TENS Explained Chapter

17Transcutaneouselectrical nervestimulation (TENS)Mark Johnson

CHAPTER CONTENTS

Introduction 259

History 260

Definition 261

Physical principles 262Conventional TENS 266Acupuncture-like TENS (AL-TENS) 266Intense TENS 267Practical implications 267

Known biological effects 268Mechanisms of action 268Analgesic effects 271

Known efficacy: the clinical effectiveness ofTENS 271

TENS and acute pain 272TENS and chronic pain 275

Principles underlying application 277Electrode positions 277Electrical characteristics 277Timing and dosage 278Giving a patient a trial of TENS for thefirst time 278

Declining response to TENS 279

Hazards and contraindications 280Contraindications 280Hazards 281

Summary 282

INTRODUCTION

Transcutaneous electrical nerve stimulation(TENS) is a simple, non-invasive analgesictechnique that is used extensively in health-caresettings by physiotherapists, nurses and mid-wifes (Johnson, 1997; Pope, Mockett and Wright,1995; Reeve, Menon and Corabian, 1996;Robertson and Spurritt, 1998). It can be adminis-tered in the clinic by health-care professionals orat home by patients who have purchased aTENS device directly from manufacturers. TENSis mainly used for the symptomatic manage-ment of acute and non-malignant chronic pain(Box 17.1, Walsh, 1997a; Woolf and Thompson,1994). However, TENS is also used in palliativecare to manage pain caused by metastatic bonedisease and neoplasm (Thompson and Filshie,1993). It is also claimed that TENS hasantiemetic and tissue-healing effects although itis used less often for these actions (Box 17.1,Walsh, 1997b).

During TENS, pulsed currents are generatedby a portable pulse generator and deliveredacross the intact surface of the skin via conduct-ing pads called electrodes (Fig. 17.1). The con-ventional way of administering TENS is to useelectrical characteristics that selectively activatelarge diameter touch fibres (A) without acti-vating smaller diameter nociceptive fibres (Aand C). Evidence suggests that this will producepain relief in a similar way to rubbing the painbetter (see Mechanisms of action). In practice,conventional TENS is delivered to generate a

259

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strong but comfortable paraesthesia within thesite of pain using frequencies between 1 and 250pulses per second (p.p.s.) and pulse durationsbetween 50 and 1000 s.

In medicine, TENS is the most frequently usedelectrotherapy for producing pain relief. It ispopular because it is non-invasive, easy toadminister and has few side-effects or druginteractions. As there is no potential for toxicityor overdose, patients can administer TENSthemselves and titrate the dosage of treatment asrequired. TENS effects are rapid in onset formost patients so benefit can be achieved almostimmediately. TENS is cheap when comparedwith long-term drug therapy and some TENSdevices are available for less than 30.00.

HISTORYThere is evidence that ancient Egyptians usedelectrogenic fish to treat ailments in 2500BC,although the Roman Physician ScriboniusLargus is credited with the first documentedreport of the use of electrogenic fish in medicinein AD46 (Kane and Taub, 1975). The develop-ment of electrostatic generators in the eighteenthcentury increased the use of medical electricity,although its popularity declined in the nine-teenth and early twentieth century due to vari-able clinical results and the development ofalternative treatments (Stillings, 1975). Interestin the use of electricity to relieve pain wasreawakened in 1965 by Melzack and Wall (1965)who provided a physiological rationale for elec-troanalgesic effects. They proposed that trans-mission of noxious information could beinhibited by activity in large diameter peripheralafferents or by activity in pain-inhibitory path-ways descending from the brain (Fig. 17.2). Walland Sweet (1967) used high-frequency percuta-neous electrical stimulation to activate largediameter peripheral afferents artificially andfound that this relieved chronic pain in patients.Pain relief was also demonstrated when electri-cal currents were used to stimulate the periaque-ductal grey (PAG) region of the midbrain(Reynolds, 1969), which is part of the descend-ing pain-inhibitory pathway. Shealy, Mortimer

260 LOW-FREQUENCY CURRENTS

Figure 17.1 A standard device delivering TENS to the arm.There is increasing use of self-adhesive electrodes ratherthan black carbon-rubber electrodes that require conductivegel and tape as shown in the diagram.

Box 17.1 Common medical conditions that TENShas been used to treat

Analgesic effects of TENSRelief of acute pain Postoperative pain Labour pain Dysmenorrhoea Musculoskeletal pain Bone fractures Dental procedures

Relief of chronic pain Low back Arthritis Stump and phantom Postherpetic neuralgia Trigeminal neuralgia Causalgia Peripheral nerve injuries Angina pectoris Facial pain Metastatic bone pain

Non-analgesic effects of TENSAntiemetic effects Postoperative nausea associated with opioid

medication Nausea associated with chemotherapy Morning sickness Motion/travel sickness

Improving blood flow Reduction in ischaemia due to reconstructive surgery Reduction of symptoms associated with Raynauds

disease and diabetic neuropathy Improved healing of wounds and ulcers

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and Reswick (1967) found that electrical stimula-tion of the dorsal columns, which form thecentral transmission pathway of large diameterperipheral afferents, also produced pain relief.TENS was used to predict the success of dorsalcolumn stimulation implants until it was realisedthat it could be used as a successful modality onits own (Long, 1973, 1974).

DEFINITIONBy definition, any stimulating device whichdelivers electrical currents across the intactsurface of the skin is TENS, although the techni-cal characteristics of a standard TENS deviceare given in Table 17.1 and Figure 17.3. Develop-ments in electronic technology have meant that

TRANSCUTANEOUS ELECTRICAL NERVE STIMULATION (TENS) 261

Periphery

PAIN

Injury

Pain gate

Brain

Periphery Spinal cord Brain

Rubbing

Afibres

TENS

Pain gateclosed

A andC fibres

RubbingTingling

Tissuedamage

(segm

enta

l)Sp

inal i

nter

neur

ons

(extrase

gmen

tal)

Inhibitory

pathw

ays

Descend

ingpa

in

PAIN

Figure 17.2 The Pain Gate. A: Under normal physiological circumstances, the brain generates pain sensations by processingincoming noxious information arising from stimuli such as tissue damage. In order for noxious information to reach the brain itmust pass through a metaphorical pain gate located in lower levels of the central nervous system. In physiological terms, thegate is formed by excitatory and inhibitory synapses regulating the flow of neural information through the central nervoussystem. This pain gate is opened by noxious events in the periphery. B: The pain gate can be closed by activation ofmechanoreceptors through rubbing the skin. This generates activity in large diameter A afferents, which inhibits the onwardtransmission of noxious information. This closing of the pain gate results in less noxious information reaching the brain reduc-ing the sensation of pain. The neuronal circuitry involved is segmental in its organisation. The aim of conventional TENS is toactivate A fibres using electrical currents. The pain gate can also be closed by the activation of pain-inhibitory pathways whichoriginate in the brain and descend to the spinal cord through the brainstem (extrasegmental circuitry). These pathways becomeactive during psychological activities such as motivation and when small diameter peripheral fibres (A) are excited physiologi-cally. The aim of AL-TENS is to excite small diameter peripheral fibres to activate the descending pain-inhibitory pathways.

Table 17.1 Typical features of TENS devices

Weight dimensions 50 250 g 6 5 2 cm (small device) 12 9 4 cm (large device)

Cost 30150Pulse waveform (fixed) Monophasic

Symmetrical biphasicAsymmetrical biphasic

Pulse amplitude (adjustable) 150 mA into a 1 k loadPulse duration (often fixed) 101000 s Pulse frequency (adjustable) 1250 p.p.s.Pulse pattern Continuous, burst (random

frequency, modulated amplitude, modulated frequency, modulated pulse duration)

Channels 1 or 2 Batteries PP3 (9 V), rechargeableAdditional features Timer

Most devices deliver constant current output

A

B

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a variety of TENS-like devices are available onthe market (Table 17.2). However, the clinicaleffectiveness of these TENS-like devices is notknown owing to a lack of randomised controlledclinical trials (RCTs). Unfortunately, the increas-ing number of TENS-like devices has created aliterature littered with inconsistent and ambigu-ous terminology and this has led to confusion innomenclature. Nevertheless, the main types ofTENS described in the literature are conven-tional TENS, acupuncture-like TENS (AL-TENS)and intense TENS (Table 17.3, Walsh, 1997c;Woolf and Thompson, 1994). At present, conven-tional TENS remains the most commonly usedmethod for delivering currents in clinical prac-tice (Johnson, Ashton and Thompson 1991a).

