Surveys show that TENS is one of the most frequently used electrotherapies for pain relief and that it is available throughout the world (Johnson 1997, Pope et al 1995, Reeve et al 1996, Robertson & Spurritt 1998). TENS is popular because it is cheap, non-invasive, easy to administer, has few side effects and has no drug interactions. There is no potential for toxicity or overdose. TENS is commonly prescribed by doctors, physiotherapists, nurses and midwives; patients in the UK can also buy a TENS device for their own use. It is preferable for patients to administer TENS for themselves, following appropriate instruction by a healthcare professional. TENS effects are rapid in onset for most patients, so benefit can be achieved almost immediately.

During TENS, pulsed electrical currents are generated by a portable pulse generator and delivered across the intact surface of the skin via conducting pads called electrodes (Fig. 16.1). The conventional way of administering TENS is to use electrical characteristics that selectively activate large-diameter non-noxious afferents (Aβ) without activating smaller-diameter nociceptive fibres (Aδ and C). Evidence suggests that this will produce pain relief in a similar way to ‘rubbing the pain better’. In practice, conventional TENS is delivered to generate a strong but comfortable paraesthesia within, or close to, the site of pain using frequencies anywhere between 1 and 200 pulses per second (pps) and pulse durations anywhere between 50 and 500μs.

Figure 16.1 A standard device delivering TENS to the arm. There is increasing use of self-adhesive electrodes rather than black carbon-rubber electrodes that require conductive gel and tape as shown in the diagram.

HISTORY

The use of electricity to relieve pain is an age-old technique. The ancient Egyptians used electrogenic fish to treat ailments in 2500 bc, although the Roman physician Scribonius Largus is credited with the first written report of the use of electrogenic fish in medicine in AD46 (Kane & Taub 1975). The development of electrostatic generators in the eighteenth century increased the use of medical electricity, although popularity declined in the nineteenth and early twentieth centuries owing to variable clinical results and the development of pharmacological treatments (Stillings 1975). Interest in the use of electricity to relieve pain was re-awakened in 1965 by Melzack and Wall, who provided a physiological rationale for electroanalgesic effects. They proposed that transmission of noxious information could be inhibited by activity in large-diameter peripheral afferents or by activity in pain inhibitory pathways descending from the brain (Fig. 16.2). Wall and Sweet (1967) used high-frequency percutaneous electrical stimulation to artificially activate large-diameter peripheral afferents and found that this relieved chronic pain in patients. Pain relief was also demonstrated when electrical currents were used to stimulate the periaqueductal grey (PAG) region of the midbrain (Reynolds 1969), which is part of the descending pain inhibitory pathway. Shealy et al (1967) found that electrical stimulation of the dorsal columns, which form the central transmission pathway of large-diameter peripheral afferents, also produced pain relief. TENS was used to predict the success of dorsal column stimulation implants until it was realised that TENS could be used as a successful modality on its own (Long 1973, 1974).

Figure 16.2 The ‘pain gate’. Under normal physiological circumstances, the brain generates pain sensations by processing incoming noxious information arising from stimuli such as tissue damage. If noxious information is to reach the brain it must pass through a metaphorical ‘pain gate’, which is located in lower levels of the central nervous system. In physiological terms, the gate is formed by excitatory and inhibitory synapses regulating the flow of neural information through the central nervous system. This ‘pain gate’ is opened by noxious events in the periphery; it can be closed by activation of mechanoreceptors through ‘rubbing the skin’. This generates activity in large-diameter Aβ afferents, which inhibits the onward transmission of noxious information. This closing of the ‘pain gate’ results in less noxious information reaching the brain, which results in a reduction in the sensation of pain. The neuronal circuitry involved is segmental in its organization. The aim of conventional TENS is to activate Aβ fibres using electrical currents. The ‘pain gate’ can also be closed by the activation of pain inhibitory pathways, which originate in the brain and descend to the spinal cord through the brain stem (extrasegmental circuitry). These pathways become active during psychological activities, such as motivation, and when small-diameter peripheral fibres are excited physiologically. The aim of AL-TENS is to excite small-diameter peripheral fibres to activate the descending pain inhibitory pathways.

