neuromodulation || transcutaneous electrical nerve stimulation (tens)
TRANSCRIPT
C H A P T E R
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335Neuromodulation © 2008, Elsevier Ltd.2009
Transcutaneous Electrical Nerve Stimulation (TENS): A Review
Kathleen A. Sluka, Howard S. Smith, and Deirdre M. Walsh
Introduction 335
TENSTerminology 336
GeneralPrinciplesofApplicationofTENS 336
TheoriesofTENSAnalgesiaandEffectsofTENSinAnimalModels 338
AnalgesicMechanismsofTENS 338High Frequency (50–100 Hz) TENS 338
Low Frequency (10 Hz) TENS 340Autonomic Effects of Low Frequency TENS 340
TranslationofMechanismsofTENSAnalgesiatotheClinic 340
TheClinicalEfficacyofTENS 341
SummaryPoints 342
References 342
o u T L i N E
INTRoduCTIoN
Transcutaneous electrical nerve stimulation (TENS) involves the application of electrical currents to the skin primarily for the purposes of pain relief. It is a safe, non-invasive treatment that can be self-administered.
Natural forms of electricity have been used as a method of pain relief since the Egyptian era with early prototypes of TENS units available by the late 1800s (Walsh, 1997). However, the use of electrical currents for pain relief was met with a degree of skepticism until a theoretical foundation for this electroanalgesia was established. This came in the form of Melzack and Wall’s gate control theory of pain (Melzack and Wall, 1965), which proposed that a gate existed in the dorsal horn of the spinal cord which could regulate the amount of incoming nociceptive traffic via small diameter afferent
nerve fibers. This gate could be closed by a range of stimuli which activate large diameter afferent fibers such as touch, pressure, and electrical currents.
Shortly after the theory was published, initial stud-ies emerged which showed the effective use of percutaneous electrical stimulation for chronic neuropathic pain (Wall and Sweet, 1967). However, it was Dr Norman Shealy who made a significant discovery for the use of transcutaneous electrical nerve stimulation for pain relief.
Around this time, dorsal column stimulation (DCS), a new technique for pain relief, was developed. DCS, now called spinal cord stimulation or SCS, involved the surgical implantation of electrodes over the dor-sal columns of the spinal cord which were then acti-vated by an external battery-operated device (Shealy et al., 1967). Today, SCS involves the placement of
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either percutaneous or laminotomy electrode lead arrays within the epidural space overlying the dor-sal columns of the spinal cord, which are activated by either an external battery source to an implanted radio receiver (RF device), an implanted neuropulse generator (IPG) with either an externally rechargeable or a non-rechargeable battery. Shealy used an early TENS device as a screening tool prior to proceeding with DCS for the management of his patients with chronic pain (Shealy, 1974). Interestingly, Shealy dis-covered that some of his patients responded better to the TENS therapy than when he used DCS, and so TENS subsequently emerged as a viable modality for the management of pain.
Meyer and Fields (1972) were among the first to report on the clinical use of TENS for the relief of chronic pain. Technological advances have subse-quently produced today’s wide range of stimulators with an even wider range of stimulation parameters for clinicians to choose from. Despite widespread use, the clinical efficacy of TENS remains ambiguous. This chapter provides an overview of the pertinent research relating to the theory and clinical application of TENS.
TENSTERMINoloGy
A TENS unit may be considered as any device gen-erating appropriate cutaneously applied pulsed current through surface electrodes to overcome the impedance of the skin’s conductive barrier and result in excita-tion of peripheral nerves (see Figure 24.1). There are many types of transcutaneous currents that fall under the umbrella term of TENS, e.g. interferential currents, H-wave therapy etc. Interferential currents involve the application of two medium frequency currents (typi-cally around 4000 Hz) to the skin to theoretically pro-duce an amplitude modulated low frequency current (range 1–150 Hz) within the tissues. Medium frequency currents are applied in order to overcome skin imped-ance which is inversely proportional to the frequency of the applied current. It is suggested that the resulting low frequency amplitude modulated current can stim-ulate deeper tissues as less current is required to over-come skin resistance; however, evidence supporting the theory behind this is lacking (Ozcan et al., 2004). In contrast, H-wave therapy employs a biphasic expo-nentially decaying waveform with a fixed pulse dura-tion (approximately 15 ms) delivered at frequencies ranging from 2 Hz to 60 Hz. Research to date on the hypoalgesic effects of both types of electrical current
remains equivocal (McDowell et al., 1999; Adedoyin et al., 2002; Cheing and HuiChan, 2003; Johnson and Tabasam, 2003; Reichstein et al., 2005). For the purposes of this chapter, the term TENS will be used to describe those types of electrical current with a frequency of less than 200 Hz and a pulse duration less than 400 s.