PHYSICAL PRINCIPLESThe electrical characteristics of TENS are chosenwith a view to selectively activate differentpopulations of nerve fibres as this is believed to

produce different analgesic outcomes (Table 17.3).A standard TENS device provides a range ofpossible ways that TENS currents could bedelivered so it is important to review the princi-ples of nerve fibre activation (Fig. 17.3). Largediameter nerve fibres such as A and A havelow thresholds of activation to electrical stimuliwhen compared with their small diametercounterparts (A and C). The current amplitudeneeded to excite a nerve fibre declines withincreasing pulse duration and increasing pulsefrequency. Pulse durations of 101000 s providethe greatest separation (and sensitivity) of pulseamplitudes required to selectively activate largediameter afferents, small diameter afferents andmotor efferents (Fig. 17.4, Howson, 1978). Thus,to activate large diameter fibres (A) withoutactivating smaller diameter nociceptive fibres(A and C) one would select low-intensity, high-frequency (10250 p.p.s.) currents with pulsedurations between 10 and 1000 s (see Howson,1978; Walsh, 1997d; Woolf and Thompson, 1994


Frequency Pattern

High (250pps) Low (1pps)

High

Low

Amplitude Duration

LongShort

Burst

Continuous

Amplitude modulated

B

C

M

12

3

456

7

8

910

I

12

3

456

7

8

910

F

12

3

456

7

8

910

DOn

Off

Figure 17.3 Schematic diagram of the output characteristics of a standard TENS device (topographic view, each vertical linerepresents one pulse). The intensity control dial (I) regulates the current amplitude of individual pulses, the frequency controldial (F) regulates the rate of pulse delivery (pulses per second p.p.s.) and the pulse duration control dial (D) regulatesthe time duration of each pulse. Most TENS devices offer alternative patterns of pulse delivery such as burst, continuous andamplitude modulated.

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for discussion). Increasing the pulse durationwill lead to the activation of small diameterfibres at lower pulse amplitudes.

In practice, it is difficult to predict the exactnature and distribution of currents when theyare passed across the intact surface of the skindue to the complex and non-homogeneousimpedance of the tissue. However, as the skinoffers high impedance at pulse frequencies usedby TENS it is likely that currents will remainsuperficial stimulating cutaneous nerve fibresrather than deep-seated visceral and musclenerve fibres. Moreover, different TENS devicesuse a variety of pulse waveforms. Generally,these can be divided into monophasic andbiphasic waveforms (Fig. 17.5). It is the cathode

(usually the black lead) that excites the axon soin practice the cathode is placed proximal to theanode to prevent the blockade of nerve trans-mission due to hyperpolarisation (Fig. 17.6).Devices which use biphasic waveforms withzero net current flow will alternate the cathodeand anode between the two electrodes. Zero netcurrent flow may prevent the build-up of ionconcentrations beneath electrodes, preventingadverse skin reactions due to polar concentra-tions (Kantor, Alon and Ho, 1994; Walsh, 1997d).

The introduction of novel features on devices,such as modulated amplitude, modulated fre-quency and modulated duration (Fig. 17.7),enable manufacturers to gain a competitive edgein the market-place but are rarely supported by


Table 17.2 Characteristics of TENS-like devices

Device Experimental Manufacturers claim Typical stimulating characteristicswork

Action potential Odendaal and Pain relief Monophasic square pulse with exponential decaysimulation (APS) Joubert (1999) Improve mobility Delivered by two electrodes

Improve circulation Pulse amplitude low (< 25 mA), duration longReduce inflammation (800 s6.6 ms), frequency fixed at 150 p.p.s

Codetron Pomeranz and Pain relief Square waveNiznick (1987) Reduce habituation Delivered randomly to one of six electrodes Fargas-Babjak Pulse amplitude low, duration long (1 ms), et al. (1989; frequency low (2 p.p.s.)1992).

H wave McDowell et al. Pain relief 'Unique' biphasic wave with exponential decaystimulation (1995; 1999) Improve mobility Delivered by two electrodes

Improve circulation Pulse amplitude low (< 10 mA), duration long Reduce inflammation (fixed at 16 ms), frequency low (260 p.p.s.)Promote wound healing

Interference See Chapter 18 Pain relief Two out-of-phase currents which interfere currents Improve mobility with each other to produce an amplitude-modulated wave

Improve circulation Traditionally, delivered by four electrodes Reduce inflammation Pulse amplitude low, amplitude-modulated frequency Promote wound healing 1200 Hz (carrier wave frequencies approximately Muscle re-education 24 kHz)

Microcurrent Johannsen Promote wound Modified square direct current with monophasic or et al. (1993) healing biphasic pulses changing polarity at regular intervalsJohnson et al. (0.4 s)(1997) Pain relief Delivered by two electrodes

Other indications Pulse amplitude low (1600 A with no paraesthesia),often claimed frequency depends on manufacturer (15000 p.p.s.)

Many variants exist (e.g. transcranial stimulation formigraine and insomnia)

Transcutaneous Macdonald and Pain relief, Differentiated wavespinal Coates (1995) especially allodynia Delivered by two electrodes positioned electroanalgesia and hyperalgesia on spinal cord at T1 and T12 or straddling C3C5(TSE) due to central Pulse amplitude high (although no paraesthesia),

sensitisation duration very short (1.54 s, frequency high(60010 000 p.p.s.)

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Tabl

e 17

.3Th

e ch

aract

eris

tics

of d

iffere

nt t

ypes

of T

ENS

Aim

of

Mai

n fib

re-

Des

ired

Opt

imal

El

ectro

deAn

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sicD

uratio

nM

ain

curr

ents

type

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ctric

al

posit

ion

prof

ileof

mech

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spon

sible

patie

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tics

treat

men

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effe

cts

expe

rienc

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ion

Conv

entio

nal

Activ

ate

A,

Stro

ngH

igh

frequ

ency

/O

ver

site

Rap

id o

nset

Co

ntin

uousl

ySe

gmen

tal

TEN

Sla

rge

mech

ano-

com

forta

ble

low

inte

nsity

of p

ain

1h

musc

lew

ithin

burs

tpa

inafte

r sw

itch-

off

twitc

h le

adin

gPa

ttern

bu

rst

Myo

tom

alto

act

ivatio

nof s

mal

l di

amet

er n

on-

nox

ious

musc

le

affe

rents

(GIII)

Inte

nse

Activ

ate

sm

all

A,

Hig

hest

Hig

h fre

quen

cy /

Ove

r si

teR

apid

ons

et~

15 m

in/

Perip

hera

l TE

NS

diam

eter

noci

cept

ors

inte

nsity

high

inte

nsity

of p

ain

or

1000

s

main

ner

ve>

1h

afte

rco

ntra

ctio

n Fr

equ

ency

~

200

p.p.

s.bu

ndl

esw

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off

Patte

rn

contin

uous

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exp

erie

nce

hypo

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hesia

F07216-17.qxd 15/9/01 9:34 PM Page 264


Stim

ulus

am

plit

ude

(arb

itrar

y un

its)

10 100 1000

Pulse duration (s)

Small diameter afferentMotor efferentLarge diameter afferent

Figure 17.4 Strengthduration curve for fibre activation. As pulse duration increases less current amplitude is needed toexcite an axon to generate an action potential. Small pulse durations are unable to excite nerve axons even at highcurrent amplitudes. Large diameter axons require lower current amplitudes than small diameter fibres. Thus, passing pulsedcurrents across the surface of the skin excites large diameter non-noxious sensory nerves first (paraesthesia), followed bymotor efferents (muscle contraction) and small diameter noxious afferents (pain). Alteration of pulse duration is one means ofhelping the selective recruitment of different types of nerve fibre. For example, intense TENS should use long pulse durations( 1000 s) as they activate small diameter afferents more readily. During conventional TENS pulse durations ~ 100200 s areused as there is a large separation (difference) in the amplitude needed to recruit different types of fibre. This enables greatersensitivity when using the intensity (amplitude) dial so that a strong but comfortable paraesthesia can be achieved withoutmuscle contraction or pain.

Monophasicpulses

Symmetricalbiphasicpulses

Asymmetricalbiphasic

pulse

Spike-likebiphasic

pulse

Cur

rent

Time

Figure 17.5 Common pulse waveforms used in TENS.

SkinNervefibre

Hyperpolarised Depolarised

Potentially blocked byhyperpolarisation Nerve impulse

CathodeAnode

+ + + ++ + + +

+ + + ++ + + +

Figure 17.6 Fibre activation by TENS. When devices usewaveforms which produce net DC outputs which are notzero, the cathode excites (depolarisation) the axon and thenerve impulse will travel in both directions down the axon.The anode tends to inhibit the axon (hyperpolarisation)and this could extinguish the nerve impulse. Thus, duringconventional TENS the cathode should be positioned proxi-mal to the anode so that the nerve impulse is transmitted tothe central nervous system unimpeded. However, duringAL-TENS the cathode should be placed distal, or over themotor point, as the purpose of AL-TENS currents is toactivate a motor efferent.