DEFINITION

By strict definition, TENS is anything that delivers electricity across the intact surface of the skin to activate underlying nerves. Healthcare professionals use the term ‘TENS’ to describe a ‘standard TENS device’ (see Fig. 16.1), although literature about TENS uses ambiguous and inconsistent nomenclature. This has resulted in inappropriate clinical practice.

THE STANDARD TENS DEVICE

Standard TENS devices are characterized by their technical output characteristics, and in the UK they retail at between £30 and £150 (Table 16.1). Standard TENS devices vary between manufacturers but these variations are minor and have a limited impact on the physiological effects produced. Standard TENS devices usually deliver biphasic pulsed currents in a repetitive manner using pulse durations of between 50 and 500 μs and pulse frequencies of between 1 and 200 pulses per second (pps) [American Physical Therapy Association (APTA) 1990, Barlas & Lundeberg 2006, Mannheimer & Lampe 1987, Sjölund et al 1990, Walsh 1997a, Woolf & Thompson 1994]. Pulses are usually delivered in a continuous pattern, although most modern devices have other patterns available, such as burst, modulated amplitude, modulated frequency, and modulated pulse duration (Fig. 16.3).

Table 16.1 Typical features on TENS devices

Feature	Examples
Weight	50-250g
Dimensions	6 × 5 × 2 em (small device) 12 × 9 × 4cm (large device)
Cost	£30-150
Pulse waveform (fixed)	Monophasic Symmetrical biphasic Asymmetrical biphasic
Pulse amplitude (adjustable)	1-50mAintoa 1 kO,load
Pulse duration (often fixed)	50-500 μ5
Pulse frequency (adjustable)	1-200pps
Pulse pattern	Continuous, burst (random frequency, modulated amplitude, modulated frequency, modulated pulse duration)
Channels	1 or 2
Batteries	PP3 (9V), rechargeable
Additional features	Timer Most devices deliver constant current output

Figure 16.3 The output characteristics of a standard TENS device (topographic view, each vertical line represents one pulse). The intensity control dial (I) regulates the current amplitude of individual pulses; the frequency control dial (F) regulates the rate of pulse delivery (pulses per second; pps) and the pulse duration control dial (D) regulates the time duration of each pulse. Most TENS devices offer patterns of pulse delivery such as burst, continuous and amplitude modulated.

TENS-LIKE DEVICES

Developments in electronic technology have flooded the market with a variety of TENS-like devices (Table 16.2). TENS-like devices have been defined as any stimulating device that delivers electrical currents across the intact surface of the skin and whose technical output specifications and generic name differ from those of TENS (Johnson 2003). This includes interferential current therapy (IFT), microcurrent electrical therapy (MET), high-voltage pulsed (galvanic) current (HVPC), transcutaneous spinal electroanalgesia (TSE), action potential simulation (APS), H-wave therapy (HWT), Pain^®Gone and transcranial electrical stimulation (TCES), to name but a few. TENS-like devices often have design features that are unique to a particular device and this makes categorization difficult. For example, some MET devices deliver currents at the site of pain using standard TENS electrodes; others deliver MET transcranially (e.g. as a type of TCES), and others deliver MET using a pen-like electrode. Claims about the relative effectiveness of TENS-like devices are overambitious and practitioners should use a standard TENS device in the first instance (Johnson 2003). The remainder of this chapter will focus on TENS administered using a standard TENS device.

Table 16.2 Characteristics of some of the commercially available TENS-like devices

PHYSICAL PRINCIPLES

TENS is a technique-based intervention. Hence, outcome is dependent on the appropriateness of the TENS procedure, which is determined by the user. Users need to select the appropriate location to stimulate, type and number of electrodes, electrical characteristics of currents (i.e. pulse amplitude, frequency, pattern and duration) and dosing regimens (i.e. how often and for how long). To do this the TENS user needs to consider the physiological intention of the TENS treatment.

The physiological intention of TENS when used for pain relief is to selectively activate different populations of nerve fibres to initiate antinociceptive mechanisms that produce clinically meaningful pain relief. The electrical characteristics of TENS determines which population of nerve fibres is activated, so it is important that the TENS user is familiar with the basic principles of nerve fibre activation and how this relates to TENS technique (this is described in Chapter 13).