GENERAlPRINCIPlESofAPPlICATIoNofTENS
The clinical application of TENS involves the deliv-ery of a low voltage electrical current from a small battery-operated device to the skin via surface elec-trodes. The majority of TENS devices offer variable frequency (pulse rate), pulse duration, intensity (ampli-tude), and type of output (the pattern in which the pulses are delivered: burst, continuous, or modulated). A modulated output is produced by varying pulse dura-tion, frequency, and/or amplitude in a regular and cycli-cal manner with the hope of avoiding accommodation of nerve fibers to a constant stimulus (e.g. amplitude modulation involves a cyclical modulation in amplitude from zero increasing gradually to a preset peak level, and then decreasing gradually back to zero again).
TENS devices typically use a pulsed current with a rectangular shaped waveform; waveforms are usually monophasic, symmetrical biphasic, or asymmetrical biphasic. The amplitude is directly related to the mag-nitude or intensity of the current being delivered. Intensity is measured in milliamperes (mA) (or milli-volts if the device is designed to deliver constant volt-age) and generally ranges from 30 to 100 mA, often yielding sensations of tingling or pins-and-needles. The pulse duration is the length of time during which
fIGuRE24.1 Select TENS unit(Courtesy of Empi, St Paul, MN)
GENERAL PRiNCiPLES of APPLiCATioN of TENS 337
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each pulse is delivered. Longer pulse durations give rise to increases in the total electrical charge delivered. As the pulse duration is increased in the usual range from 40 to 400 microseconds (s), the patient may feel a spreading/radiating and/or deepening/penetrating sensation. The pulse rate (frequency) is the number of pulses delivered per second (Hz). The range of pulse rate is generally 1 Hz to 200 Hz. Combinations of these different stimulation parameters are used to produce four main modes of TENS (Walsh, 1997): Conventional or high frequency TENS (frequency typically above 100 Hz, short pulse duration (50–80 s), low intensity); Acupuncture-like or low frequency TENS (frequency usually 1–4 Hz, long pulse duration (200 s), high intensity); Burst TENS (high internal frequency trains of pulses (100 Hz) delivered at a low frequency, typi-cally 1–4 Hz); and Brief–Intense TENS (high frequency and long pulse duration pulses delivered at a high intensity) (see Figure 24.2).
Conventional TENS (high frequency TENS with frequencies typically above 100 Hz, short pulse dura-tion (50–80 s), low intensity) stimulates large diameter afferents and produces paresthesia in the area under the electrodes whereas the production of muscle twitches is desirable with Acupuncture-like TENS (low frequency TENS with frequencies usually between 1 and 4 Hz with long pulse durations of ~200 s, high intensity). In Acupuncture-like TENS, the electrodes should be positioned to produce visible non-painful muscle con-tractions (twitching type) (e.g. over a myotome related to the painful area). Burst TENS, consisting of high fre-quency trains of pulses delivered at low frequencies,
may produce more comfortable muscle contractions. Brief–Intense TENS, which consists of high frequency (100–150 Hz) and long duration (150–250 s) pulses delivered at the patient’s highest tolerable intensity for short periods of time (15 minutes), is sometimes used for painful procedures (e.g. skin debridement) (see Figure 24.2).