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proven improvements in clinical effectiveness.Unfortunately, the ever-increasing complexity ofTENS devices has led to confusion about themost appropriate way to administer TENS.Therefore it is important to summarise theprinciples for the main types of TENS.

Conventional TENSThe aim of conventional TENS is to activateselectively large diameter A fibres withoutconcurrently activating small diameter A and C(pain-related) fibres or muscle efferents (Fig. 17.8).Evidence from animal and human studiessupports the hypothesis that conventional TENSproduces segmental analgesia with a rapid onsetand offset and which is localised to the der-matome (see Mechanisms of action). Theoretically,high-frequency, low-intensity pulsed currentswould be most effective in selectively activating

large diameter fibres, although in practice thiswill be achieved whenever the TENS user reportsthat they experience a comfortable paraesthesiabeneath the electrodes.

During conventional TENS currents are usu-ally delivered between 10 and 200 p.p.s., and100200 s with pulse amplitude titrated toproduce a strong comfortable and non-painfulparaesthesia (Table 17.3). As large diameterfibres have short refractory periods they cangenerate nerve impulses at high frequencies.This means that they are more able to generatehigh-frequency volleys of nerve impulses whenhigh-frequency currents are delivered. Thus,greater afferent barrages will be produced inlarge diameter nerve fibres when high frequen-cies (10200 p.p.s.) are used. The pattern of pulsedelivery is usually continuous, although con-ventional TENS can also be achieved by deliver-ing the pulses in bursts or trains and this hasbeen described by some authors as pulsed orburst TENS (Walsh, 1997c; Woolf andThompson, 1994). It is likely that continuousTENS and burst TENS produce similar effectswhen delivered at a strong but comfortable levelwithout concurrent muscle twitches.

Acupuncture-like TENS (AL-TENS)The majority of commentators believe that AL-TENS should be defined as the induction of force-ful but non-painful phasic muscle contractions


TENS currents

A-segmental

C

A

CathodeAnode

TENSelectrodes

Muscle

Figure 17.8 The aim of conventional TENS is to selectivelyactivate A afferents producing segmental analgesia.

fr

Continuous

Burst

Amplitudemodulated

Frequencymodulated

Durationmodulated

Randomequency

Figure 17.7 Novel pulse patterns available on TENSdevices. Modulated patterns fluctuate between upper andlower limits over a fixed period of time and this is usually preset in the design of the TENS device.

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at myotomes related to the origin of the pain(Eriksson and Sjlund, 1976; Johnson, 1998;Meyerson, 1983; Sjlund, Eriksson and Loeser,1990; Walsh, 1997c; Woolf and Thompson, 1994).The purpose of AL-TENS is to selectively acti-vate small diameter fibres (A or group III)arising from muscles (ergoreceptors) by theinduction of phasic muscle twitches (Fig. 17.9).Thus, TENS is delivered over motor pointsto activate A efferents to generate a phasicmuscle twitch resulting in ergoreceptor activity(Table 17.3). Patients report discomfort whenlow-frequency pulses are used to generate mus-cle twitches so bursts of pulses are used instead(Eriksson and Sjlund, 1976). Evidence sug-gests that AL-TENS produces extrasegmentalanalgesia in a manner similar to that suggestedfor acupuncture (see Mechanisms of action).However, there is inconsistency in the use ofthe term, AL-TENS, as some commentatorsdescribe AL-TENS as the delivery of TENSover acupuncture points irrespective of muscleactivity (Lewers et al., 1989; Lewis et al., 1990;Longobardi et al., 1989; Rieb and Pomeranz,1992). A critical review of AL-TENS can befound in Johnson (1998).

Intense TENSThe aim of intense TENS is to activate smalldiameter A cutaneous afferents by deliveringTENS over peripheral nerves arising from thesite of pain at an intensity which is just tolerableto the patient (Jeans, 1979; Melzack, Vetere andFinch, 1983, Fig. 17.10). Thus, TENS is deliveredover the site of pain or main nerve bundle aris-ing from the pain using high-frequency andhigh-intensity currents which are just bearableto the patient (Table 17.3). As intense TENS actsin part as a counterirritant it can be delivered foronly a short time but it may prove useful forminor surgical procedures such as wound dress-ing and suture removal. Activity in cutaneousA afferents induced by intense TENS hasbeen shown to produce peripheral blockade ofnociceptive afferent activity and segmental andextrasegmental analgesia (see Mechanisms ofaction).

Practical implicationsThe theoretical relationship between pulse fre-quency, duration and pattern may break downas currents follow the path of least resistancethrough the underlying tissue. So in clinicalpractice a trial and error approach is usedwhereby patients titrate current amplitude, fre-quency and duration to produce the appropriate


TENS currents

Contraction

GIII-extrasegmental

A-segmentalGI

AnodeCathode

TENSelectrodes

Motor point

Muscle

Muscle

Figure 17.9 The aim of AL-TENS is to selectively activategroup I (GI) efferents producing a muscle contraction, whichresults in activity in ergoreceptors and group III (GIII)afferents. GIII afferents are small in diameter and have beenshown to produce extrasegmental analgesia through theactivation of descending pain inhibitory pathways. Aafferents will also be activated during AL-TENS producingsegmental analgesia. Note the position of the cathode.

TENS currents

A-segmental

C

A-extrasegmental

CathodeAnode

TENSelectrodes

Muscle

Figure 17.10 The aim of intense TENS is to selectively acti-vate A afferents leading to extrasegmental analgesia. Aafferents will also be activated producing segmental analgesia.

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outcome. The patients report of the sensationproduced by TENS is the easiest means ofassessing the type of fibre active. A strong non-painful electrical paraesthesia is mediated bylarge diameter afferents and a mildly painfulelectrical paraesthesia is mediated by recruit-ment of small diameter afferents. The presenceof a strong non-painful phasic muscle contrac-tion is likely to excite muscle ergoreceptors.

KNOWN BIOLOGICAL EFFECTS TENS effects can be subdivided into analgesicand non-analgesic effects (Box 17.1). In clinicalpractice, TENS is predominantly used for itssymptomatic relief of pain although there isincreasing use of TENS as an antiemetic and forrestoration of blood flow to ischaemic tissueand wounds. There is, however, less publishedresearch on the non-analgesic effects of TENSand some of the experimental work in the fieldis contradictory. The reader is guided to Walsh(1997b) for a discussion of the non-analgesiceffects of TENS. In contrast, the mechanism bywhich TENS produces pain relief has receivedmuch attention.

Mechanisms of actionStimulation-induced analgesia can be cate-gorised according to the anatomical site ofaction into peripheral, segmental and extraseg-mental. In general, the main action of conven-tional TENS is segmental analgesia mediated byA fibre activity. The main action of AL-TENS isextrasegmental analgesia mediated by ergore-ceptor activity. The main action of intense TENSis extrasegmental analgesia via activity in smalldiameter cutaneous afferents. Conventional andintense TENS are also likely to produce peri-pheral blockade of afferent information in thefibre type that they activate.

Peripheral mechanismsThe delivery of electrical currents over a nervefibre will elicit nerve impulses that travel inboth directions along the nerve axon, termed

antidromic activation (Fig. 17.11). TENS-inducednerve impulses travelling away from the centralnervous system will collide with and extinguishafferent impulses arising from tissue damage.For conventional TENS, antidromic activationis likely to occur in large diameter fibres andas tissue damage may produce some activity inlarge diameter fibres conventional TENS maymediate some of its analgesia by peripheralblockade in large diameter fibres. TENS-inducedblockade of peripheral nerve transmission hasbeen demonstrated by Walsh et al. (1998) inhealthy human subjects. They found that TENSdelivered at 110 p.p.s. significantly increased thenegative peak latency of the compound actionpotential and this suggests that there was aslowing of transmission in the peripheral nerve.Nardone and Schieppati (1989) have alsoreported that the latency of early somatosensoryevoked potentials (SEPs) was increased duringTENS in healthy subjects and concluded thatconventional TENS could produce a busy line-effect in large afferent fibres.

The contribution of peripheral blockade onanalgesia is likely to be greater during intenseTENS. Impulses travelling in A fibres induced


TENS currents

Skin

TENS inducedimpulses travelto CNS

Impulsesgeneratedby noxiousevent

Antidromicactivationof axon by

TENS

TENSelectrodes

Tissuedamage

TENS induced impulseextinguishes impulsearising from noxiousstimulus

Antidromic collision

Figure 17.11 TENS-induced blockade of peripheral trans-mission. Impulses generated by TENS will travel in bothdirections down an axon (antidromic activation) leading to acollision with noxious impulses travelling toward the centralnervous system (CNS).

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by intense TENS will collide with nociceptiveimpulses, also travelling in A fibres. Ignelzi andNyquist (1976) demonstrated that electrical stim-ulation (at intensities likely to recruit A fibres)can reduce the conduction velocity and ampli-tude of A, A and A components of the com-pound action potential recorded from isolatednerves in the cat. The greatest change was foundin the A component. However, Levin and Hui-Chan (1993) have shown that healthy subjectscannot tolerate direct activation of A afferentsby TENS and therefore intense TENS is adminis-tered for only brief periods of time in clinicalpractice.

