The International Association for the Study of Pain (IASP) defines conventional TENS as high frequency (50–100 Hz), low intensity (paraesthesia, not painful), small pulse width (50–200 μs) (Charlton 2005). Conventional TENS is used to activate low-threshold, large diameter myelinated afferent fibres (Aβ) normally transmitting information related to non-painful touch and pressure (Fig. 12.3). This inhibits onward transmission of nociceptive information at synapses in the central nervous system (see Mechanism of Action). Patients are instructed to increase TENS pulse amplitude until a strong, comfortable, non-painful paraesthesia is experienced beneath the electrodes, indicating large diameter myelinated afferent fibre activity. A painful TENS paraesthesia beneath the electrodes is not appropriate. Theoretically, high-frequency (∼10–200 pulses per second (pps)) currents are optimal because they generate a large afferent barrage leading to greater inhibition of nociceptive transmission. Pulse durations between 50 and 200 μs allow optimal precision in achieving the desired intensity when titrating pulse amplitude.

Figure 12.3 The physiological intention of conventional TENS

Arrows indicate direction of TENS-induced nerve impulses; PNS = peripheral nervous system; CNS = central nervous system.

AL-TENS was developed to harness the mechanisms of action of TENS and acupuncture by activating segmental and extrasegmental mechanisms (descending pain inhibitory pathways) (Eriksson & Sjölund 1976). IASP define AL-TENS as a form of hyperstimulation achieved using currents that are low frequency (2–4 Hz), higher intensity (to tolerance threshold), and longer pulse width (100–400 μs) (Charlton 2005). Intermittent trains or bursts (2–4 Hz) of high-frequency pulses (100–200 pps) are often used in clinical practice to reduce discomfort experienced using high-intensity single pulses. The intention of AL-TENS is to stimulate small diameter, higher threshold afferents (Aδ) using high-intensity, low-frequency TENS. Research suggests that small muscle afferents produce greatest analgesia so some practitioners administer AL-TENS to generate non-painful muscle twitches which indirectly generates impulses in small diameter muscle afferents (Fig. 12.4). Electrodes are positioned at the site of pain, over myotomes, muscles, acupuncture points, and trigger points. AL-TENS is used to treat patients who are resistant to conventional TENS and patients are advised to administer it less frequently than conventional TENS, e.g. 20 minutes, 3 times a day (Eriksson & Sjölund 1976). AL-TENS can also be used for muscle and visceral pain arising from deep-seated structures, radiating neuropathic pain, and in situations where prolonged analgesia is required (Johnson 1998).

Figure 12.4 The physiological intention of acupuncture-like TENS.

Arrows indicate direction of TENS-induced nerve impulses; PNS = peripheral nervous system; CNS = central nervous system.

Intense TENS is a counterirritant and is delivered for short periods of time over nerve bundles close to the site of pain. High-frequency (up to 200 pps), high-intensity currents that are painful but tolerable are used. The intention of intense TENS is to stimulate small diameter, higher threshold cutaneous afferents (Aδ) to block transmission of nociceptive information in peripheral nerves (Fig. 12.5). Intense TENS activates diffuse noxious inhibitory controls (Le Bars et al 1979), and can be used for minor procedures such as wound dressing and suture removal.

Figure 12.5 Intense TENS

Arrows indicate direction of TENS-induced nerve impulses and direction of nerve impulses arising from damaged tissue

Manufacturers list cardiac pacemakers, epilepsy, and pregnancy as contraindications because it may be difficult to exclude TENS as a potential cause from a medico-legal perspective. The Chartered Society for Physiotherapy (CSP) suggest that TENS can be used in pregnancy and in epilepsy providing electrodes are placed well away from the abdomen, sacrum, and neck respectively (i.e. local contraindication) (CSP 2006). The CSP also lists bleeding tissue as a contraindication and suggests that TENS should not be delivered over active epiphysis or over an active, treatable tumour.

TENS should not be administered over the anterior neck, eyes, and testes or through the chest using anterior and posterior positions. TENS may interfere with foetal and cardiac monitoring equipment and should not be administered close to transdermal drug delivery systems. There is no known evidence that adverse events occur when TENS is used with metal implants, stents, percutaneous central catheters, or drainage systems. It should not be used while driving and should only be given internally using devices designed for that purpose (e.g. incontinence or dental analgesia). TENS devices with timers that automatically switch off are useful to aid sleep and may be used by children with success (Lander & Fowler-Kerry 1993; Merkel et al 1999).

Serious adverse events from TENS occur but are extremely rare (Mann 1996; Rosted 2001). It has been known to exacerbate pain and occasionally causes nausea and light-headedness, but retains an excellent safety and toxicity profile. No major drug interactions occur; therefore it can be combined with analgesics to reduce dosage and drug-related side effects. It has been claimed that caffeine may inhibit TENS effects (Marchand et al 1995).

