondylitis. In contrast, Hatten et al. reported that Rebox did not provide significant pain relief in patients 130 A placebo-controlled trial by Nussbaum and Gabison showed no differences between active and placebo with daily treatments of Rebox on experimentally-induced delayed onset muscle soreness in 30 healthy volunteers. 131 High-voltage pulsed currents HVPC, also known as high-voltage galvanic stimulation and high-voltage pulsed galvanic stimulation, have been used for muscle strengthening, wound healing and pain relief since the 1940s. 99,127, 128,132 Until recently HVPC devices were relatively large and resided in physiotherapy clinics, although, with advances in electronic technology, modern HVPC devices can be similar in size to standard TENS devices and, as a consequence, are being marketed for pain and wound management. HVPC devices deliver direct current with twin monophasic spiked pulses of V (500 ohm load) with a short pulse duration (microseconds) to increase penetration of tissue, leading to greater selectivity in recruiting motor nerves in innervated muscle and improved comfort for the patient. Pulses are delivered at double pulse frequencies of between 1 and 120 per second via a variety of types of electrodes including sponge, traditional carbon rubber and hand-held point electrodes. 127,132 Much of the experimental work on HVPC has focused on claims that it assists wound healing and is out of the scope of this discussion. 99,128 Two RCTs of note have reported that HVPC assists the rate of healing of ulcers. Kloth and Feedar 133 delivered HVPC to patients with decubitis ulcers and reported that the healing rate was faster when compared with sham. Griffin et al. 134 reported that HVPC significantly increase the healing rate of pressure ulcers in the pelvic region when given at 100 pps and an intensity of 200 V for 1 h a day for 20 consecutive days. Little experimental work on the effects of HVPC on pain relief was found. A comparison of the analgesic effects of HVPC with different types of TENS on electrically induced pain threshold and tolerance in 14 healthy people found no significant differences between the groups. 135 Morris and Newton 136 investigated the effects of HVPC on 28 patients with symptoms of pain and discomfort in the perirectal or rectal region (levator ani syndrome). HVPC were administered using a rectal probe for 1 h at a frequency of 120 Hz and at the maximum intensity that the patients could tolerate. They reported that 50% of these patients had pain or symptom relief after an average of eight treatments, although the study lacked a control group. Clearly, more experimental work is needed. TENS-pens A variety of hand-held pain relieving pens are available on the market, which deliver electrical currents to the intact surface of the skin using a single point electrode. TENS-pens are available as stand-alone battery operated devices or attached to battery operated pulse generators via a lead. The single point electrode used in TENSpens encourages users to deliver currents to discrete points on the surface of the body. Acupuncture points are often used as sites for stimulation and some devices incorporate an acupuncture point finder that detects low resistance on the skin. However, advertizing material recommends that TENS-pens can also be used to stimulate trigger points or the site of pain. The user needs to hold the pen during stimulation, so treatment times tend to be short and often less than a minute. It is therefore assumed that painrelieving effects occur predominantly poststimulation. The technical specifications of TENS-pens vary considerably between manufacturers, with available pens delivering currents in both milliampere (i.e. using a standard TENS pulse generator) and

24 Efficacy of TENS and TENS-like devices in pain relief 29 microampere (i.e. using a standard MET generator) ranges. Milliampere pens that deliver pulsed currents at strong but nonnoxious intensities are likely to activate large diameter nerve fibres and mimic the actions of conventional TENS. It is not known whether there are differences in outcome when conventional TENS currents are delivered to acupuncture points rather than to the site of pain because experimental evidence is lacking. A review of studies that assessed the pain relieving effects of TENS when delivered to acupuncture points using traditional electrode pads reported conflicting results. 33 Furthermore, it is not known whether delivering nonnoxious pulsed currents via a pen produces different outcomes to those obtained by using self-adhesive surface electrodes. Nevertheless, as the post-stimulation effects of nonnoxious pulsed currents (i.e. conventional TENS) are short lived, the delivery of currents intermittently would be of limited benefit. MET devices sometimes use pen electrodes to administer microampere currents. Whether this produces different treatment outcomes to those seen when MET is administered using pad electrodes is not known. Recently, high-voltage single-pulse TENS-pens (e.g. Pain Gone) have appeared on the UK market for treatment of minor ailments and painful conditions such as arthritis, back pain, headache and sports injuries. 