Segmental mechanismsConventional TENS produces analgesia pre-dominantly by a segmental mechanism wherebyactivity generated in A fibres inhibits ongoingactivity in second-order nociceptive (pain related)neurons in the dorsal horn of the spinal cord(Fig. 17.12). Workers have shown that activity inlarge diameter afferents will inhibit nociceptivereflexes in animals when the influence of pain-inhibitory pathways descending from the brainhas been removed by spinal transection(Sjlund, 1985; Woolf, Mitchell and Barrett, 1980;Woolf, Thompson and King, 1988). Garrison andForeman (1994) showed that TENS could signi-ficantly reduce ongoing nociceptor cell activityin the dorsal horn cell when it was applied tosomatic receptive fields. Follow-up work afterspinal cords had been transected at T12 demon-strated that spontaneously and noxiouslyevoked cell activities were still reduced duringTENS. This demonstrates that the neuronalcircuitry for conventional TENS analgesia islocated in the spinal cord and it is likely that acombination of pre- and postsynaptic inhibitiontakes place (Garrison and Foreman, 1996).

Studies using the opioid receptor antagonistnaloxone have failed to reverse analgesia fromhigh-frequency TENS, suggesting that non-opioid transmitters may be involved in thissynaptic inhibition (see Thompson (1989) forreview). Studies by Duggan and Foong (1985)using anaesthetised cats suggest that the

inhibitory neurotransmitter gamma aminobu-tyric acid (GABA) may play a role. The clinicalobservation that conventional TENS producesanalgesia that is short lasting and rapid in onsetis consistent with synaptic inhibition at a seg-mental level.

A number of workers have shown that TENS-induced activity in A fibres during intenseTENS can lead to long-term depression (LTD)of central nociceptor cell activity for up to 2hours. Low-frequency stimulation of A-fibres(1 p.p.s., 0.1 ms) has been shown to produce LTDin animals which is not influenced by bicu-culline, which is a GABA receptor antagonist, butis abolished by D-2-amino-5-phosphonovalericacid, which is a N-methyl-D-aspartate (NMDA)receptor antagonist (Sandkhler, 2000; Sandkhler


ConventionalTENS

A fibres

Nociceptoractivity

A & C fibres

Spinal cord

SP

VIP

Dor

sal c

olum

n

Paraesthesia

Sp

inot

hala

mic

and

sp

inor

etic

ular

GABA

SG T

+

+ +

PAIN

Figure 17.12 Neurophysiology of conventional TENS analge-sia. Activity in A and C fibres from nociceptors leads to exci-tation () of interneurons in the substantia gelatinosa (SG)of the spinal cord via neurotransmitters like substance P (SP,cutaneous nociceptors) or vasoactive intestinal peptide (VIP,visceral nociceptors). Central nociceptor transmission neu-rons (T) project to the brain via spinoreticular and spinothala-mic tracts to produce a sensory experience of pain.TENS-induced activity in A afferents leads to the inhibition() of SG and T cells (dotted line) via the release of gammaamino butyric acid (GABA, black interneuron). Paraesthesiaassociated with TENS is generated by information travellingto the brain via the dorsal columns.

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et al., 1997). This suggests that glutamate ratherthan GABA may be involved in LTD induced byintense TENS. The time course of latency andamplitude changes in SEPs after high-frequency(200 p.p.s.) electrical stimulation of the digitalnerves in healthy subjects supports the conceptthat TENS can produce LTD of central nociceptivecells (Macefield and Burke, 1991). One practicaloutcome of this work may be introduction ofsequential TENS where conventional TENS isadministered at a strong but comfortable level inthe first instance followed by a brief period ofintense TENS leading to longer post-stimulationanalgesia (Sandkhler, 2000).

Extrasegmental mechanisms TENS-induced activity in small diameter affer-ents has also been shown to produce extra-segmental analgesia through the activation ofstructures which form the descending pain-inhibitory pathways, such as periaqueductalgrey (PAG), nucleus raphe magnus and nucleusraphe gigantocellularis. Antinociception inanimals produced by stimulation of cutaneousA fibres is reduced by spinal transection, sug-gesting a role for extrasegmental structures(Chung et al., 1984a, b; Woolf, Mitchell andBarrett, 1980). Phasic muscle contractions pro-duced during AL-TENS generates activity insmall diameter muscle afferents (ergoreceptors)leading to activation of the descending pain-inhibitory pathways (Fig. 17.13). The importanceof muscle afferent activity in this effect has beenshown in animal studies by Sjlund (1988) whofound that greater antinociception occurredwhen muscle rather than skin afferents wereactivated by low-frequency (2 bursts per second)TENS. Duranti, Pantaleo and Bellini (1988)confirmed this in humans by demonstratingthat there was no difference in analgesia pro-duced by currents delivered through the skin(e.g. AL-TENS) compared to currents which bypassed the skin (e.g. intramuscular electricalnerve stimulation; IENS).

There is growing evidence that AL-TENSbut not conventional TENS is mediated by

endorphins. Sjlund, Terenius and Eriksson,(1977) reported that AL-TENS increased cere-brospinal (CSF) endorphin levels in ninepatients suffering chronic pain and that AL-TENS analgesia was naloxone reversible(Sjlund and Eriksson, 1979). However, nalox-one failed to reverse analgesia produced by conventional TENS in pain patients (Abram,Reyolds and Cusick, 1981; Hansson et al., 1986;Woolf et al, 1978). Claims that conventionalTENS can elevate plasma -endorphin and -lipotrophin in healthy subjects (Facchinetti et al.,1986) have not been confirmed (Johnson et al.,1992) and it seems unlikely that -endorphinwould cross the bloodbrain barrier owing to itslarge size.


AL-TENS Descending paininhibitory pathways

A/GIII

A & C fibres

Spinal cord

Sp

inot

hala

mic

and

sp

inor

etic

ular

E

SG T

+

+ +

PAIN

Muscle twitchand paraesthesia

Muscletwitch

PAG

+

++

nRM

Figure 17.13 Neurophysiology of AL-TENS analgesia.Actvity in A and C fibres from nociceptors leads to excita-tion () of central nociceptor transmission neurons (T) whichproject to the brain to produce a sensory experience of pain.TENS-induced activity in small diameter muscle afferents(A, GIII) leads to the activation of brainstem nuclei such asthe periaqueductal grey (PAG) and nucleus raphe magnus(nRM). These nuclei form the descending pain inhibitorypathways which excite interneurons which inhibit () SG andT cells (dotted line) via the release of met-enkephalin (E,black interneuron). It is likely that paraesthesia and sensa-tions related to the muscle twitch are relayed to the brain viathe dorsal columns.

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Analgesic effectsAs different mechanisms contribute to analgesiaproduced by different types of TENS it is plausi-ble that they will have different analgesic pro-files. In fact this is the rationale for the use ofdifferent types of TENS. Evidence from labora-tory and clinical studies show that TENS analge-sia is maximal when the stimulator is switchedon irrespective of the type of TENS used(Fishbain et al., 1996; Johnson et al., 1991a; Walsh,1997c; Woolf and Thompson 1994). This explainsthe finding that long-term users of TENS admin-ister conventional TENS continuously through-out the day to achieve adequate analgesia(Chabal et al., 1998; Fishbain et al., 1996; Johnsonet al., 1991a; Nash, Williams and Machin, 1990).Poststimulation analgesia has been reportedto occur in some patients and this may be dueto LTD and activation of descending paininhibitory pathways. Reports of the duration ofthese poststimulation effects vary widely from18 hours (Augustinsson, Carlsson and Pellettieri,1976) to 2 hours (Johnson et al., 1991a). It is pos-sible that natural fluctuations in symptoms andthe patients expectation of treatment effectsmay have contributed to some extent to theseobservations.

There are remarkably few studies which havesystematically investigated the analgesic profilesof a range of TENS pulse frequencies, pulsedurations and pulse patterns when all otherstimulating characteristics are fixed. There is anextensive literature of studies which have com-pared the analgesic effects of two pulse frequen-cies (usually high ~ 100 p.p.s. and low ~ 2 p.p.s.)in animals, healthy humans and patients in pain.However, the TENS characteristics used in manyof these studies appear to have been chosenad hoc, which makes synthesis of the findingsbetween groups almost impossible (see tables inWalsh 1997a and e).

Sjlund (1985) delivered seven different stim-ulation frequencies (10, 40, 60, 80, 100, 120 and160 p.p.s.) to a dissected skin nerve in lightlyanaesthetised rats and reported that a stimulationfrequency of 80 p.p.s. gave the most profoundinhibition of the C-fibre-evoked flexion reflex.