PRINCIPLES OF NERVE FIBRE ACTIVATION

When the pulse amplitude (intensity) of TENS is increased, non-nociceptive nerve fibres are activated first and the user experiences a non-painful tingling under the electrodes. This is because large-diameter, non-nociceptive (touch) nerve fibres (Aα and Aβ fibres) have lower thresholds of activation to electrical stimuli than their small-diameter nociceptive counterparts (Aδ and C fibres; see Chapters 5 and 6). If the pulse amplitude (intensity) of TENS is increased further, nociceptive nerve fibres are activated and the user experiences a painful tingling under the electrodes.

Pulse durations between 50 and 500 μs enable the user titrate pulse amplitude with greater precision (Howson 1978). Pulsed currents with small amplitudes (i.e. low intensity) and durations between 50 and 500 μs would be best to activate large-diameter fibres (Aβ) without activating small-diameter nociceptive fibres (Aδ and C) (Fig. 16.4). Increasing the pulse duration above 500 μs will lead to the activation of small-diameter fibres at lower pulse amplitudes. The number of nerve impulses generated by TENS will increase according to the frequency of the pulsed currents, although this is limited by the absolute and relative refractory periods for the axon. Theoretically, delivering TENS at high pulse frequencies (up to ∼200 pps) should be optimum (for a discussion, see Howson 1978, Walsh 1997e, Woolf & Thompson 1994).

Figure 16.4 Strength–duration curve for fibre activation. As pulse duration increases, less current amplitude is needed to excite an axon to generate an action potential. Small pulse durations are unable to excite nerve axons even at high current amplitudes. Large-diameter axons require lower current amplitudes than small-diameter fibres. Thus, passing pulsed currents across the surface of the skin excites large-diameter non-noxious sensory nerves first (paraesthesia), followed by motor efferents (muscle contraction) and small-diameter noxious afferents (pain). Alteration of pulse duration is one means to help in the selective recruitment of different types of nerve fibre. For example, intense TENS should use long pulse durations (>50 μs)as they activate small-diameter afferents more readily. During conventional TENS pulse durations ∼100–200 μs are used as there is a large separation (difference) in the amplitude needed to recruit different types of fibre. This enables greater precision when using the Intensity (amplitude) dial so that a strong but comfortable paraesthesia can be achieved without muscle contraction or pain.

The type of pulsed current waveform used in TENS devices varies between manufacturers. Generally, these can be divided into monophasic or biphasic waveforms (Fig. 16.5). If the TENS device delivers a monophasic pulsed wave, the cathode should be placed proximal to the anode. This is because the cathode activates the axonal membrane, leading to an action potential, and placing the anode proximal could block nerve transmission due to hyperpolarization (Fig. 16.6, and see Chapter 13). The cathode is usually, but not always, the black lead. Many TENS devices use biphasic waveforms whereby the cathode and anode alternate between the two electrodes during stimulation. Biphasic waveforms, which result in zero net current flow, may prevent the build-up of ion concentrations beneath electrodes and reduce adverse skin reactions due to polar concentrations (Kantor et al 1994, Walsh 1997e). Electrode positioning with these biphasic waveforms is less critical given the zero net DC element and therefore orientation of positive/negative is not thought to make any significant clinical difference.

Figure 16.5 Common pulse waveforms used in TENS.

Figure 16.6 Fibre activation by TENS. When devices use waveforms that produce net DC outputs that are not zero, the cathode excites (depolarization) the axon and the nerve impulse will travel in both directions down the axon. The anode tends to inhibit the axon (hyperpolarization), which could extinguish the nerve impulse. Thus, during conventional TENS the cathode should be positioned proximal to the anode so that the nerve impulse is transmitted to the central nervous system unimpeded. However, during AL-TENS the cathode should be placed distally or over the motor point, as the purpose of AL-TENS currents is to activate a motor efferent. Table 16.3 The characteristics of different types of TENS

The theoretical relationship between output characteristics of electrical stimuli delivered and nerve fibre activation can break down in clinical practice. It has been shown that the exact nature and distribution of currents is difficult to predict when they are passed across the intact surface of the skin, due to the complex and non-homogeneous impedance of the tissue underlying electrodes (Demmink 1993). Furthermore, the skin offers high impedance at pulse frequencies used by TENS, so it is likely that currents will remain superficial, stimulating cutaneous nerve fibres rather than the more deep-seated visceral and muscle nerve fibres. Interestingly, however, recent evidence from animal studies suggests that activity in deeper afferents may be responsible for the antinociceptive effects of TENS (Radhakrishnan & Sluka 2005).