In terms of application, the clinician has four different electrode placement sites to choose from: the painful area; the peripheral nerve supply to the painful area, spinal nerve roots dermatomal distribution, and acupunc-ture/motor/trigger points. Self-adhesive electrodes are most commonly used although some clinicians still use a carbon rubber electrode and gel application. If tape is required to secure the latter type of electrode in place, care must be taken to ensure the tape is applied evenly to ensure uniform distribution of the current.
Relatively few adverse effects have been reported with TENS. Precautions for and contraindications to TENS are mostly empirical, reflecting “common sense” and include: impaired sensation, impaired alertness/cognition, use in the region of the anterior neck or eyes (e.g. where carotid sinuses are located), history of contact allergy to the electrode gel (which commonly contains propylene glycol) or tape, epilepsy, use over broken or irritated skin, use while operating machin-ery, or pregnancy (however, TENS is frequently used for pain relief during labor). In addition, TENS has been shown to interfere with some types of pacemak-ers (Broadley, 2000; Pyatt et al., 2003).
The successful application of TENS involves a degree of trial and error. Several attempts are typically required
Time � 1 s
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before the optimal stimulation parameters and electrode site are determined for a patient. It is recommended that the first trial of TENS involve Conventional TENS applied over the painful area as the paresthesia experi-enced is usually more comfortable for the patient. Following this initial trial, other modes of TENS should also be sequentially tried to determine which produces the maximum pain relief. The application time should be kept to 30 minutes for the first trial to allow moni-toring for adverse effects and subsequently increased to one hour at a time, repeated as many times as neces-sary. A 30 minute break between applications over the same skin area is recommended to avoid skin irrita-tion associated with prolonged use. The intensity of the TENS should be increased to produce what the patient feels is a “strong but comfortable” sensation. As muscle contraction is desirable with Acupuncture-like TENS, the intensity should be increased until muscle twitching is observed. Due to perceived accommodation of nerve fibers, the intensity can be increased during treatment to maintain this subjective sensation of being “strong but comfortable.”
However, the effect of perceived accommodation has not been rigorously examined in the clinical set-ting. A recent study by Defrin et al. (2005) on inter-ferential currents suggests that it is not necessary to adjust current intensity during treatment to obtain pain relief. No significant differences in treatment out-comes were found between groups of patients with chronic pain in which the current intensity was con-stantly adjusted to prevent fading of sensation versus those in which the intensity was not adjusted and in which patients reported fading of sensation.
ThEoRIESofTENSANAlGESIAANdEffECTSofTENSINANIMAlModElS
Two theories are commonly utilized to support the use of TENS. The gate control theory of pain is most commonly utilized to explain the inhibition of pain by TENS. According to the gate control theory of pain, stimulation of large diameter A afferents inhibits nocic-eptive C-fiber evoked responses within the dorsal horn. There is now much more detailed data on mechanisms of actions of TENS that includes anatomical pathways, neurotransmitters and their receptors, and the types of neurons involved in the inhibition. Release of endog-enous opioids has been used to explain the actions of TENS, particularly low frequency stimulation. Recent data support this theory for low frequency TENS as well as for high frequency TENS stimulation (Sluka et al., 1999; Kalra et al., 2001).
Early studies on the mechanisms of action of TENS were performed in normal, uninjured animals. These studies provided valuable information regarding poten-tial mechanisms of action of TENS. More recent studies have translated and extended these data by examin-ing mechanisms of action of TENS in animal models of pain. The studies in animal models of pain have revealed pharmacological and anatomical pathways that mediate the reduction of pain produced by TENS. The current data suggest that different frequencies of TENS produce analgesia through actions on different neurotransmitters and receptors. Below we describe the neurotransmitters and receptors involved in TENS analgesia.