In a follow-up study they reported that a pulse-train repetition rate of around 1 Hz was mosteffective in inhibition of the C-fibre-evoked flex-ion reflex. Johnson et al. (1989) assessed the anal-gesic effects of five stimulating frequencies (10,20, 40, 80 and 160 p.p.s.) on cold-induced pain inhealthy subjects. TENS frequencies between 20and 80 p.p.s. produced greatest analgesia whendelivered at a strong but comfortable intensity,with 80 p.p.s. producing the least intersubjectvariation in response (e.g. the most reliable effectamong subjects). Thus, when trying out conven-tional TENS on a patient for the first time it seemssensible to start with frequencies around 80 p.p.s.

Johnson et al. (1991) systematically investi-gated the analgesic effects of burst, amplitude-modulated, random (frequency of pulse delivery)and continuous TENS delivered at a strong butcomfortable level on cold-induced pain inhealthy subjects. All pulse patterns elevated ice-pain threshold but there were no significant differences between the groups when all otherstimulating characteristics were fixed. Tulgaret al. (1991a) demonstrated that a variety ofpatterns of pulse delivery were equally effectivein managing patients pain. However, patientspreferred modulated patterns of TENS such asfrequency modulation and burst rather thancontinuous (Tulgar et al., 1991b). This seems tocontrast with Johnson, Ashton and Thompson(1991a) who found that the majority of long-termusers of TENS preferred continuous rather thanburst mode. More systematic investigationswhich compare the analgesic effects of a range of(i.e. more than two) stimulating characteristicswhen all other variables are fixed are clearlyneeded.

KNOWN EFFICACY: THE CLINICALEFFECTIVENESS OF TENSThere is an extensive literature on the clinicaleffectiveness of TENS although the majority ofreports are anecdotal or of clinical trials lackingappropriate control groups. These reports are oflimited use in determining the clinical effec-tiveness as they do not take account of normal


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fluctuations in the patients symptoms, thetreatment effects of concurrent interventions orthe patients expectation of treatment success.Placebo-controlled clinical trials should be usedto determine the absolute effectiveness of treat-ments so that the effects due to the active ingre-dient (e.g. the electrical currents for TENS) canbe isolated from the effects associated with theact of giving the treatment. Placebo or shamTENS is usually achieved by preventing TENScurrents from reaching the patient, for exampleby cutting wires within the device. Failure toblind patients and investigators to the differenttreatment groups in placebo-controlled trials, aswell as failure to randomise the sample popula-tion into treatment groups, will markedly over-estimate treatment effects (see McQuay andMoore, 1998a; Schulz et al., 1995 for discussion).Unfortunately, there are many practical difficul-ties in designing and blinding treatment groupsin studies which examine technique-based inter-ventions like TENS (Bjordal and Greve, 1998;Deyo et al., 1990a; Thorsteinsson, 1990).

Carroll et al. (1996) demonstrated the impact ofusing non-randomised trials in determining TENSeffectiveness; 17 of 19 non-randomised controlledtrials (non-RCTs) reported that TENS had apositive analgesic effect whereas 15 of 17 ran-domised controlled trials (RCTs) reported thatTENS had no effect for postoperative pain.Carroll et al. (1996) concluded that non-ran-domised studies on TENS, or any other treatment,will overestimate treatment effects. Therefore, in a climate of evidence-based medicine thefindings of systematic reviews of randomisedcontrolled clinical trials will be used to determineeffectiveness (Table 17.4).

TENS and acute painPostoperative painHymes et al. (1974) were the first to report thesuccess of conventional TENS for acute painresulting from surgery using sterile electrodesstraddling the incision (Fig. 17.14). Potentially,TENS could relieve pain and reduce concurrentopioid consumption and associated adverse


Table 17.4 Outcomes of systematic reviews

Condition Existing reviews

Acute pain Reeve, Menon and Corabian (1996)

Range of conditions (dysmenorrhea, dental, cervical, orofacial, sickle cell disease)

TENS effective 7/14 RCTsReviewers conclusion: evidence inconclusivepoor RCTmethodology in field

Postoperative pain Reeve, Menon and Corabian(1996)

TENS effective 12/20 RCTsReviewers conclusion: evidence inconclusivepoor RCTmethodology in field

Carroll et al. (1996)TENS effective in 2/17 RCTsReviewers conclusion: limited evidence of effectiveness

Labour pain Reeve, Menon and Corabian (1996)TENS effective 3/9 RCTsReviewers conclusion: evidence inconclusivepoor RCT methodology in field

Carroll et al. (1997a)TENS effective 3/8 RCTsReviewers conclusion: limited evidence of effectiveness

Carroll et al. (1997bupdate of Carroll et al. (1997a) review)

TENS effective 3/10 RCTsReviewers conclusion: limited evidence of effectiveness

Chronic pain Reeve, Menon and Corabian(1996)

Range of conditions (low back, pancreatitis, arthritis, angina)

TENS effective 9/20 RCTsReviewers conclusion: evidence inconclusivepoor RCT methodology in field

McQuay and Moore (1998b)Range of conditions (low back, pancreatitis, osteoarthritis, dysmenorrhea)

TENS effective 10/24 RCTsReviewers conclusion: evidence inconclusivepoor RCT methodology in field

TENS dosage too lowFlowerdew and Gadsby (1997)/ Gadsby and Flowerdew (1997)

Low back pain (6 RCTs)Odds ratio vs. placebo, conventional TENS (1.62), AL-TENS (7.22)Reviewers conclusion: TENS effectivepoor RCT methodology in field

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A

B

Neck pain bilateral unilateral

Stump pain

Hip pain

Stump and phantom limb

Postherpetic neuralgia above affected dermatome across affected dermatome

Tendonitis

Low back pain or dysmenorrhoea

Peripheral vascular disease

Shoulder pain

Sciatica

Anterior shoulder

Angina

Rib metastasis

Postoperative pain large electrodes if appropriate

Phantom limb pain contralateral site-median nerve

Postoperative pain (saphenous vein) large electrodes if appropriate

Knee pain (osteoarthritis) dual channel if appropriate

Thalamic pain where pain is most pronounced

Postherpetic neuralgia above affected dermatome across affected dermatome

Phantom limb pain over main nerve bundle arising from phantom

Dysmenorrhoea (women)

Trigeminal neuralgia

Ankle pain

Figure 17.14 A: Electrode positions for common pain conditionsanterior view. B: Electrode positions for common painconditionsposterior view.

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events such as respiratory depression. Clinicaltrials have shown that TENS reduces pain andadditional analgesic intake and improves respi-ratory function (Ali, Yaffe and Serrette, 1981;Bayindir et al., 1991; Benedetti et al., 1997; Chiuet al., 1999; Schuster and Infante, 1980; Warfield,Stein and Frank, 1985). However, the existingliterature has been reviewed systematically byCarroll et al. (1996) who found that 15 of 17 RCTsreported that TENS produced no significantbenefit when compared with placebo; thisgroup concluded that TENS was not effective forthe management of postoperative pain. A sys-tematic review on acute pain, including post-operative pain, by Reeve, Menon and Corabian(1996) reported that 12 of 20 RCTs found thatTENS was beneficial in postoperative pain,suggesting that TENS may be of some benefit(Table 17.4).

Closer examination reveals discrepancies inthe judgements of individual RCT outcome bythe reviewers, which may undermine confidencein their findings. For example, the RCT by Connet al. (1986) was judged as a negative outcomestudy by Carroll et al. (1996) and a positive out-come study by Reeve, Menon and Corabian(1996). Conn et al. (1986) concluded that its(TENS) use in this situation (postappendicectomypain) cannot be recommended. Difficulties in mak-ing judgements about trial outcome may arisewhen multiple outcome measures have beenused, leading to combinations of positive andnegative effects. This makes summary judge-ments of effectiveness by reviewers difficult. Inaddition, Benedetti et al. (1997) has shown thatTENS was effective for mild to moderate painassociated with thoracic surgical procedures butineffective for severe pain. However, reductionsin mild pain are harder to detect than reductionsin severe pain, and studies which include onlythose patients with mild to moderate pain willlose sensitivity in the detection of outcomemeasure, while TENS trials attempting to opti-mise trial sensitivity by including only patientswith severe pain would bias the study towardnegative outcome. This may be overlooked in sys-tematic reviews, so it would be hasty to accept thefindings of the systematic reviews on TENS and

postoperative pain without further scrutiny(Bjordal and Greve, 1998; Johnson, 2000).