PRINCIPLES UNDERPINNING TYPES OF TENS TECHNIQUE

The most common types of TENS technique described in the literature are conventional TENS, acupuncture-like TENS (AL-TENS) and intense TENS (Table 16.3; Barlas & Lundeberg 2006, Mannheimer & Lampe 1987, Sjölund et al 1990, Walsh 1997a, Woolf & Thompson 1994). These TENS techniques suggest the electrical characteristics of the currents that are necessary to activate different types of nerve fibre, although the emphasis on electrical characteristics has resulted in the use of inflexible ‘prescriptive protocols’ in clinical practice and research. In addition, the emphasis on the electrical characteristics of TENS has driven the development of TENS devices with novel output characteristics (Fig. 16.7), which are based more on technological advances in electronics than on proven physiological effect. In clinical practice in the UK, conventional TENS is most commonly used by patients whereas AL-TENS and intense TENS tend to be used only in specific situations. It is important to establish the physiological intention of each TENS technique to appreciate their relative merits.

Table 16.3 The characteristics of different types of TENS

Figure 16.7 Novel pulse patterns available on TENS devices. Modulated patterns fluctuate between upper and lower limits over a fixed period of time; this is usually preset in the design of the TENS device.

CONVENTIONAL TENS

The physiological intention of conventional TENS is to selectively activate large-diameter Aβ fibres (touch related) without concurrently activating small-diameter Aδ and C fibres (pain-related) or muscle efferents (Fig. 16.8). Theoretically, high-frequency (∼10–200 pps), low-intensity (non-painful) currents with pulse duration between 50 and 500 μs would be most efficient in selectively activating Aβ fibres (see Table 16.3). As large-diameter fibres have short refractory periods, they can generate nerve impulses at high frequencies. This means that they are more able to generate high-frequency volleys of nerve impulses when high-frequency currents are delivered, resulting in a greater afferent barrage into the central nervous system.

Figure 16.8 The aim of conventional TENS is to selectively activate Aβ afferents producing segmental analgesia.

Large-diameter afferent activity is occurring when the user reports ‘strong but comfortable’ non-painful electrical paraesthesia beneath the electrodes. Animal studies have demonstrated that TENS-induced large-diameter afferent activity inhibits ongoing transmission of nociceptive information in the spinal cord and reduces central sensitization (Garrison & Foreman 1994, 1996, Ma & Sluka 2001, Sandkühler 2000). This effect is often rapid in onset and offset. Studies on healthy human participants exposed to experimentally induced pain in laboratory settings demonstrate that TENS applied at a strong but comfortable level produces hypoalgesic effects over and above those seen with sham TENS (Johnson & Tabasam 1999, 2003, Walsh 1997d). Hence users should be trained to titrate pulse amplitude so that it is strong enough to generate a non-noxious paraesthesia (Aβ activity) without frank pain (Aδ or C fibre activity). It is claimed that the magnitude of analgesia achieved during conventional TENS depends in part on the pulse frequency. This is supported by studies in animals, which demonstrate that high- and low-frequency TENS may generate differential antihyperalgesic effects, which use different neurochemicals (Radhakrishnan & Sluka 2003, Radhakrishnan et al 2003, Sjölund 1985, Sluka et al 2000, 2005). However, the results of experimental studies in healthy human subjects and patients are inconsistent, with some demonstrating frequency-dependent effects (De Tommaso et al 2003, Johnson et al 1989, Walsh et al 1995) and others not (Barr et al 1986, Cramp et al 2000, Foster et al 1996, Walsh et al 2000).

ACUPUNCTURE-LIKE TENS (AL-TENS)

AL-TENS is often described – in vague terms – to be the delivery of low-frequency (e.g. <10 pps), high-intensity TENS. This has led to confusion about its physiological intention (Johnson 1998). Most, but not all, opinion leaders define AL-TENS as the induction of forceful but non-painful phasic muscle contractions at myotomes related to the origin of the pain (Eriksson & Sjölund 1976, Johnson 1998, Meyerson 1983, Sjölund et al 1990, Walsh 1997a). However, most research reports articulate AL-TENS as low-frequency, high-intensity TENS without any reference to the presence or absence of muscle contractions. Some commentators describe AL-TENS as the delivery of TENS over acupuncture points irrespective of muscle activity (Lewers et al 1989, Lewis et al 1990, Longobardi et al 1989, Rieb & Pomeranz 1992). It is possible that there are differences in physiological action when TENS is applied over acupuncture points in this way (see reviews by Johnson 1998, Walsh 1996).