In animals without tissue injury, the behavioral responses to noxious thermal stimuli are increased (Woolf et al., 1977; Woolf et al., 1980) and dorsal horn neuron activity is reduced (Lee et al., 1985; Sjolund, 1985, 1988; Garrison and Foreman, 1994, 1997) by either high or low frequency TENS. In animal models of cutaneous, joint or muscle inflammation, primary and/or secondary hyperalgesia is reversed by either low frequency (4 Hz) or high frequency (100 Hz) TENS at sensory intensities (Sluka et al., 1998; Gopalkrishnan and Sluka, 2000; deResende et al., 2004; Ainsworth et al., 2006; Vance et al., 2007). Interestingly, when bilateral hyperalgesia occurs, application of TENS to the inflamed or the contralateral non-inflamed mus-cle equally reduces the hyperalgesia (Ainsworth et al., 2006; Sabino et al., 2008). Furthermore, increased responsiveness of dorsal horn neurons that occurs after peripheral inflammation is also reduced by either high or low frequency TENS (Ma and Sluka, 2001). In animal models of neuropathic pain, either high or low frequency TENS reduces hyperalgesia that normally occurs in these models (Somers and Clemente, 1998; Nam et al., 2001). Similarly, the responsiveness of spinal neurons to innocuous mechanical stimulation is inhib-ited by TENS in neuropathic animals (Leem et al., 1995).
ANAlGESICMEChANISMSofTENS
highfrequency(50–100hz)TENS
In animals that were spinalized to remove descend-ing inhibitory pathways, inhibition of the tail flick by high frequency TENS still occurs but is reduced by about 50% (Woolf et al., 1980). Thus, these studies sug-gest that both spinal and descending inhibition are involved in the analgesia produced by high frequency TENS. A later study showed that high frequency TENS prevents the antihyperalgesia by blockade of -opioid
ANALGESiC mECHANiSmS of TENS 339
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receptors in the rostral ventral medial medulla (RVM) further supporting a role for supraspinal pathways in TENS analgesia (Kalra et al., 2001).
Pharmacologically, opioid peptides mediate the effects of high frequency TENS. Concentrations of -endorphins increase in the bloodstream and cere-brospinal fluid, and methionine–enkephalin in the cere-brospinal fluid were found in human subjects following administration of high frequency TENS (Salar et al., 1981; Han et al., 1991). Blockade of -opioid receptors in the spinal cord or the RVM reverses the antihyperalge-sia produced by high frequency TENS in animals with carrageenan induced knee joint inflammation (Sluka et al., 1999; Kalra et al., 2001). Repeated application of high frequency, motor intensity TENS produces tolerance (reduced effectiveness) to the antihyperal-gesic effects of TENS and at spinal -opioid receptors (Chandran and Sluka, 2003). In addition, the excita-tory neurotransmitters glutamate and substance P are decreased in the spinal dorsal horn by high frequency TENS (Sluka et al., 2005; Liu et al., 2007); this decrease in glutamate is mediated through activation of -opioid receptors (Sluka et al., 2005). Other neurotransmit-ters commonly involved in spinal inhibition are also involved in TENS inhibition: muscarinic receptors (M1, M3) in the spinal cord also prevent the antihyperalgesia produced by high frequency TENS (Radhakrishnan and
Sluka, 2003). There is also an increased release of GABA in response to high frequency TENS, and the antihy-peralgesia is reduced by blockade of GABAA receptors in the spinal cord (Maeda et al., 2007). However, block-ade of serotonin or noradrenergic receptors in the spi-nal cord has no effect on the reversal of hyperalgesia produced by high frequency TENS (Radhakrishnan et al., 2003). Thus a complicated neural circuitry is acti-vated in response to high frequency TENS that uti-lizes descending opioid inhibitory pathways to reduce excitability of dorsal horn neurons through decreasing release of glutamate and increasing release of GABA to result in reduction of nociception and consequently pain (see Figure 24.3).
TENS could have effects on autonomic function, blood flow, and peripheral afferent fibers (reviewed in Sluka and Walsh, 2003). The reported effects of high frequency TENS at different sensory or motor intensi-ties are mixed with some studies showing increases in blood flow, and others showing no change (Indergand and Morgan, 1994; Wikström et al., 1999; Cramp et al., 2000; Chen et al., 2007; Sandberg et al., 2007). The primary afferent neuropeptide, substance P, which is normally increased in injured animals is reduced in dorsal root ganglia neurons by high frequency, sen-sory intensity TENS in animals injected with the inflammatory irritant, formalin (Rokugo et al., 2002).
