Electrotherapy



Electrotherapy


Tim Watson



Introduction


Electrotherapy has been a component of physiotherapy practice since the early days of the profession. Modern electrotherapy use deserves to be evidence-based and the modalities used judiciously. When used appropriately, electrotherapy modalities have a demonstrable capacity to achieve significant benefit. Used unwisely, they will either do no good at all or, worse still, aggravate the clinical condition. In addition to the delivery skills of each modality, there is a critical skill in making the appropriate clinical decision as to which modality to use and when.


It is commonly argued that electrotherapy has little, or no, value in modern physiotherapy practice, and that it lacks an evidence base and so should be ‘left out’ of the treatment options. It is not uncommon to find whole clinics or departments that have decided not to deliver electrotherapy any longer on this premise. As you will see from the information in this chapter (which is not a full review of the evidence – just a brief summary of it), there is a substantial evidence base behind these modalities and if we were to deliver ‘evidence-based therapy’, then electrotherapy would be included – not for everything – but certainly for a significant range of musculoskeletal and associated presentations.


With regard the mechanisms by which each modality achieves its effects, it is important to realise that it is not the modality per se which brings about the therapeutic benefit. The applied energy stimulates or induces a physiological response. It is the physiological response which, in turn, brings about the therapeutic effects. The key to the application of electrotherapy is the relationship between these concepts. The therapist working with an electrotherapy modality is using, or manipulating, the physiological changes in order to achieve the desired effect. It may seem to be a pedantic argument, but it is a critical point. The outcome of the therapy is achieved through physiological manipulation and this concept applies to the application of all electrotherapy modalities.


A further very important concept is that the electrotherapy intervention is only a component of the overall treatment package. It is rarely appropriate for a patient to receive electrotherapy in isolation. It is most effectively combined with a range of manual therapies, exercise, advice and education. The elements of the treatment package need to be complementary. Careful construction of treatment programmes will enable the most effective outcome. It may be that the electrotherapy components are only utilised for the first few sessions or, indeed, only for the sessions later in a series. Electrotherapy is not an essential component for all patients and should only be used when and where appropriate.



Electrotherapy versus electrophysical agents


There is a general shift to move away from the term ‘electrotherapy’ toward a more encompassing term of ‘electro-physical agents’ (EPAs). This is largely to be welcomed in that electrotherapy, in the strictest sense of the term, would only apply to those modalities which involved the delivery of electrical, and possibly also electromagnetic, energies (e.g. transcutaneous electrical nerve stimulation (TENS), interferential therapy (IFT)). Ultrasound, light, vibration and various heat therapies, for example, would not fit this more narrow definition. As a term, EPA is certainly more inclusive and thus a more accurate reflection of the wide range of modalities employed in physiotherapy (Watson 2010). A general shift away from the term electrotherapy and towards a more common use of EPAs is anticipated over the next few years. The term electrotherapy in the context of this chapter will be used in its broad, inclusive sense rather than as a strict definition of the energy applied.



Scope


The aim of this chapter is to enable the reader to identify the key issues in electrotherapy, using commonly employed modalities as examples. It does not purport to fully examine or explain the evidence for every modality, and there are many ‘modalities’ that will hardly be mentioned. This does not mean that they are unimportant or worthless: it is a realistic reflection of the complexity of modern electrotherapy practice and the limits of what can be achieved in a single chapter. Further details are available in the standard texts (e.g. Robertson et al. 2006; Watson 2008b; Belanger 2010) and reference will be made in this chapter to further useful research articles and resources. In terms of clinical practice, the most widely used modalities in the UK are ultrasound, IFT and TENS, with pulsed shortwave with laser and neuromuscular electrical stimulation (NMES) following close behind. Microcurrent therapy, shockwave therapy, low intensity pulsed ultrasound (LIPUS) and some new radio frequency (RF) applications have been added to this chapter as evidenced and emerging interventions.



Model of electrotherapy


Electrotherapy modalities follow a very straightforward model that is presented below. The model (Figure 19.1) identifies that the delivery of energy from a machine or device is the starting point of the intervention. The energy delivery to the tissues results in a change in one or more physiological events – some of which are very specific while others are multifaceted or more general. The capacity of the applied energy to influence physiological events is key to the process. The physiological shift that results from the energy delivery is used in practice to generate what is commonly referred to as therapeutic effects.



The clinical application of the model is best achieved by what appears to be a reversal of this process. Starting with the patient and their problems, identified from the clinical assessment, the treatment priorities can be established and the rationale for the treatment determined. Having established the therapeutic target (or aim), move one step back through the model and identify which physiological events/processes need to be activated or stimulated in order to achieve the outcome. Once the required physiological changes have been identified, moving one step further back will enable a modality decision to be made. This is based on the existing evidence relating to which modalities stimulate which physiological effects. If there is no electrotherapy modality that is capable of achieving the intended effects, then electrotherapy would have no rational use in the management of this particular patient. The effects of electrotherapy appear to be modality dependent. This is a critical decision, in that each modality has a limited subset of effects which are fundamentally different from another modality. It is certainly not the case that some modalities are universally ‘better’ than others – it is the case that some modalities are more effective at achieving particular therapeutic effects.