The physiological intention of AL-TENS, as described by the opinion leaders who developed it, is to generate activity in small-diameter muscle afferents (Aδ or group III) that arise from ergoreceptors and respond to muscle contraction (Eriksson & Sjölund 1976, Sjölund et al 1990). This is achieved indirectly by activating Aα efferents to produce a forceful but non-painful phasic muscle twitch (Anderson et al 1973, Eriksson & Sjölund 1976, Eriksson et al 1979; Fig. 16.9). AL-TENS is administered over muscles or motor points at high but non-painful intensities using low-frequency pulses (∼1–10 pps) or low-frequency bursts of pulses (∼2–5 bursts per second of 100 pps). Burst patterns of pulse delivery were originally incorporated on TENS devices as they were found to be more comfortable than low-frequency single pulses in producing muscle twitches (Eriksson & Sjölund, 1976). The impulses generated in small-diameter muscle afferents initiate extrasegmental antinociceptive mechanisms and the release of endogenous opioid peptides in a manner similar to that suggested for acupuncture (Meyerson 1983, Sjölund et al 1977). It should be remembered that currents delivered during AL-TENS will also activate Aβ fibres during their passage through the skin leading to segmental analgesia (see Table 16.3).

Figure 16.9 The aim of AL-TENS is to selectively activate group I (GI/Aδ) efferents, producing a muscle contraction that results in activity in ergoreceptors and group III (GIII) afferents. GIII afferents are small in diameter and have been shown to produce extrasegmental analgesia through the activation of descending pain inhibitory pathways. Aβ afferents will also be activated during AL-TENS producing segmental analgesia. Note the position of the cathode.

INTENSE TENS

The physiological intention of intense TENS is to activate small-diameter Aδ afferents by delivering TENS over peripheral nerves arising form the site of pain at an intensity that is just tolerable to the patient (Jeans 1979, Melzack et al 1983; Fig. 16.10). Currents are administered at high frequencies (up to 200 pps) to prevent phasic muscle twitches that would be too forceful for the patient to tolerate (see Table 16.3). Cutaneous Aδ afferent activity has been shown to block transmission of nociceptive information in peripheral nerves and to activate extrasegmental antinociceptive mechanisms (Chung et al 1984a, 1984b, Ignelzi & Nyquist 1976, 1979, Woolf et al 1980). Intense TENS will also activate Aβ fibres, producing segmental antinociceptive effects. As intense TENS acts in part as a counterirritant, it can be delivered for only a short time; however, it may prove useful for minor surgical procedures such as wound dressing and suture removal (Mannheimer & Lampe 1987). 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.

Figure 16.10 The aim of intense-TENS is to selectively activate Aδ afferents leading to extrasegmental analgesia. Aβ afferents will also be activated, producing segmental analgesia.

PRINCIPLES UNDERLYING APPLICATION

The patient’s report of the sensation produced by TENS is the easiest means of assessing the type of fibre activity. A strong, non-painful electrical paraesthesia is mediated by large-diameter cutaneous afferents, as intended with conventional TENS; a painful electrical paraesthesia is mediated by small-diameter cutaneous afferents, as intended by intense TENS; a strong, non-painful phasic muscle contraction is likely to excite muscle ergoreceptors, as intended by AL-TENS. Patients should be trained to titrate the amplitude, frequency and duration of pulsed currents to produce the appropriate outcome. Before patients use TENS for the first time it is important to assess their pain and to instruct them on the safe use of TENS (Box 16.1).

CONTRAINDICATIONS AND PRECAUTIONS

There are very few reported cases of adverse events associated with TENS in the literature. Neverthe-less, patients need to be informed of potential hazards so that they can give valid consent to treatment. Decisions on whether to prescribe TENS to a patient are taken according to the professional judgement of the healthcare professional. In difficult cases it is always wise to seek the views of medical and allied health profession colleagues.