Labour painThe popularity of TENS for labour pain is due inpart to published reports of patient satisfactionand trials demonstrating TENS success withoutappropriate control groups (Augustinsson et al.,1977; Bundsen et al., 1978; Grim and Morey,1985; Kubista, Kucena and Riss, 1978; Miller-Jones, 1980; Stewart, 1979; Vincenti, Cervellinand Mega, 1982). Augustinsson et al. (1976) pio-neered the use of TENS in obstetrics by applyingTENS to areas of the spinal cord which corre-spond to the input of nociceptive afferents asso-ciated with the first and second stages of labour(e.g. T10L1 and S2S4 respectively, Fig. 17.15).They reported that 88% of 147 women obtainedpain relief using this method although thestudy failed to include a placebo control group(Augustinsson et al., 1977). Manufacturers mar-ket specially designed obstetric TENS deviceswhich have dual channels and a boost controlbutton for contraction pain.

Two systematic reviews on TENS and labourpain concluded that evidence for TENS analge-sia during labour is weak (Carroll et al., 1997a;Reeve, Menon and Corabian, 1996; Table 17.4).Reeve, Menon and Corabian (1996) reported thatseven of nine RCTs showed no differencesbetween TENS and sham TENS or conventionalpain management (Bundsen and Ericson, 1982;Chia et al., 1990; Lee et al., 1990; Nesheim, 1981;Thomas et al., 1988). Carroll et al. (1997a)reported that five of eight RCTs showed nobenefits from TENS and this was confirmed inan updated review that included two additionalRCTs (Carroll et al., 1997b). Interestingly, Carrollet al. (1997b) reported that the odds ratio fortrials recording additional analgesic interventionwas 0.57, suggesting that analgesic interventionmay be less likely with TENS, although number-needed-to-treat was high (14, 95% confidenceinterval 7.311.9). RCTs that used analgesicintake as an outcome measure would have com-promised the validity of pain relief scores aspatients in both sham and active TENS groups


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would consume analgesics to achieve maximalpain relief. Thus, differences in pain relief scoresbetween TENS and sham are less likely, whichwill bias outcome towards no differencebetween groups.

In systematic reviews credence is given totrials with high methodological scores such asvan der Ploeg et al. (1996), Harrison et al. (1986)and Thomas et al. (1988). Van der Ploeg et al.(1996) reported no significant differencesbetween active and sham TENS in 94 women foradditional analgesic intervention or pain reliefscores. Harrison et al. (1986) conducted an RCTon 150 women and reported no differencesbetween active and sham TENS users for painrelief or additional analgesic intervention. TheRCT by Thomas et al. (1988) on 280 parturientsfound no significant differences between activeand sham TENS for analgesic intervention orpain scores. Interestingly, under double-blindconditions women favoured active TENS whencompared with sham TENS in studies byHarrison et al. (1986) and Thomas et al. (1988).

The evidence is weak for the continued use ofTENS in the management of labour pain.However, this conflicts with the clinical experi-ence of midwives and with patient satisfactionon the use of TENS (Johnson, 1997). It is possiblethat methodological problems associated withRCTs examining technique-based interventionsmay seriously bias the outcome of the systematic

reviews (Bjordal and Greve, 1998). The self-report of pain relief may be unreliable whenpatients are experiencing fluctuating emotionaland traumatic conditions as in the differentstages of labour. Responses solicited at the endof childbirth, when women are relaxed and maybe in a better position to judge and reflect onthe effects of the intervention, may be moreappropriate. Moreover, RCTs by Champagneet al. (1984) and Wattrisse et al. (1993) usedtranscranial TENS administered via electrodesplaced on the temple. Transcranial TENS deliv-ers electrical currents with markedly differentcharacteristics to those of conventional obstetricTENS (Table 17.2) and it could be argued thatthese studies should not have been included inthe review. Interestingly both of these studiesdemonstrated beneficial effects. Nevertheless,this raises questions about the appropriateness of the treatment protocols used in some RCTsincluded in the reviews. It would be unreason-able to dismiss the use of TENS for labour painuntil the discrepancy between clinical experienceand clinical evidence is resolved (Johnson, 2000).

TENS and chronic painThe widespread use of TENS for chronic pain issupported by a large number of clinical trialsthat suggest that TENS is useful for a wide rangeof chronic pain conditions. Conditions include


All four electrodes are active

Boostbutton

During contractions

T10-L1

S2-S4

Electrodes positioned to targetafferents active during distensionof cervix and lower uterine segment

(1st stage labour)

Electrodes positioned to targetafferents active during distensionof pelvis and perineum

(2nd stage labour)

high intensity/high frequency(continuous)

Between contractions

low intensity/low frequency(burst)

Figure 17.15 The position of electrodes and electrical characteristics of TENS when used to manage labour pain.

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chronic neuropathies (Thorsteinsson et al., 1977),postherpetic neuralgia (Nathan and Wall, 1974),trigeminal neuralgia (Bates and Nathan, 1980),phantom limb and stump pain (Finsen et al.,1988; Katz and Melzack, 1991; Thorsteinsson,1987), musculoskeletal pains (Lundeberg, 1984)and arthritis (Mannheimer and Carlsson, 1979;Mannheimer, Lund and Carlsson, 1978). Myers,Woolf and Mitchell (1977) and Sloan et al. (1986)have shown that TENS relieves pain associatedwith fractured ribs.

Systematic reviews of TENS and chronic painconclude that it is difficult to determine TENSeffectiveness due to the lack of good qualitytrials (Flowerdew and Gadsby, 1997; Gadsbyand Flowerdew, 1997; McQuay and Moore,1998b; Reeve, Menon and Corabian, 1996).Reeve, Menon and Corabian (1996) reported thatnine of 20 RCTs provided evidence that TENSwas more effective than sham TENS (n 7) orno treatment (n 2) for a range of conditions(Table 17.4). Eight of 20 RCTs showed evidencethat TENS was no more effective than shamTENS (n 6) or acupuncture. It was not possibleto classify the outcome of three RCTs. Reeve,Menon and Corabian (1996) concluded that theevidence was inconclusive and that the method-ological quality of these trials was poor.

McQuay et al. (1997) also reported that therewas limited evidence to assess the effectivenessof TENS in outpatient services for chronic pain.Ten of 24 RCTs provided evidence that TENSeffects were better than sham TENS, placebopills or control points such as inappropriate elec-trode placements (McQuay and Moore, 1998b).Fifteen RCTs compared TENS with an activetreatment and only three reported that TENS pro-vided benefit greater than the active treatment.However, over 80% of trials included in thereview by McQuay and Moore (1998b) deliveredTENS for less than 10 hours per week and 67% oftrials delivered less than ten TENS treatmentsessions. McQuay and Moore (1998b) concludedthat TENS may provide some benefit in chronicpain patients if large enough (appropriate) dosesare used.

Perhaps the most common use for TENS is inthe management of low back pain. However,

contradictory findings are found in the litera-ture. Marchand et al. (1993) concluded that con-ventional TENS was significantly more efficientthan placebo TENS in reducing pain intensity butnot pain unpleasantness in 42 patients with backpain. In contrast, a RCT by Deyo et al. (1990b)concluded that treatment with TENS was nomore effective than treatment with a placebo in145 patients with chronic low back pain. Asystematic review by Flowerdew and Gadsby(Flowerdew and Gadsby, 1997; Gadsby andFlowerdew, 1997) included only six RCTs; 62trials were excluded as they were either non-randomised or failed to compare active TENSwith a credible placebo. The meta-analysisshowed that more patients improved withAL-TENS (86.70%) than with conventional TENS(45.80%) or placebo (36.40%), with greater oddsratios for AL-TENS vs. placebo (7.22) than con-ventional TENS vs. placebo (1.62). However, theodds ratio for AL-TENS was based on the find-ings of only two studies, neither of which appliedAL-TENS to produce muscle contractions(Gemignani et al., 1991; Melzack, Vetere andFinch, 1983, see Johnson (1998) for critical review).Flowerdew and Gadsby (1997) concluded thatTENS reduces pain and improves the range ofmovement in patients suffering chronic low backpain although a definitive RCT is still necessaryin the field. Thus, at present the evidence forTENS effectiveness for chronic pain as generatedfrom systematic reviews is inconclusive.