137 High-voltage TENS-pens generate a single pulse when two crystals (piezoelectric elements) are forced together by a plunger. Each pulse has a high voltage (claimed to be 15,000 V) and short pulse duration, resulting in a 6 µa shock. Advocates claim that the high-voltage TENS-pens deliver currents in the microampere range, yet their output characteristics clearly differ from MET and are probably more akin to HVPC. High-voltage TENS-pens are claimed to generate low-frequency stimulation (1 2 pps), although the frequency of pulse delivery will be dependent on the rate of button pressing by the user, and is more likely to be ad hoc and asynchronous. Manufacturers recommend that patients should click the stimulating button times over acupuncture points or over the site of pain as this will result in effects that are similar to TENS and acupuncture. Descriptions of this mechanism of action are superficial and incomplete. The shock produces a sensation that resembles a pinprick and can be mildly painful, depending on the body site stimulated. This suggests that cutaneous Aδ fibres are active. Aδ afferents are believed to have a role in acupuncture analgesia and are known to trigger diffuse noxious inhibitory controls and release endorphins. Thus, high-voltage TENS-pens may initiate acupuncture-like mechanisms (on acupuncture points) or counter-irritation (on remote body sites) or both. It is not known whether the effects of high-voltage TENS-pens are dependent on the site of application. Information on the clinical effectiveness of high-voltage TENS-pens is lacking. One unpublished manuscript of an open uncontrolled clinical trial on 25 patients was identified. 137 Each patient received 25 clicks of the high-voltage TENS-pen once a day for 3 5 days, either over or just above the most painful area. Good to excellent pain relief that occurred immediately after treatment and lasted for some hours was reported by 76% of patients. Similar results were obtained in an uncontrolled trial on 36 patients with chronic musculoskeletal pain. 138 There was no placebo control group in either study, so it is possible that the pain relieving effects were produced by the act of giving the treatment rather than the electrical currents. Clearly, randomized controlled clinical trials are needed. Transcutaneous cranial electrical stimulation TCES has been used for over 30 years in rehabilitation medicine in the USA for insomnia, anxiety, depression, drug withdrawal and pain relief, and to reduce consumption of analgesics and anaesthetics. 139,140 Other names for TCES include cranial electrotherapy stimulation, transcranial electrotherapy, neuroelectric therapy, transcranial electrostimulation, and electrosleep. Electrode positions for TCES give the technique its identity and include: (1) attaching an electrode to each earlobe; or (2) attaching electrodes to each temple; or (3) attaching two positive electrodes in the retromastoid region and a negative electrode between the eyebrows (Limoge currents). TCES treatment usually lasts for minutes and is repeated once or twice daily. It

25 30 MI Johnson uses MET-like currents with current amplitudes below 1 ma. Pulse repetition rates of 100 pps are popular, although they can range from 0.5 pps to 15,000 pps, depending on the device. There does not seem to be a general consensus about the specific mechanism by which TCES could alleviate pain. Advocates claim that the output characteristics of TCES devices enable currents to reach the brain directly from the site of stimulation and that the currents affect brain function through direct action on neuronal activity and/or endogenous pharmacology. Animal and human studies have implicated endorphins, serotonin, cortisol and many other agents as potential mediators of TCES effects Experimental work suggests that TCES may potentiate the effects of opiates, neuroleptics and anxiolytics, allowing reductions in drug medication during anaesthetic procedures The findings of clinical trials on TCES are encouraging. A multicentre double-blind RCT on 100 patients with tension headache reported that TCES significantly reduced pain intensity when compared with placebo. 154 RCTs have also found positive effects of TCES on stress-related symptoms in people with closed head injury 155 and in reducing anxiety during routine dental procedures, 156 which may indirectly reduce pain. Recently, Scherder et al. 157 reported that TCES produced no improvements in cognition and (affective) behaviour in 18 patients with Alzheimer s disease when compared with placebo. One meta-analysis on the clinical effectiveness of TCES versus sham was found. 158 Eighteen RCTS were identified, out of which 14 had sufficient data to pool. TCES was significantly more effective than sham treatment for anxiety (eight trials) and headache (two trials), but not significant for brain dysfunction (two trials) and insomnia (two trials). TCES using Limoge currents has attracted attention for use in anaesthesic procedures because it has been claimed to reduce consumption of analgesics and anaesthetics. 140,144,145,148,149,153 One group claims to have administered TCES using Limoge currents in over 30,000 major interventions and also to aid drug withdrawal in 4000 opioid addicted patients, without any adverse events Limoge currents are high-frequency pulses (166 khz; on-time = 1 ms) interrupted with a repetitive low-frequency pulse (83 Hz; on-time = 4 ms) and delivered at low intensities of approximately 2 ma Each pulse has a high-amplitude, short duration (1.7 µs) positive phase and this is followed by a lowamplitude, long duration negative phase (6.3 µs). These pulses are delivered in trains (bursts). Some Limoge devices deliver currents at 167 khz, interrupted with an intermittent current of 77 Hz, 83 Hz or 100 Hz. Two studies included in systematic reviews on TENS and labour pain found that Limoge currents reduced additional analgesic intervention in women experiencing labour pain when compared with a sham device. 