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The antihyperalgesia produced by high frequency TENS in animals with joint inflammation is reduced in 2A-noradrenergic receptor knockout mice, and pre-vented by peripheral blockade of 2-noradrenergic receptors (but not by spinal or supraspinal blockade) (King et al., 2005). Thus, evidence is beginning to emerge that some of the analgesic effects of TENS may be mediated through actions on primary afferent fibers and modulation of autonomic activity.
lowfrequency(10hz)TENS
Low frequency TENS antihyperalgesia is prevented by blockade of -opioid receptors in the spinal cord or the RVM (Sluka et al., 1999; Kalra et al., 2001). Repeated application of TENS produces tolerance to the anti-hyperalgesic effects of TENS and of spinal -opioid receptors (Chandran and Sluka, 2003). The effects of low frequency, sensory intensity TENS is also reduced by blockade of GABAA, serotonin 5-HT2A and 5-HT3, and muscarinic M1 and M3 receptors in the spinal cord (Radhakrishnan and Sluka, 2003; Radhakrishnan et al., 2003; Maeda et al., 2007). Similarly, serotonin is released during low frequency TENS in animals with joint inflammation (Sluka et al., 2006). Taken together, these studies suggest that low frequency TENS utilizes classical descending inhibitory pathways which utilize opioid, GABA, serotonin and muscarinic receptors in the spinal cord to reduce dorsal horn neuron activity, nociception and the consequent pain (see Figure 24.3).
AutonomicEffectsoflowfrequencyTENS
The effect of low frequency intensity TENS on cold allodynia is reduced by administration of systemic phentolamine to block -adrenergic receptors (Nam et al., 2001). Using laser Doppler, blood flow increases during low frequency TENS applied over a periph-eral nerve or the trapezius muscle (Wikstrom et al., 1999; Cramp et al., 2000; Chen et al., 2007; Sandberg et al., 2007). Similarly, the antihyperalgesia produced by low frequency TENS in animals with joint inflamma-tion is reduced in 2A-noradrenergic receptor knock-out mice, and prevented by peripheral blockade of 2-noradrenergic receptors (but not by spinal or supraspinal blockade [King et al., 2005]). Transient increases in blood flow with low frequency, burst-mode (2 Hz) TENS were observed at the area of stimulation if intensity was 25% above the motor threshold, but not just below (sensory intensity) or just above motor threshold (Sherry et al., 2001). Thus, peripheral effects of TENS may involve changes in sympathetic activity uti-lizing local 2A-noradrenergic receptors.
TRANSlATIoNofMEChANISMSofTENSANAlGESIAToThEClINIC
Clinically, TENS will more than likely not be the only treatment the patient is receiving. TENS is a complemen-tary and adjunct treatment to control pain. Medically, the patient will more than likely be taking prescription medications such as nonsteroidal anti-inflammatories (NSAIDs), opioids (e.g. fentanyl, oxycodone, etc.), 2-adrenergic agonists (e.g. clonidine) and/or muscle relaxants (e.g. cyclobenzaprine).
The most common procedural interventions in phys-ical therapy are therapeutic exercise and functional training. Physical therapists that treat pain, particularly chronic pain, utilize a combination of exercise and func-tional training. Electrotherapeutic modalities, or TENS, are utilized by physical therapists as an adjunct to modulate and reduce pain, and the use of TENS in the absence of other interventions is not considered physi-cal therapy.
However, in some conditions and patients, pain lim-its the ability of a patient to perform an adequate exer-cise program. Once the pain is controlled, the patient should be better able to perform an active exercise program, activities of daily living or return to work. Understanding the mechanisms will better assist the clinician in the appropriate choice of pain control treat-ment. Parameters of stimulation can be based on the basic knowledge and use of a particular modality such as electrical stimulation can be utilized in a more edu-cated manner. Specific examples will be given below to address these issues.