Contraindications

In clinical practice, healthcare practitioners have to assess the risk of using TENS against the risk associated with using other available treatments, including drug medication. TENS usually evaluates favourably. Contraindications to TENS are few and mostly hypothetical (Table 16.4). TENS manufacturers list cardiac pacemakers, pregnancy and epilepsy as contraindications. From a legal perspective it may be difficult to exclude TENS as a potential cause of a problem in these patient groups. However, it is possible to use TENS in these patients providing TENS is not applied locally and the patient’s progress is carefully monitored throughout (Watson 2006). Most importantly, the situation must been discussed with the patients physician.

Table 16.4

Contraindication	Advice
‘Undiagnosed pain’	Ensure patient has undergone assessment by a healthcare professional
Cardiac pacemakers	Seek approval from patient’s cardiologist Apply over chest with extreme caution; cardiologist should monitor progress frequently
Pregnancy	Seek approval from patient’s cardiologist, obstetrician and midwife Do not apply over anywhere over abdomen/pelvic region
Epilepsy	Seek approval from patient’s neurologist Do not apply on neck or head
Non-adherent patients	Assess patient for competency If necessary, seek approval from patient’s psychiatrist
Malignancy	Seek approval from patient’s palliative care specialist In general, do not apply over active malignancy
Cardiovascular problems	Seek approval from patient’s cardiologist Apply over chest with extreme caution; cardiologist should monitor progress frequently
Dermatological conditions or frail skin	Apply electrodes to healthy skin at appropriate dermatomes
Inappropriate electrode sites	Do not apply on anterior neck, around the eyes, overtestes, through the chest (i.e. anterior and posterior electrode positions) or over damaged skin or open wounds
Precaution	Advice
Dysaesthesia/hypoaesthesia	Assess skin sensation using sharp/blunt test prior to first TENS treatment. Apply TENS on skin with normal sensation so that patient reports presence of TENS paraesthesia. Carefully monitor to ensure TENS does not aggravate dysaesthesia
Allodynia/hyperalgesia	Test skin sensation prior to first TENS treatment. Apply TENS on skin with normal sensation in first instance as TENS may, but not always, exacerbate the pain
Contact dermatitis	Change type of electrodes – use hypoallergenic electrodes
Autonomic reactions to TENS	Supervise first TENS treatment and assess severity of symptoms with a view to withdrawing TENS
Operating hazardous equipment	Advise patients never to use TENS when driving or when using hazardous equipment

Cardiac pacemakers

Patients with cardiac pacemakers should not use TENS unless this has been discussed with their cardiologist. This is because the electrical field generated by TENS could interfere with implanted electrical devices. Rasmussen et al (1988) found that TENS did not interfere with pacemaker performance in 51 patients. However, Pyatt et al (2003) reported that TENS did interfere with pacemaker function in a patient who had received a biventricular ICD for malignant ventricular arrhythmias and medically refractory cardiac failure. The patient experienced bradycardia and dizziness. TENS has been reported to induce artefacts on monitoring equipment (Hauptman & Raza 1992, Sliwa & Marinko 1996). Chen et al (1990) reported two cases of a Holter monitor detecting interference with a cardiac pacemaker by TENS; in both instances the sensitivity of the pacemaker was reprogrammed to resolve the problem. Careful evaluation and extended cardiac monitoring should be performed when using TENS with pacemakers.

Pregnancy

There is much debate about whether TENS should be contraindicated in pregnancy and a recent review of the subject found no reports of deleterious effects of TENS on pregnancy (Crothers 2003). TENS should not be administered over the abdomen or pelvis during pregnancy because the effects of TENS on fetal development are still unknown. TENS should not be administered over a pregnant uterus because currents could inadvertently cause uterine contractions and induce premature labour. Potential hazards when using TENS away from these sites seem minimal, although it would be wise to regularly monitor progress. It is known that obstetric TENS interferes with fetal monitoring equipment (Bundsen & Ericson 1982).