54,55 Stimulators to overcome tolerance to TENS Reports have suggested that some patients become tolerant to the pain-relieving effects of currents delivered by a standard TENS device, which may result from nervous system habituation to repetitive monotonous stimuli In an attempt to overcome nervous system habituation and the resultant TENS tolerance, some devices now have output characteristics that fluctuate between preset limits during stimulation. One common approach to TENS tolerance has been to fluctuate pulse frequency (i.e. frequency modulation) between upper and lower boundaries in a similar manner to that described for IFT. Frequency modulation on TENS devices has proved popular with patients and has been shown to be effective in relieving pain However, it is not known whether frequency modulation produces clinically meaningful reductions in the incidence of TENS tolerance. Another approach has been to deliver pulses randomly (i.e. random frequency). Random frequency TENS has been shown to elevate the experimental pain threshold in healthy people when compared with placebo, but the magnitude of the change was no different to that seen with other modes of TENS. 14 No clinical studies were found that had assessed the effects of random frequency TENS on TENS tolerance. An alternative approach to overcome nervous system habituation has been to deliver TENS

26 Efficacy of TENS and TENS-like devices in pain relief 31 pulses randomly to different body sites. Codetron is a TENS-like device that delivers low-frequency (2 4 pps) square waves with a pulse duration of 1 ms in a random order to one of six active electrode pads, which are usually positioned on acupuncture points. A small direct current of opposite polarity follows each pulse in order to avoid polarization of tissue. Codetron has been shown to increase the amplitude of cortical evoked potentials, indicative of a reduction in nervous system habituation, in healthy volunteers when compared with pulses delivered using conventional TENS. 166 Manufacturers also claim that Codetron mimics the effects of electroacupuncture and AL-TENS. Patients are advised to administer Codetron currents at the highest intensity that they can tolerate, providing they do not produce frank pain It is plausible, therefore, that Codetron generates activity in small diameter nerve fibres, resulting in extrasegmental analgesia, in a manner similar to electroacupuncture. Clinical trials of Codetron have produced conflicting results. It has been shown to provide over 30% pain relief in 107 of 137 patients with a variety of painful conditions. 168 A double-blind randomized sham controlled trial in 37 patients with osteoarthritis of the knee found that Codetron significantly improved pain when compared with sham (low-intensity TENS). 69 It has also been reported that Codetron reduces musculoskeletal pain to the same extent as electroacupuncture when delivered to acupuncture points at 4 pps and 200 pps at intensities just below pain. 170 Patients were given 20-minute treatment sessions, once or twice a week for a maximum of 12 treatments, depending on need. Telephone interviews 4 8 months after the end of the study showed that patients in the Codetron group reported greater improvement when compared with those in the electroacupuncture group. In contrast, a RCT that examined the effect of adding Codetron to an exercise programme in 58 low back pain patients found no differences between actual or placebo (no current) stimulation for disability or pain scores. 166 Patients did improve with exercise. Transcutaneous spinal electroanalgesia TSE, which has attracted much attention in the UK since its introduction in 1995, is indicated for minor aches and pains, migraine and stress. 171 Preliminary data suggest that TSE may help to reduce general practitioner consultation rates and that patients are satisfied with its effects. 172,173 TSE delivers pulsed currents with a high frequency (600 10,00 pps), high voltage and short pulse duration (1.5 4 µs) via two pad electrodes positioned either at T1 and T12 or straddling C3 C5. The intention of TSE is to activate excitable tissue in the spinal cord in order to reduce central sensitization by resetting central nervous system neuronal activity back to its presensitized state. 171 Physiological studies suggest that conventional TENS may reduce central sensitization, although there have so far been no experiments investigating the effects of TSE. If proved, TSE could be useful in the management of hyperalgesia and allodynia. The output characteristics of TSE devices are designed to overcome skin resistance so that currents bypass the skin and directly affect spinal cord circuitry. Patients do not usually experience electrical paraesthesia during TSE, so it is likely that cutaneous nerves are not activated to any appreciable extent and therefore the mechanism of action is different from conventional TENS mechanisms (i.e. activation of Aβ afferents). It is also claimed that, because peripheral nerve input converges at the spinal cord, TSE effects will be widespread over the body. 171 Studies on the effects of TSE are sparse. The initial promise of TSE was based on observations that it reduced pain by 60% in over two-thirds of a sample of 100 pain patients. 171 A preliminary RCT on eight patients suffering musculoskeletal pain found that TSE produced significantly greater reductions in pain measures than TENS. Each patient received one 20-minute treatment of TSE (10 khz, 1.5 µs) and one 20-minute treatment of TENS (100 pps, 200 µs) in a randomized, double-blind cross-over fashion. Both TSE and TENS were applied over T1 and T12. The authors recognized that this was not the normal way of administering TENS and that the study lacked power owing to the small sample size.