There is an increasing use of TENS for angina,dysmenorrhoea, pain associated with cancer andpain in children. Conventional TENS is used forangina with electrodes placed directly over thepainful area of the chest (Brjesson et al., 1997;Mannheimer et al., 1982; Fig. 17.14). Mannheimeret al. (1985); Mannheimer, Emanuelsson andWaagstein (1990) have shown that TENSincreases work capacity, decreases ST segmentdepression, and reduces the frequency of angi-nal attacks and nitroglycerin consumption whencompared with control groups. A variety oftypes of TENS have been reported to be success-ful in the management of dysmenorrhea(Dawood and Ramos, 1990; Kaplan et al., 1994;Lewers et al., 1989; Milsom, Hedner and


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Mannheimer, 1994; Neighbors et al., 1987). Mostoften electrodes are applied over the lowerthoracic spine and sometimes on acupuncturepoints (Fig. 17.14, see Walsh (1997a, p. 86) forreview). Success with TENS has also beenreported in the palliative care setting with bothadults (Avellanosa and West, 1982; Hoskin andHanks, 1988) and children (Stevens et al., 1994).TENS can be used for metastatic bone disease,for pains caused by secondary deposits and forpains due to nerve compression by a neoplasm(see Thompson and Filshie (1993) for review). Inthese circumstances electrodes should be placedon healthy skin near to the painful area ormetastatic deposit providing sensory functionis preserved or alternatively the affected der-matome. TENS has been shown to be useful inthe management of a variety of pains in childrenincluding dental pain (Harvey and Elliott, 1995;Oztas, Olmez and Yel, 1997; teDuits et al., 1993),minor procedures such as wound dressing(Merkel, Gutstein and Malviya, 1999) andvenipuncture (Lander and Fowler-Kerry, 1993).

PRINCIPLES UNDERLYINGAPPLICATIONThe basic principles of the practical application ofelectrical stimulation are described in Chapter 15.

Electrode positionsAs conventional TENS is operating via a seg-mental mechanism TENS electrodes are placedto stimulate A fibres which enter the samespinal segment as the nociceptive fibres associ-ated with the origin of the pain. Thus, electrodesare applied so that currents permeate the site ofpain and this is usually achieved by applyingelectrodes to straddle the injury or painful area(Fig. 17.14). Electrodes should always be appliedto healthy innervated skin. If it is not possible todeliver currents within the site of pain, due toabsence of a body part following amputation, askin lesion or altered skin sensitivity, electrodescan be applied proximally over the main nervetrunk arising from the site of pain. Alternatively,electrodes can be applied over the spinal cord at

the spinal segments related to origin of pain.Electrodes can also be applied at a site whichis contralateral to the site of pain in conditionssuch as phantom limb pain and trigeminal neu-ralgia where the affected side of the face may besensitive to touch.

Accurate placement of pads can be time con-suming. Berlant (1984) has described a usefulmethod of determining optimal electrode sitesfor TENS. The therapist applies one TENS elec-trode to the patient at a potential placement site.The second electrode is held in the hand of thetherapist who uses the index finger to probe theskin of the patient to locate the best site to placethe second electrode. When the TENS device isswitched on and the amplitude slowly increasedthe patient or therapist, or both, will feel TENSparaesthesia when the circuit is made by touch-ing the patients skin. As the therapist probes thepatients skin with the index finger the intensityof TENS paraesthesia will increase whenevernerves on the patients skin run superficial. Thiswill help to target an effective electrode site.

Dual-channel devices using four electrodes orlarge-sized electrodes should be used for painscovering large areas. However, if the pain is gen-eralised and widespread over a number of bodyparts it may be more appropriate to use AL-TENSat a relevant myotome as this may produce amore generalised analgesic effect (Johnson,1998). Dual-channel stimulators are useful forpatients with multiple pains such as low backpain and sciatica or for pains which change intheir location and quality as during childbirth.

Electrical characteristicsThe efficiency of different electrical characteris-tics of TENS to selectively activate different typesof fibre was discussed earlier. For conventionalTENS, selective activation of A fibres is deter-mined through the report of a strong but com-fortable electrical paraesthesia without musclecontraction. Pulse frequencies anywhere between1 and 250 p.p.s. can achieve this although clinicaltrials consistently report frequencies between 10and 200 p.p.s. to be effective and popular with


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patients. In practice, each patient may have anindividual preference for pulse frequencies andpulse patterns and will turn to these settings onsubsequent treatment sessions (Johnson, Ashtonand Thompson, 1991b). As no relationshipbetween pulse frequency and pattern used bypatients and the magnitude of analgesia or theirmedical diagnosis has yet been found it is likelythat encouraging patients to experiment withTENS settings will produce the most effective out-come (Johnson, Ashton and Thompson, 1991a).

Timing and dosageClinical trials report that maximum pain reliefoccurs when the TENS device is switched onand that analgesic effect usually disappearsquickly once the device is switched off. Thus,patients using conventional TENS patientsshould be encouraged to use TENS wheneverthe pain is present. For ongoing chronic painthis may mean that patients use TENS overthe entire day. In a study of long-term users ofTENS Johnson, Ashton and Thompson (1991a)reported that 75% used TENS on a daily basisand 30% reported using TENS for more than 49hours a week. When TENS is used continuouslyin this way it is wise to instruct the patient tomonitor skin condition under the electrodes on aregular basis and take regular (although short)breaks from stimulation. It is advisable to applyelectrodes to new skin on a daily basis. If TENSis administered in an outpatients clinic a dosing

regimen of 20 minutes at daily, weekly ormonthly intervals is likely to be ineffective.

Some patients report poststimulation analge-sia although the duration of this effect varieswidely, lasting anywhere between 18 hours(Augustinsson, Carlsson and Pellettieri, 1976)and 2 hours (Johnson, Ashton and Thompson,1991a). This may reflect natural fluctuations insymptoms and the patients expectation oftreatment duration rather than specific TENS-induced effects. It is believed that post-TENSanalgesia is longer for AL-TENS than forconventional TENS and this is supported byinitial findings in experimental studies (Johnson,Ashton and Thompson, 1992a). However, morework is needed to establish the time course ofanalgesic effects of different types of TENS.

Giving a patient a trial of TENS forthe first timeAll new TENS patients should be given a super-vised trial of TENS prior to use (Table 17.5). Thepurpose of the trial is to ensure TENS does notaggravate pain and to give careful instruction onequipment use and expected therapeutic out-come. Patients should be allowed to familiarisethemselves on the use of TENS and therapistsshould use the session to check that patients canapply TENS appropriately. The initial trial canhelp to determine whether a patient is likely torespond to TENS and it should also be seen asan opportunity to troubleshoot problems arising


Table 17.5 Suggested characteristics to use for a patient trying TENS for the first time

Conventional TENS AL-TENS Intense TENS

Electrode placement Straddling site of pain or Over muscle or motor point Straddling site of pain orover main nerve bundle myotomally related to the over main nerve bundleproximal to pain site of pain proximal to pain

Pulse pattern Continuous Burst Continuous Pulse frequency 80100 p.p.s. 80100 p.p.s. 200 p.p.s.Pulse duration 100 200 s 100 200 s 1000 sPulse amplitude Increase intensity to Increase intensity to produce Increase intensity to produce (intensity) produce a strong but a strong but comfortable an uncomfortable tingling

comfortable tingling muscle twitch which is just bearableDuration of At least 30 minutes No more than 20 minutes No more than 5 minutes stimulation in first instance

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from poor response. Ideally, the trial should lasta minimum of 3060 minutes as it may take thislength of time for a patient to respond.

When using TENS on a new patient for thefirst time it is advisable to deliver conventionalTENS as most long-term users select this type ofTENS (Table 17.5). A set of audio speakers (orheadphones) can be plugged into the outputsockets of some TENS devices to demonstratethe sound of pulses and improve patient under-standing of output characteristics of the TENSdevice. Following the initial trial, patientsshould be instructed to administer TENS in 30minute sessions for the first few times althoughonce they have familiarised themselves with theequipment they should be encouraged to useTENS much as they like. Patients should alsobe encouraged to experiment with all stimulatorsettings so that they achieve the most comfort-able pulse frequency, pattern and duration(Table 17.6).

An early review of progress, ideally within afew weeks, can serve to ensure correct applica-tion, provide further instruction and recall TENSdevices which are no longer required. Most non-responders return borrowed devices at the nextclinic visit (Johnson, Ashton and Thompson,1992b). Assessing TENS effectiveness at regularintervals is vital for tracking the location andcontinued use of devices. Some clinics andmanufacturers allow patients to borrow TENSdevices for a limited period with a view to pur-chasing the device. A point of contact shouldalways be made available for patients whoencounter problems.