Epilepsy

Practitioners should be cautious when giving TENS to patients with epilepsy as it may be difficult to exclude TENS as a potential cause of a seizure. Rosted (2001) reported a case of repetitive epileptic seizures in a post-stroke patient using TENS. Patients are more prone to epileptic seizures following a stroke, although in this case TENS seemed to trigger the repetitive seizures. TENS should be used with caution for post-stroke pain. Scherder et al (1999) reported increased frequency of seizures when TENS was used to improve memory and behaviour in a child with a severe psychomotor disorder and epilepsy. For these reasons, TENS should not be applied on the neck or head in patients with epilepsy.

Non-adherent patients

Patients who refuse TENS treatment, do not cooperate with instructions or do not comprehend instructions should not be given TENS. This may include patients with learning difficulties, mental illness or phobiasabout electricity. TENS can be used on childrenproviding they understand what to expect during TENS.

Malignancy

TENS should not be applied directly over an area where there is active malignancy, except in palliative care and under the supervision of a palliative care specialist. This is because electrical currents are known to promote cell growth in vitro, although there are no known reports of TENS increasing malignancy in clinical practice.

Cardiovascular problems

Therapists wishing to administer TENS to a patient with a history of cardiac disturbances, such as dysrhythmias, should always discuss the situation with a cardiologist. It is likely that TENS can be used in most cases provided it is to be applied away from the chest area. TENS is often used over the chest for angina, with much success (see the section Known efficacy: the clinical effectiveness of TENS). However, Mann (1996) reported a case of a patient who had inappropriately applied TENS electrodes on the anterior and posterior areas of the chest for unstable angina and severely compromised pulmonary ventilation owing to excessive stimulation of the intercostal muscles. The situation was resolved as soon as TENS was switched off. TENS should not be delivered through the chest using anterior and posterior electrode positions. In addition, TENS should not be applied over areas where there has been recent haemorrhage because the currents may cause further bleeding. TENS should never be applied over areas where there is ischaemic tissue and/or thrombosis, as there may be a potential for an embolism.

Dermatological conditions or frail skin

TENS electrodes should not be applied on areas of broken or damaged skin, such as open wounds, although they can be placed over healthy skin surrounding a wound. Electrodes should not be applied on frail skin (e.g. some elderly patients) or on conditions such as eczema, because they may cause skin damage on removal. TENS can be placed over healthy skin around, or proximal to, an area of eczema. TENS has been used successfully for pruritus (Hettrick et al 2004, Tang et al 1999, Tinegate & McLelland 2002, Ward et al 1996).

Inappropriate electrode sites

TENS should not be delivered over the anterior neck because currents may stimulate the carotid sinus leading to an acute hypotensive response via a vasovagal reflex. TENS at this site may also stimulate laryngeal nerves leading to a laryngeal spasm. TENS should not be delivered over the eyes because it may cause an increase in intraocular pressure. TENS should not be delivered through the chest using anterior and posterior electrode positions as it may affect the electrical conductivity of the heart and compromise breathing. TENS should not be administered internally except in specific circumstances and using TENS devices designed for dental, vaginal and anal stimulation. There are no known adverse effects of administering TENS on skin over or close to metal implants.

Precautions

Dysaesthesia (hyperaesthesia and hypoaesthesia)

If TENS is applied to skin with diminished sensation it is likely that it will be less effective because of the presence of nerve damage. Furthermore, the patient may be unaware that high-intensity currents are being administered, and this may result in skin irritation. If TENS is applied to skin with heightened sensation (e.g. allodynia and/or hyperalgesia) it may aggravate the pain, although this is not always the case. In both cases, electrodes should be positioned on healthy skin proximal to the site of pain, and for this reason practitioners should ensure that a patient has normal skin sensation prior to using TENS.

Contact dermatitis

Patients may experience minor skin irritation with TENS, such as reddening beneath or around the electrodes. This is often due to dermatitis at the site of contact with the electrodes resulting from the constituents of electrodes, electrode gel or adhesive tape (Corazza et al 1999, Meuleman et al 1996). The development of hypoallergenic electrodes has markedly reduced the incidence of contact dermatitis. Patients should be encouraged to wash the skin (and electrodes when indicated by the manufacturer) after TENS and to apply electrodes to fresh skin on a daily basis. Occasionally, patients may manifest severe contact dermatitis. There are anecdotal reports (not written up in the literature) of cases where TENS has caused an allergic response, with severe redness, blotching and irritation at local and distant sites to TENS. It is difficult to confirm the validity of such reports and whether symptoms are a direct result of TENS.