27 32 MI Johnson However, these initial findings suggested that the output characteristics of TSE produced more pain relief than those from a standard TENS device when administered at spinal sites. Subsequent reports have been less encouraging. Towell et al. 178 conducted a study using 60 healthy people to investigate the effects of TSE on mood and mechanical pain tolerance. When applied to the spinal cord for 30 minutes, it reduced tolerance to mechanical pain when compared with sham TSE, suggesting that TSE had made the experimental pain worse. However, TSE was found significantly to elevate mood. A second experiment by the same group applied TSE to the shoulder joint and found no differences in mood or pain tolerance to experimental pain in healthy people when compared with sham. Studies reported in conference proceedings confirm the lack of analgesic effect. Hilberstadt et al. 179 reported that TSE did not alter pain when administered in 10-day periods to two patients with low back pain. A series of double-blind placebo controlled trials conducted by Heffernan and Rowbotham 180 found that TSE did not reduce pain or the need for additional analgesic interventions when compared with sham TSE in acute and chronic pain settings. At least one other RCT on the effects of TSE for pain after breast cancer treatment was found, although the current status of this trial is unknown. 181 Action potential simulation It is claimed that APS provides pain relief, reduces inflammation and swelling, enhances local blood circulation, increases mobility, regenerates cell and bone growth, and generates adenosine triphosphate (ATP) The term action potential simulation derives from claims that APS devices generate electrical currents that are similar in shape to nerve action potentials. 101,184 It is unclear whether APS currents are designed to trigger action potentials or whether they simulate changes in membrane potentials resulting from neural activity. APS delivers monophasic square waves with exponential decay and a DC offset that remains at 5 V. APS uses a long pulse duration between 800 µs and 6.6 ms, a pulse frequency fixed at ~150 pps and a pulse amplitude between 0 and 24.4 ma into a 500 ohm load. It is claimed that APS is a unique type of MET, although most articles on APS do not make this explicit. 101,113 In some experiments APS was delivered using lowcurrent amplitudes (e.g. between 0.70 ma and 1.7 ma) with patients being unable to perceive currents. 182,185 However, it has also been delivered at doses that appear to be strong and producing electrical paraesthesia. 182,186 APS is administered using two electrodes attached close to the site of pain and protocols used in some published trials seem to focus on treatment times in multiples of 8 min (e.g. 8 and 16 min) although the rationale for this approach is vague. 182,183 Descriptions of the hypothetical mechanism of action of APS are ambiguous, and general statements that APS leads to excitation of the central nervous system and the release of neurohormones are common. 184,187 It is claimed that the DC offset in the APS waveform increases production of ATP and also creates tissue polarization, resulting in increased levels of oxygen and catabolism and leading to the removal of waste substances of tissue damage. 101,113,184 It is interesting that tissue polarization is seen as an adverse effect for Codetron but is considered to be of benefit for APS. The majority of experimental work on the effects of APS originates in South Africa, where the device was originally designed. A doubleblind placebo controlled study found that APS increased plasma levels of L-enkephalin and melatonin and reduced Beta-endorphin when compared with sham APS (no current) in patients suffering chronic low back pain. 187 No changes in plasma serotonin or cortisol levels were found and the authors speculated that decreasing plasma beta-endorphin would help to reduce inflammation, although this could not be proved within their experiment because they did not record changes in inflammation directly. Experimental evidence on the effect of TENS on plasma opioids is conflicting An open trial using 285 patients with a variety of chronic pain conditions found that APS improved pain and mobility, although the study lacked a control group. 183 Odendaal and Joubert 185 examined the effects of APS in a placebo controlled trial on 76

28 Efficacy of TENS and TENS-like devices in pain relief 33 patients with chronic low back pain and claimed that APS may be an effective treatment. The data suggested that there were no differences between APS and sham APS groups, although the authors argued that the trial population was too small to conduct a between-group analysis. Berger and Matzner 182 examined the effects of APS, TENS and placebo on mobility and swelling in 99 patients suffering from osteoarthritis, reporting that APS, TENS and placebo improved pain. However, no effects were found between treatment groups, suggesting that the act of giving APS, rather than the electrical currents delivered by APS, produced the reduction in pain. The authors argued that APS was a viable treatment option because it was unacceptable to administer placebos in clinical practice. A more appropriate interpretation of these findings would be that APS, TENS and placebo are equally ineffective in the treatment of patients with osteoarthritis. Double-blind studies on the effects of APS, TENS and IFT on skin temperature and mechanical pain threshold in healthy people have also failed to find any significant differences between groups. 85,191 H-wave therapy Manufacturers claim that HWT is useful in the treatment of soft tissue injuries by promoting healing, reducing inflammation and oedema, and relieving pain. It has been used to treat neuropathic pain associated with diabetes 192 and for dental anaesthesia. 