Declining response to TENSSome TENS users claim that the effectivenessof TENS declines over time although the exactproportion of patients is not known (see Table92-1 in Sjlund, Eriksson and Loeser (1990) for


Table 17.6 Suggested advice following the initial trial

Conventional TENS AL-TENS Intense TENS

Electrode positions Straddle site of pain but Over muscle belly at site of pain Straddle site of pain but if notif not successful try main nerve but if not successful try motor successful try over main bundle, across spinal cord or point at site of pain, contralateral nerve bundlecontralateral positionsdematomal positionsmyotomal

Pulse pattern Patient preference Burst but if not successful Continuous but if not or uncomfortable try amplitude successful or uncomfortablemodulated try frequency or duration

modulatedPulse frequency Patient preference, Above fusion frequency of High, e.g. 200 p.p.s.

usually 10200 p.p.s. muscle 80100 p.p.s. within the burst

Pulse duration Patient preference, Patient preference, usually Highest possible but ifusually 100250 s 100 250 s uncomfortable gradually

reduce durationPulse amplitude Strong but comfortable sensation Strong but comfortable Highest tolerable sensation(intensity) without visible muscle contraction sensation with visible muscle with limited muscle

contraction contraction Dosage As much and as often About 30 minutes at a time as 15 minutes at a time as the

as is requiredhave a break fatigue may develop with ongoing stimulation may be every hour or so muscle contractions uncomfortable

Analgesic effects Occur when stimulator on Occur when stimulator on and Occur when stimulator on andfor a while once the stimulator for a while once the stimu-has been switched off lator has been switched off

May exacerbate pain May exacerbate pain General advice Experiment with settings to Experiment with settings Experiment with settings to

maintain strong comfortable (except burst) to maintain a maintain highest tolerable sensation phasic twitch sensation

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summary of studies). Eriksson, Sjlund andNielzen (1979) found that effective pain reliefwas achieved by 55% of chronic pain patients at2 months, 41% at 1 year and 30% at 2 years.Loeser, Black and Christman (1975) reportedthat only 12% of 200 chronic pain patientsobtained long-term benefits with TENS despite68% of patients achieving initial pain relief.Woolf and Thompson (1994) suggest that themagnitude of pain relief from TENS may declineby up to 40% for many patients over a period ofa year.

There may be many reasons for the decline inTENS effects with time including dead batteries,perished leads or a worsening pain problem.However, there is evidence that some patientshabituate to TENS currents owing to a progres-sive failure of the nervous system to respondto monotonous stimuli. Pomeranz and Niznick(1987) have shown that repetitive delivery ofTENS pulses at 2 p.p.s. produces habituation oflate peaks ( 50 ms) of SEPs. This implies thatfor some people the nervous system filters outmonotonous stimuli associated with TENS.However, they found that delivering currentsrandomly to six different points on the bodyusing a TENS-like device called a Codetronmarkedly reduced the habituation response(Table 17.2). Fargas-Babjak and colleagues(Fargas-Babjak, Rooney and Gerecz, 1989;Fargas-Babjak, Pomeranz and Rooney, 1992)performed a 6 week double-blind randomisedplacebo controlled pilot trial of the effectivenessof Codetron on osteoarthritis of the hip/kneeand reported beneficial effects. Some TENSmanufacturers have tried to overcome the prob-lem of habituation by including random pulsedelivery or frequency-modulated pulse deliverysettings to their standard TENS devices. However,these devices have met with varied success.

If patients report that they are responding lesswell to TENS over time it may be worth experi-menting with the electrical characteristics ofTENS or with electrode placements to try andimprove analgesia. It may also be worth consid-ering temporary withdrawal of TENS treatmentso that an objective assessment of the contribu-tion of TENS to pain relief can be made. When

this is done patients may report that their painworsens in the absence of TENS, demonstratingthat TENS was in fact beneficial.

HAZARDS ANDCONTRAINDICATIONSContraindicationsContraindications to TENS are few and mostlyhypothetical (Box 17.2) with few reported casesof adverse events associated with TENS in theliterature. Nevertheless, therapists should becautious when giving TENS to certain groups ofpatients.

Those suffering from epilepsy (Scherder, VanSomeren and Swaab, 1999): if the patient were toexperience a problem while using TENS, from alegal perspective it might be difficult to excludeTENS as a potential cause of the problem.

Women in the first trimester of pregnancy:TENS effects on fetal development are as yetunknown (although there are no reports of it beingdetrimental). To reduce the risk of inducing labour,TENS should not be administered over a pregnantuterus although TENS is routinely administeredon the back to relieve pain during labour.

Patients with cardiac pacemakers: this isbecause the electrical field generated by TENScould interfere with implanted electrical devices.


Box 17.2 Contraindications

Undiagnosed pain (unless recommended by amedical practitioner)

Pacemakers (unless recommended by a cardiologist) Heart disease (unless recommended by a

cardiologist) Epilepsy (unless recommended by a medical

practitioner) Pregnancy:

first trimester (unless recommended by a medicalpractitioner)

over the uterus

Do not apply TENS: over the carotid sinus on broken skin on dysaesthetic skin Internally (mouth)

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Rasmussen et al. (1988) reported that TENS didnot interfere with pacemaker performance in 51patients although TENS may induce artifacts inmonitoring equipment (Hauptman and Raza,1992; Sliwa and Marinko, 1996). Chen et al.(1990) reported two cases of a Holter monitordetecting interference of a cardiac pacemaker byTENS and in both instances the sensitivity of thepacemaker was reprogrammed to resolve theproblem. These authors suggest that carefulevaluation and extended cardiac monitoringshould be performed when using TENS withpacemakers. Therapists wishing to administerTENS to a patient with a cardiac pacemaker orany cardiac problem should always discuss thesituation with a cardiologist.

TENS should not be applied internally(mouth), or over areas of broken or damaged skin.

Therapists should ensure that a patient hasnormal skin sensation prior to using TENS as ifTENS is applied to skin with diminished sensa-tion the patient may be unaware that they areadministering high-intensity currents and thismay result in a minor electrical skin burn.

TENS should not be delivered over theanterior part of the neck as currents may stimu-late the carotid sinus leading to an acutehypotensive response via a vasovagal reflex.TENS currents may also stimulate laryngealnerves, leading to a laryngeal spasm.

Hazards Patients may experience skin irritation with

TENS such as reddening beneath or aroundthe electrodes. This is commonly due to dermati-tis at the site of contact with the electrodesresulting from the constituents of electrodes,electrode gel or adhesive tape (Corazza et al.,1999; Fisher, 1978; Meuleman, Busschots andDooms Goossens, 1996a, b). The developmentof hypoallergenic electrodes has markedlyreduced the incidence of contact dermatitis.Patients should be encouraged to wash the skin(and electrodes when indicated by the manufac-turer) after TENS and to apply electrodes tofresh skin on a daily basis.

It is crucial that patients are educatedon the appropriate administration of TENS.For example, patients (and therapists) should beencouraged to follow set safety procedureswhen applying and removing TENS (Box 17.3)to reduce the chance of an electric shock. Ifpatients are to borrow a TENS device from aclinic they should be informed that they shouldnot use TENS while operating vehicles or poten-tially hazardous equipment. In particular, dri-vers of motor vehicles should never use TENSwhile driving as a sudden surge of current maycause an accident. From a legal perspective itwould be wise for TENS users to place theirTENS device in a glove compartment wheneverdriving as the cause of an accident may beattributed to TENS if it were attached to a dri-vers belt (even if it was switched off). TENS canbe used at bedtime providing the device has atimer so that it automatically switches off.Patients should be warned not to use TENS in


Box 17.3 Safety protocols for TENS

Protocol for the safe application of TENS Check contraindications with patient. Test skin for normal sensation using blunt/sharp test. TENS device should be switched off and electrode

leads disconnected. Set electrical characteristics of TENS while device is

switched off (see Tables 17.5 and 17.6). Connect electrodes to pins on lead wire and position

electrodes on patients skin. Ensure TENS device is still switched off and connect

the electrode wire to the TENS device. Switch the TENS device ON. Gradually (slowly) increase the intensity until the

patient experiences the first tingling sensation fromthe stimulator.

Gradually (slowly) increase the intensity further untilthe patient experiences a strong but comfortabletingling sensation.

This intensity should not be painful or cause musclecontraction (unless intense TENS or AL-TENS arebeing used).

Protocol for the safe termination of TENS Gradually (slowly) decrease the intensity until the

patient experiences no tingling sensation. Switch the TENS device OFF. Disconnect the electrode wire from the TENS device. Disconnect electrodes from the pins on lead wire. Remove the electrodes from the patients skin.

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the shower or bath and keep TENS appliancesout of the reach of children.

SUMMARYTENS is used extensively in health care to man-age painful conditions because it is cheap, safeand can be administered by patients themselves.Success with TENS depends on appropriateapplication and therefore patients and therapistsneed an understanding of the principles ofapplication. When used in its conventional formTENS is delivered to selectively activate Aafferents leading to inhibition of nociceptivetransmission in the spinal cord. It is claimedthat the mechanism of action and analgesic pro-file of AL-TENS and intense TENS differ from

conventional TENS and they may prove usefulwhen conventional TENS is providing limitedbenefit. Systematic reviews of RCTs report thatthere is weak evidence to support the use ofTENS in the management of postoperative andlabour pain. However, these findings have beenquestioned as they contrast with clinical experi-ence and it would be inappropriate to dismissthe use of TENS in acute pain until the reasonsfor the discrepancy between experience andpublished evidence is fully explored. Systematicreviews are more positive about the effective-ness of TENS in chronic pain. However, better-quality trials are required to determinedifferences in the effectiveness of different typesof TENS and to compare the cost effectiveness ofTENS with conventional analgesic interventionsand other electrotherapies.


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