Use of TENS (in combination with other therapies) will allow the patient to increase activity level, reduce hospital stay and improve function. Indeed, treatment with TENS increases joint function in patients with arthri-tis (Mannheimer et al., 1978; Mannheimer and Carlsson, 1979; Kumar and Redford, 1982; Abelson et al., 1983; Zizic et al., 1995). In patients with chronic low back pain, improvements in the physical and mental component summary of the SF-36 quality of life survey occurs with TENS (Ghoname et al., 1999). Postoperative TENS treat-ment in patients following thoracic surgery reduces recovery room stay and improves pulmonary function as measured by postoperative PO2, vital capacity, and functional residual capacity when compared to sham controls (Ali et al., 1981; Warfield et al., 1985; Rakel and Frantz, 2003). Thus, decreasing pain with TENS may increase function and allow the patient to tolerate other therapies and activities, resulting in an improved qual-ity of life.
One should be aware of the medication a person is taking and the effects of these medications on the
THE CLiNiCAL EffiCACy of TENS 341
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effects of TENS. By understanding the mechanisms of action of TENS, more appropriate treatment strategies can be tried. If a patient is taking opioids (currently those available activate -opioid receptors), high fre-quency TENS may be more appropriate. Repeated application of opioids produces tolerance to the opioid such that a higher dose is necessary to produce the same effect. This is based on the fact that low frequency TENS, but not high frequency, is ineffective if given in animals tolerant to morphine (Sluka et al., 2000). Clinically, Solomon et al. (1980) demonstrated that in patients who had taken enough opioids to become tol-erant to morphine, TENS was ineffective in reducing postoperative pain. Furthermore, daily treatment with either low frequency or high frequency TENS in ani-mals with knee joint inflammation produces tolerance to TENS and a cross tolerance to either spinally admin-istered - or -opioid agonists, respectively (Sluka and Chandran, 2002). Thus, TENS is ineffective if morphine tolerance is present and shows opioid tolerance with repeated use.
It might be possible to enhance the analgesic effects of TENS clinically if given in combination with certain agonists or antagonists. Either high or low frequency TENS is more effective in reducing primary hyperal-gesia if given in combination with acute administra-tion of morphine (Sluka, 2000) or clonidine (Sluka and Chandran, 2002). Synergism between -adrenergic ago-nists and opioid agonists (- and -) has been shown in pharmacological studies (Fairbanks, Nguyen et al., 2000; Fairbanks, Posthumus et al., 2000). Since low frequency TENS works by activation of -opioid receptors, this enhanced antihyperalgesia is probably a result of syn-ergistic interaction between 2-noradrenergic receptors and endogenous opioids. Use of TENS in combination with morphine or clonidine should reduce the dosage of morphine or clonidine necessary to reduce hyper-algesia and thus reduce side effects of morphine and increase analgesia. In fact, clinically, intake of opioids is reduced in patients using TENS (Rosenberg et al., 1978; Solomon et al., 1980; Smith et al., 1983; Wang et al., 1997; Ghoname et al., 1999). Further there is a reduction in nausea, dizziness, pruritis associated with morphine intake when taken in association with TENS (Wang et al., 1997).
Based on the known pharmacology presented above, one could hypothesize that selective serotonin norepine-phrine reuptake inhibitors would prolong the effects of low frequency TENS; combining NSAIDs with TENS could enhance the effectiveness of TENS, or patients taking ACE inhibitors for cardiac disease might have a reduced effectiveness of TENS.
Therefore, understanding the neurotransmitters and pathways involved in TENS antihyperalgesia could
help explain conflicting data with respect to the patient population studies and TENS. It will further assist the clinician in the treatment choice for a particular patient. The clinical use of TENS and further clinical outcome studies should be carefully evaluated with respect to the current medication of the patient. Combinations of com-monly administered pharmaceutical agents and TENS should be addressed in a clinical population.
ThEClINICAlEffICACyofTENS
Several non-analgesic applications of TENS have been reported including effects on circulation (e.g. soft tissue healing) and antiemetic effects (Burssens et al., 2005; Kabalak et al., 2005). However, TENS is most commonly used for the management of both acute and chronic pain.