193 It seems that HWT was developed to reproduce the H-reflex (Hoffman reflex), hence the name, although a suitable explanation of the relationship between the Hoffman reflex, HWT and analgesia could not be found. Manufacturers state that HWT currents pass through the skin without causing discomfort and that the output characteristics mimic those found in the body, although how this relates to physiological mechanisms is not clear. HWT is administered by using two electrodes at the site of pain, over acupuncture points or over muscle bellies surrounding tissue. It delivers a biphasic exponentially decaying wave of long pulse duration (16 ms) with pulse frequencies limited to either 2 pps or 60 pps. Marketing material argues that frequencies above 60 pps are not needed because HWT uses a bipolar waveform and 60 pps equates to 120 pps monophasic waveforms, although this rationale appears to be weak because most standard TENS devices delivering biphasic waveforms offer frequencies beyond 200 pps. Manufacturers also recommend that highfrequency HWT (i.e. 60 pps) generating a strong but comfortable electrical paraesthesia within the site of pain will provide the best pain relief. The mechanism of action of this effect is likely to be similar to that found for conventional TENS. Low-frequency HWT (i.e. 2 pps) generating muscle twitches is recommended for inflammation and oedema because it will produce muscle pumping actions that will compress surrounding circulatory vessels and improve blood flow and fluid drainage. Clinical studies on HWT are few. It was shown significantly to reduce pain associated with peripheral neuropathy in 31 patients with type 2 diabetes when compared with sham stimulation. 194 HWT was administered to the lower extremities for 30 min daily for four weeks using pulses with a frequency of 2 pps or greater and a duration of 4 ms. A follow-up study reported that HWT improved neuropathic pain associated with diabetes in 76% of respondents to a postal questionnaire, although this study failed to include a control group. 195 Workers have reported that HWT increases the mechanical pain threshold 196 and reduces McGill Pain Questionnaire scores for experimentally-induced ischaemic pain in healthy people when compared with sham HWT, 197 although this latter finding was not confirmed in a follow-up study. 198 HWT was also shown to increase the latency of the compound action potential in the superficial radial nerve of 32 healthy volunteers, suggesting that it reduces nerve conduction in peripheral nerves. 199 HWT delivered at 2 pps, but not 60 pps, has also been shown to increase skin blood flow and skin temperature when compared with placebo, providing indirect evidence that it may be useful in facilitating tissue repair through improved circulation. 200

29 34 MI Johnson Discussion It is hardly surprising that confusion exists about the effectiveness of TENS and TENS-like devices with such an array of available stimulators and claims of efficacious treatment protocols. The theoretical principles underpinning many of the TENS-like devices described in this review have their traditional roots in physiotherapy and rehabilitation medicine. Until recently, treatment with many TENS-like devices could be obtained only under the supervision of a trained therapist in a clinic setting and working under the constraints of the clinical rota. Advances in electronic technology have reduced the cost, size and dangers of TENS-like devices and companies now market to a broader section of health care professionals. Increasing numbers of cheap handheld devices have placed greater emphasis on self-treatment and the general public can purchase many TENS-like devices directly from manufacturers. It is important that the interests of the public, patients and health care professionals are protected by scrutinizing claims made by manufacturers about the relative effectiveness of the different devices. The findings of systematic reviews and metaanalyses on the effectiveness of TENS for pain relief can be summarized as follows: TENS will relieve pain associated with knee osteoarthritis 65 and primary dysmenorrhoea, 64 but will not relieve postoperative pain 42 or labour pain. 50,51 Evidence is inconclusive for post-stroke shoulder pain 58 and chronic pain in general. 57 Evidence for the efficacy of TENS in chronic low back pain has produced conflicting results These clinical bottom lines are particularly attractive to practitioners when making decisions about treatment interventions. However, the term TENS encompasses a range of output characteristics, application procedures and dosing regimens, and this has resulted in the use of inconsistent and sometimes inappropriate criteria to differentiate types of TENS, such as conventional TENS and AL- TENS. Furthermore, the evaluation models used in some reviews have been challenged. Trials in the acute pain setting use pain scores as the primary outcome measure, despite patients having access to additional analgesic drugs, which will compromise pain scores because patients in both sham and active TENS groups will titrate analgesic consumption to achieve effective relief and minimal pain. Analgesic consumption would be the most clinically meaningful outcome in these trials, although this has been dismissed as being of secondary importance by some authors. 51 The correct way to administer TENS is also contentious and some systematic reviews include RCTs that used protocols that underdose or use TENS-like devices that differ from standard TENS devices. When these issues are taken into account they can change the outcome of systematic reviews. 