TENS is potentially useful for any type of pain including that of nociceptive, neuropathic, and musculoskeletal origins (Table 12.2). Clinical experience suggests it provides long-term benefit for low back pain (LBP), osteoarthritis (OA), localized muscle pain, and neuropathic pains of peripheral origin such as postherpetic and trigeminal neuralgias, amputee pain, entrapment neuropathies, and radiculopathies (Barlas & Lundeberg 2006). TENS may also benefit metastatic bone disease, nerve compression by a neoplasm, and post-mastectomy and post-thoracotomy pains (Berkovitch & Waller 2005).

Table 12.2 Clinical Indications

TENS is ideal when treatment needs to be dynamic as effects are usually rapid in onset and offset, and are maximal during stimulation. Electrodes are left in situ and TENS may be administered intermittently throughout the day on an as-needed basis. Patients can leave TENS switched on for long periods of time and should increase intensity for breakthrough or incident pain. It should be administered before pain becomes moderate or severe but skin hygiene is essential as minor skin irritation under electrodes may occur.

TENS should be delivered over healthy sensate skin; therefore skin sensitivity testing should be undertaken at the site of electrode placement. Electrodes are positioned at dermatomes related to the site of pain for conventional TENS. As TENS activates nerve fibres directly beneath the electrodes the primary site for electrodes is around the site of pain (Fig. 12.6), or positioned paravertebrally at the appropriate spinal segment or on contralateral dermatomes. If it is not possible to site electrodes close to the pain because of hypersensitivity or skin damage (e.g. open wound, eczema), then electrodes should be positioned on nerves proximal to the pain. TENS may aggravate pain if electrodes are placed on skin with tactile allodynia.

Figure 12.6 Common sites for positioning electrodes during TENS

The use of TENS to supplement acupuncture analgesia over specific points, such as trigger and acupuncture points, is done sparingly within clinical application. A common misconception is that AL-TENS must be delivered at acupuncture points, which is not the case, but it may be effective. A review of research on TENS and acupuncture points concluded that it may be useful when given over acupuncture points but there were few studies that compared TENS at acupuncture points versus TENS at the site of pain (Walsh 1996).

Transcutaneous electrical acupoint stimulators (TEAS) are watch-like devices worn on the underside of the wrist over the Pericardium 6 (P6) acupuncture point (Fig. 12.6). Good quality randomized controlled trials (RCTs) have found that TEAS reduced postoperative and chemotherapy-induced nausea and vomiting (Coloma et al 2002; Zarate et al 2001).

The key determinant of TENS outcome is titration of the pulse amplitude to activate different nerve fibres (Table 12.1). For conventional TENS the user should titrate pulse amplitude to produce a strong, comfortable, non-painful paraesthesia beneath the electrodes. Practitioners should be cautious of claims about the best pulse frequencies, durations, and patterns for different pain conditions. A systematic review of studies investigating the effects of different pulse frequencies on experimental pain in healthy humans concluded that research to find optimal TENS settings for different conditions is confusing (Chen et al 2008) suggesting that the parameters may influence subjective comfort of paraesthesia rather than having clinically meaningful effects on TENS outcome (Johnson et al 1991a, b). For this reason, pulse frequency, pattern, and duration are selected by trial and error according to ‘personal comfort’ for the pain at that time. Patients are encouraged to experiment with settings within and between treatments whilst maintaining a strong but comfortable intensity.

TENS causes antridromic activation of peripheral nerves so that impulses travelling away from the central nervous system will collide and extinguish afferent impulses arising from peripheral receptors. This may lead to peripheral blockade of impulses arising from tissue damage (Fig. 12.5).

Animal studies show that conventional TENS inhibits central transmission of nociceptive information in the spinal cord when applied to somatic receptive fields (Garrison & Foreman 1994, 1996; Leem et al 1995). The inhibitory neurotransmitter gamma-amino butyric acid (GABA) appears to be critical for conventional TENS effects (Duggan & Foong 1985; Maeda et al 2007). It has also been shown to reduce inflammation-induced sensitization of dorsal horn neurons in anaesthetized rats (Ma & Sluka 2001).

Higher intensities, e.g. AL-TENS, act via extrasegmental mechanisms and activate structures on the descending pain inhibitory pathways (e.g. periaqueductal grey and ventromedial medulla) and inhibit structures on descending pain facilitatory pathways (Ainsworth et al 2006; Chung et al 1984a, b). Higher intensities cause long-term depression of central nociceptor cells for up to 2 hours post stimulation (Sandkühler et al 1997, 2000). Activation of deep tissue peripheral afferents appears to produce largest effects (Duranti et al 1988; Radhakrishnan & Sluka 2005). Brief, intense, painful TENS probably elicits counterirritant mechanisms via diffuse noxious inhibitory controls (Le Bars et al 1979).

Recent research has shown low-frequency TENS to involve mu opioid receptors and high-frequency TENS to involve delta opioid receptors (Kalra et al 2001; Sluka et al 1999, 2000). Cholinergic, adrenergic, and serotinergic systems also seem to be involved (King et al 2005; Radhakrishnan et al 2003; Sluka & Chandran 2002).