Research on TENS for pain relief has suffered from a lack of rigorous randomized controlled trials (RCTs). Several Cochrane systematic reviews (see Table 24.1) have highlighted the common problems with research to date: small numbers of participants, heterogeneous study populations, and inconsistent or lack of details on TENS application. The majority of these Cochrane reviews have, not surprisingly, been inconclusive.
Carroll et al. (2000) published a systematic review on the application of TENS for chronic pain; conditions included arthritis, low back pain, myofascial pain, and diabetic neuropathy. The authors highlighted the inad-equacy of the level of reporting in the included trials which obviously renders replication impossible. They also referred to inadequate treatment durations in the majority of the studies reviewed. A more recent RCT on TENS for chronic low back pain in people with mul-tiple sclerosis compared self-applied low frequency, high frequency, and placebo TENS (Warke et al., 2006). In contrast to previous studies, patients were instructed to apply TENS at least twice daily, for 45 minutes, and at any time a painful episode occurred over a six week time period. Changes in VAS from baseline of greater than 20 mm were interpreted as clinically important. Results showed that high frequency TENS (110 Hz) was more effective for pain relief during the 6-week period whereas low frequency TENS (4 Hz) showed a more sustained effect at the 32-week follow up; pla-cebo effects were also observed during this trial.
Although this modality is viewed primarily as an intervention for chronic pain, it is also used for acute pain conditions such as low back pain, labor pain, and postoperative pain (Carroll et al., 1997; Bertalanffy et al., 2005). TENS for labor pain is applied via two pairs of electrodes placed over the T10–L1 and S2–S4 spinal
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nerve roots to target afferent fibers coming from the uterus, cervix, and perineum. Conventional TENS is applied during contractions and Acupuncture-like TENS is applied between contractions. Early research studies on this topic demonstrated high levels of consumer sat-isfaction with TENS as it offers patients an active role in pain management (Bortoluzzi, 1989). However, Carroll et al.’s (1997) systematic review concluded that there was no significant effect of TENS on labor pain.
Clinical trials of TENS for postoperative pain have used the incision site (i.e. painful area) and correspond-ing spinal nerve roots as electrode placement sites. A recent meta-analysis of the studies published on TENS for postoperative pain (Bjordal et al., 2003) high-lighted the need to interpret the results of systematic reviews with a degree of caution. Bjordal and colleagues only included those studies that used what they termed “optimal” stimulation parameters whereas a previous systematic review by Carroll et al. (1996) did not impose this as an inclusion criterion. Carroll et al. concluded the majority of RCTs showed no benefit whereas the meta-analysis concluded that TENS can significantly reduce analgesic consumption for postoperative pain.
From the current literature, it can be concluded that further evidence is required on the efficacy, parameter- specific effects, and indeed cost-effectiveness of TENS. Optimal stimulation parameters and treatment dura-tions should be considered while interpreting the out-come of systematic reviews on TENS.
SuMMARyPoINTS
TENS is a safe, non-invasive modality widely used in clinical practice.
TENS can be used to treat both acute and chronic pain.
The clinical application of TENS involves a degree of trial and error in determining the most appropriate stimulation parameters and electrode placement sites.
Low frequency and high frequency TENS produce analgesia through different mechanisms that primarily involve central inhibitory mechanisms.
Systematic reviews have highlighted several deficiencies in TENS clinical trials including small numbers of participants, heterogeneous populations, and lack of details on TENS parameters.
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Bjordal, J.M., Johnson, M.I. and Ljunggreen, A.E. (2003) Transcutaneous electrical nerve stimulation (TENS) can reduce postoperative anal-gesic consumption. A meta-analysis with assessment of optimal treatment parameters for postoperative pain. Eur. J. Pain 7: 181–8.
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TAblE24.1 Summary of Cochrane Systematic Reviews on TENS for pain management
Authors Condition Number of studies meeting inclusion criteria
Outcome
Khadilkar et al., 2005 Chronic low back pain 2 Evidence is limited and inconsistent
Brosseau et al., 2003 Rheumatoid arthritis of the hand
3 Acupuncture-like TENS helps decrease hand pain in people with rheumatoid arthritis
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