48 An evaluation of the literature on the effectiveness of TENS-like devices revealed that: Available experimental evidence is limited in both quality and quantity. Controlled studies for different TENS-like devices, when available, suggest that some have absolute effectiveness resulting from the electrical currents rather than from the act of administering the currents. However, this is far from proven and seems to depend on the type of nerve fibre activated, irrespective of the output characteristics of the device. Available evidence suggests that there are minimal differences in the magnitude and time course of pain relief achieved with different types of device. Any potential differences in outcome appear to be linked to the type of fibre activated rather than the output characteristics of the device. The specificity of effect of different TENS-like devices is questioned. The relationship between output characteristics and physiological intention Electrical currents form the active ingredient of TENS in much the same way as a chemical structure forms the active ingredient of a drug. In this review, output characteristics were the main criteria used to differentiate standard TENS devices from nonstandard TENS-like devices. Most of the broad categories of TENS-like devices use novel electrical currents to gain identity in the market-place. Current waveform is commonly used in marketing literature to differentiate one type of device from another and all broad categories of devices (i.e. standard TENS, IFT, MET, HVPC) gain identity in part from their current

30 Efficacy of TENS and TENS-like devices in pain relief (a) (b) (c) (d) Standard TENS Codetron IFT APS HVPC Limoge currents HWT Figure 8 Figure 8 Waveform characteristics commonly used by standard TENS and TENS-like devices. Standard TENS devices often use pulse waveforms that are monophasic (a), symmetrical biphasic (b), asymmetrical biphasic (c), or spike-like biphasic (d). IFT uses an amplitude-modulated interference wave resulting from the collision of two out-of-phase sinusoidal-like currents. HVPC uses high-voltage double spike pulses. Limoge currents are wave trains of positive pulses of high intensity, short duration, followed by negative pulses of weak intensity long duration. Codetron uses biphasic square pulses. APS uses a monophasic square pulse with exponential decay. HWT uses bipolar exponentially decaying pulsed currents. No diagramatic representation of MET or TSE could be found in the literature. Whether these subtle differences in waveform characteristics translate into clinically different outcomes is not known waveform (Figure 8). A more appropriate way to differentiate devices would be according to the physiological intention of the currents. When used to generate pain relief, two main intentions dominate the literature: first, the use of currents to assist physiological processes associated with tissue healing; and secondly, the use of currents to stimulate nerve fibres in order to activate pain modulatory circuits. The main intention of microampere currents is to activate cellular processes that assist tissue healing. Experimental evidence suggests that MET can alter ATP and other biochemical markers of tissue healing in vitro. 100, However, there is doubt about whether this translates to meaningful changes in the rate of tissue healing in animals and humans. 119 The absence of MET-induced sensations such as electrical paraesthesia during stimulation suggests that MET currents have insufficient energy to activate cutaneous afferents to any appreciable extent. It is assumed that putative pain relief from MET is an indirect result of accelerated tissue healing. The main intention of milliampere currents is to stimulate neural circuitry directly to modulate nociceptive transmission at peripheral, segmental and extrasegmental levels. Experimental evidence supports the view that recruitment of different populations of fibres leads to different

31 36 MI Johnson modulatory mechanisms becoming active. Activation of large diameter cutaneous afferents (i.e. Aβ) is related to segmental analgesia. Activation of small diameter muscle afferents (i.e. Group III) through a phasic muscle twitch is related to extrasegmental analgesia. Activation of small diameter cutaneous afferents (i.e. Aδ) is related to counter-irritant effects and blockade of transmission in peripheral nociceptive nerves. Whether these various mechanisms result in clinically meaningful differences in outcome remains contentious. Theoretical models based on strength-duration curves for axonal excitation provides information about the relationship between output characteristics and fibre recruitment. However, the simplest way to recruit different fibre types is to raise current amplitude and/or alter electrode positions. This raises questions about the influence of other output characteristics in determining the magnitude and profile of segmental analgesia. The literature lacks published systematic investigations into the analgesic effects of different output characteristics (such as pulse frequency, pattern and duration) when current amplitude is kept constant. It has been suggested that patients choose pulse frequencies and patterns for reasons of comfort rather than improved analgesia. The physiological justification for incorporating novel output characteristics in many TENSlike devices is weak. Explanations of the relationship between output characteristics and physiological effect are often general and descriptions of closing pain gates or increasing levels of circulating endorphins are prevalent in manufacturers literature. It was often difficult to ascertain whether a TENS-like device was being administered with a view to activating nerve fibres selectively or to aid the healing process, or both. If the TENS-like devices are closing the pain gate, then it may be cheaper to use a standard TENS device. Clearly, manufacturers need to demonstrate that TENS-like devices produce effects that are clinically different to, or greater in magnitude than, those achieved using a standard TENS device. At present, many differences in output characteristics between TENS-like devices appear to be purely cosmetic. The diversity of ways in which electrical currents can be administered confounds categorization and the synthesis of available evidence. Electrode sites may be similar to those proposed for conventional TENS (i.e. around the site of pain for IFT and HWT) or completely different (i.e. on the head for TCES or spinal cord placement for TSE). Treatment regimens varied considerably and some seem to defy logic. For example, in one trial, APS was administered for 16 minutes followed by a 3-minute rest and a further 16 minutes. 185 Intermittent stimulation of between 20 and 60 minutes repeated a few times a day tended to predominate in published advice for many TENS-like devices, especially for nonpotable devices that reside in clinics. It is possible that such dosing regimens have developed because they fit into the clinical rota. 73 Patients who use conventional TENS are encouraged to administer treatment to produce a strong, comfortable electrical paraesthesia whenever they are in pain because this reflects activity in Aβ afferents. Evidence from animals, normal people and patients shows that the effects of conventional TENS are maximal when the device is switched on and are usually short lived once it is switched off. 15,17 19 Intermittent dosing regimens for conventional TENS are known to lead to inadequate treatment and this has been recognized as a design flaw in many TENS trials. 56,57 When TENS-like devices are used to generate a strong but comfortable electrical paraesthesia within the site of pain (e.g. HWT, IFT) a continuous dosing regimen should be used. The intermittent dosing regimens recommended for TENS-like devices need to be justified by demonstrating, through well-designed experimental trials, that post-stimulatory effects actually occur. The relationship between output characteristics and clinical effectiveness From a practical perspective, health care professionals need experimental evidence on absolute effectiveness (e.g. against a placebo control) and relative effectiveness (e.g. against other existing treatments) to inform clinical decisions. With an ever-increasing number of TENS and TENS-like devices available on the market, evidence about

32 Efficacy of TENS and TENS-like devices in pain relief 37 relative effectiveness is particularly important to inform device selection. In essence, health care professionals want to know whether changing the output characteristics of TENS devices alters clinical effects. Cost-effectiveness should also be considered and whether putative treatment effects can be achieved by using a standard TENS device rather than a more expensive TENS-like device. At present, there are few comparative studies between standard TENS and TENS-like devices or between different types of TENS-like devices. Those that exist suggest that the output characteristics of TENS-like devices, which may differ from each other markedly, do not influence clinical outcome to the extent inferred by their advocates. Clinical trials that examine the relative effectiveness of TENS-like devices with a standard TENS device are desperately needed to inform therapists about device selection. In conclusion, despite the technologically impressive appearance of many TENS-like devices, a standard TENS device probably remains the best type of stimulator for pain relief. Standard TENS devices are predominantly used to stimulate different populations of nerve fibres, with the potential to produce different analgesic outcomes. Clinical evidence does suggest that a standard TENS device, if used appropriately, can provide pain relief. The findings of systematic reviews suggesting that TENS is not effective for postoperative and labour pain have recently been challenged. Health care professionals often turn to TENS-like devices when patients fail to respond to treatment with a standard TENS device. It is important that the causes of nonresponse to standard TENS have been fully explored before the switch is made to a more expensive TENS-like device. Patients can be inappropriately labelled as nonresponders because they have been given unrealistic expectations of treatment outcome or insufficient information about the principles and practice of TENS use. Experimenting with the electrical characteristics, electrode positions or dosing regimen and/or trials of the different types of TENS, such as AL-TENS, can often overcome problems associated with treatment resistant patients. The driving force for the design of many TENS-like devices has been the ability to incorporate technologically impressive output characteristics based on hypothetical and speculative physiological rationale. At present there is a lack of good quality physiological and clinical evidence to justify so many different types of device. In future it would be more relevant to define TENS and TENS-like devices according to the physiological intention of the currents. Physiological justification for the inclusion of specific output characteristics in stimulator design should also be provided by manufacturers and supported by experimental evidence that demonstrates effects over and above those seen with a standard TENS device. Describing and defining TENS effects in relation to nerve activity will improve consistency in reporting and will enable systematic reviews to include and exclude trials according to the physiological consequences of currents rather than output characteristics. References 1 Walsh D. TENS. Clinical applications and related theory. New York: Churchill Livingstone, Johnson M. Transcutaneous electrical nerve stimulation (TENS). In: Kitchen S ed. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone, 2001: Chabal C, Fishbain DA, Weaver M, Heine LW. Long-term transcutaneous electrical nerve stimulation (TENS) use: impact on medication utilization and physical therapy costs. Clin J Pain 1998; 14: Bandolier. Transcutaneous electrical nerve stimulation (TENS) in postoperative pain. Bandolier 1999; (July). Available from: URL: trev/other/ap019.html [Accessed 16-Jan-03] 5 Bandolier. Transcutaneous electrical nerve stimulation (TENS) in labour pain. Bandolier 1999; (July). Available from: URL: trev/labour/ap001.html [Accessed 16-Jan-03] 6 Bandolier. Transcutaneous electrical nerve stimulation (TENS) in chronic pain. Bandolier 1999; (July). Available from: URL: onrev/other/cp074.html [Accessed 16-Jan-03] 7 Woolf C, Thompson J Segmental afferent fibreinduced analgesia: transcutaneous electrical nerve stimulation (TENS) and vibration. In: Wall P, Melzack R eds. Textbook of pain. Edinburgh: Churchill Livingstone, 1994: American Physical Therapy Association. Electrotherapeutic terminology in physical therapy:

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39 44 MI Johnson TENS reduces the secondary hyperalgesia observed after injection of kaolin and carrageenan into the knee joint. Pain 1998; 77: Sluka KA, Judge MA, McColley MM, Reveiz PM. Low frequency TENS is less effective than high frequency TENS at reducing inflammation-induced hyperalgesia in morphine-tolerant rats. Eur J Pain 2000; 4: Towell A, Williams D, Boyd S. High frequency non-invasive stimulation over the spine: effects on mood and mechanical pain tolerance in normal subjects. Behav Neurol 1997; 10: Hilberstadt D, Barlas P, Foster N. The effect of transcutaneous spinal electroanalgesia upon chronic pain: a single case study. Physiotherapy 2000; 86: Heffernan A, Rowbotham D. Transcutaneous spinal electroanalgesia: its effects in acute and chronic pain patients and healthy volunteers [Abstract]. In: The Pain Society of Great Britain Annual Scientific Meeting Abstracts; Bournemouth, UK; 2002 Apr 9 12: abstr Robb K. Pain after breast cancer treatment: a randomised, placebo controlled crossover trial to compare two different stimulation analgesic techniques. In: The Pain Society of Great Britain Annual Scientific Meeting Abstracts; Leicester, UK; 1998 Apr: abstr Berger P, Matzner L. Study on 99 patients with osteoarthritis (OA) of the knee to investigate the effectiveness of low frequency electrical currents on mobility and pain: action potential simulation therapy (APS) current compared with transcutaneous electrical nerve stimulation (TENS) and placebo. South Afr J Anaesthesiol Analg 1999; 5: Papendorp V, Krugere M, Maritz C, Dippenaar N. Action potential simulation therapy: self assessment by 285 patients with chronic pain. Geneeskunde The Medical Journal 2000; (Jan/Feb): Sandham J. Action potential simulation therapy. Eng Technol 2000; (July): Odendaal C, Joubert G. APS therapy a new way of treating chronic headache a pilot study. South Afr J Anaesthesiol Analg 1999; 5: Noble JG. Interferential therapy: assessment of hypoalgesic and neurophysiological effects. [Thesis]. Belfast: University of Ulster, 2000: Wet ED, Oosthuizen J, Odendaal C, Shipton E. Neurochemical mechanisms that may underlie the clinical efficacy of action potential simulation (APSim) therapy in chronic pain management. South Afr J Anaesthesiol Analg 1999; 5: Facchinetti F, Sandrini G, Petraglia F, Alfonsi E, Nappi G, Genazzani AR. Concomitant increase in nociceptive flexion reflex threshold and plasma opioids following transcutaneous nerve stimulation. Pain 1984; 19: Facchinetti F, Sforza G, Amidei M et al. Central and peripheral beta-endorphin response to transcutaneous electrical nerve stimulation. NIDA Res Monogr 1986; 75: Johnson MI, Ashton CH, Thompson JW, Weddell A, Wright Honari S. The effect of transcutaneous electrical nerve stimulation (TENS) and acupuncture on concentrations of beta endorphin, met enkephalin and 5 hydroxytryptamine in the peripheral circulation. Eur J Pain 1992; 13: Alves-Guerreiro J, Noble JG, Lowe AS, Walsh DM. The effect of three electrotherapeutic modalities upon peripheral nerve conduction and mechanical pain threshold. Clin Physiol 2001; 21: Alvaro M, Kumar D, Julka IS. Transcutaneous electrostimulation: emerging treatment for diabetic neuropathic pain. Diabetes Technol Ther 1999; 1: Meizels P. Electronic anesthesia. H-wave and TMJ treatment. J California Dent Assoc 1987; 15: Kumar D, Marshall HJ. Diabetic peripheral neuropathy: amelioration of pain with transcutaneous electrostimulation. Diabetes Care 1997; 20: Julka IS, Alvaro M, Kumar D. Beneficial effects of electrical stimulation on neuropathic symptoms in diabetes patients. J Foot Ankle Surg 1998; 37: McDowell BC, McCormack K, Walsh DM, Baxter DG, Allen JM. Comparative analgesic effects of H- wave therapy and transcutaneous electrical nerve stimulation on pain threshold in humans. Arch Phys Med Rehabil 1999; 80: Walsh DM, Baxter D, Allen JM, Bell AJ, Mokhtar B. An assessment of the analgesic effects of H-wave therapy upon experimentally-induced ischaemic pain [Abstract]. Ir J Med Sci 1992; 161: McDowell BC, Lowe AS, Walsh DM, Baxter GD, Allen JM. The lack of hypoalgesic efficacy of H- wave therapy on experimental ischaemic pain. Pain 1995; 61: McDowell BC, Lowe AS, Walsh DM, Baxter GD, Allen JM. The effect of H-wave therapy upon conduction in the human superficial radial nerve in vivo. Exp Physiol 1996; 81: McDowell BC, McElduff C, Lowe AS, Walsh DM, Baxter GD. The effect of high- and low-frequency H-wave therapy upon skin blood perfusion: evidence of frequency-specific effects. Clin Physiol 1999; 19:

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