Having identified the modality that is best able to achieve the effects required, the next clinical stage is to make a ‘dose’ selection. Not only is it critical to apply the right modality, but it needs to be applied at the appropriate ‘dose’ in order for maximal benefit to be achieved. There is a substantial and growing body of evidence that the same modality can be applied at different doses and the results will be different (Watson 2010). An example might be the use of ultrasound energy. Applied at a low ‘therapeutic’ dose, it can stimulate tissue repair and healing. Applied at a much higher dose (high intensity focussed ultrasound (HIFU) ) it can be used to ablate tumour tissue. The energy form is the same, but by varying the applied ‘dose’ the outcome is clearly different.


One might argue that this is an extreme example, which in some ways it is, but the point is that the effects of the therapy are both modality- and dose-dependent. There are ‘therapeutic windows’ in electrotherapy (as there are in almost all therapeutic interventions) and in order to achieve the ‘best’ outcome, it is essential to get as close to this window as one possibly can.


This fundamental model used to explain electrotherapy could be applied to many interventions, including drug therapy, manual therapy and exercise therapy. All involve the use of an intervention in order to achieve a physiological shift or change. It is this change that is the therapeutic tool. The treatment is just a tool to stimulate the physiological change and electrotherapy is therefore little different from manual therapy or any other intervention except that a ‘machine’ is employed as an initiator of the physiological shift. It is a tool that when applied at the right time at the right dose and for the right reason has the capacity to be beneficial. Applied inappropriately, it is not at all surprising that is has the capacity to achieve nothing or, in fact, to make things worse. The skilful practitioner uses the available evidence combined with experience to make the best possible decision, taking into account the psychosocial and holistic components of the problem – it is not a simple reductionist solution.



Therapeutic windows


Windows of opportunity are topical in many areas of therapy and medical practice, and are not a new phenomenon. It has long been recognised that the ‘amount’ of a treatment is a critical parameter. This is no less true for electrotherapy than for other interventions. There are literally hundreds of research articles that illustrate that the same modality applied at a different ‘dose’ will produce a different outcome.


Given the research evidence, there appear to be several aspects to this issue. Using a very straightforward model, there is substantial evidence, for example, that there is an amplitude or strength window. An energy delivered at a particular amplitude has a beneficial effect while the same energy at a lower amplitude may have no demonstrable effect (too low) or a tissue destruction effect (too high in therapy terms). Laser therapy offers an obvious example: one level will produce a distinct cellular response while a higher dose can be considered to be destructive. Karu (1987) demonstrated and reported these principles related to laser energy and the research produced since have served to reinforce the concept (e.g. Vinck et al. 2003). Further examples of amplitude windows can be seen easily in the work of Hill et al. (2002), Reher et al. (2002), Miller and Gies (1998), Cleary (1987) and Pereira et al. (2002), and have been more extensively reviewed in Watson (2010).


Along similar lines, ‘frequency windows’ are also apparent. A modality applied at a specific frequency or pulsing regime might have a measurable benefit, while the same modality applied using a different pulsing profile may not appear to achieve equivalent results. Examples can be found in many articles, including Young and Dyson (1990a), Young and Dyson (1990b) and Sontag (2000).


Electrical stimulation frequency windows have been proposed and there is clinical and laboratory evidence to suggest that there are frequency-dependent responses in clinical practice. TENS applied at frequency X appears to have a different outcome to TENS applied at frequency Y in an equivalent patient population. Studies by Sluka et al. (2006), Han et al. (1991) and Palmer et al. (1999) illustrate the point.


Assuming that there are likely to be more than two variables to the real world model, some complex further work needs to be invoked. There is almost certainly an energy or time-based window (e.g. Hill et al. 2002) and then another factor based on treatment frequency (number of sessions a week or treatment intervals). Work continues to identify the more and less critical parameters for each modality across a range of clinical presentations and a more detailed review of these concepts can be found in Watson (2010).



Electrotherapy modality grouping


No matter which classification of the numerous electrotherapy modalities is adopted, it can easily be criticised. The groupings used in Figure 19.2 are one way of looking at the scope of electrotherapy, although it is not presented as the ‘right’ model.



The division into three main subgroups – electrical stimulation, thermal modalities and non-thermal modalities – is the theme that will be followed in this chapter. The electrical stimulation modalities have a common mode of action in that their primary effect will be on nerve (and, in some circumstances, muscle) tissue. Commonly employed forms of electrical stimulation include TENS, IFT, various forms of muscle stimulation (e.g. NMES, functional electrical stimulation (FES)) and many others which will not be considered in any detail in this chapter. Microcurrent therapy is ‘different’ in that its primary mode of action is to influence tissue repair rather than stimulate nerve, and thereby falls into an overlap zone between categories.


The thermal modalities group includes various forms of heating that have been used for many years in therapy, including infrared therapy, conductive heating, wax therapy, hot packs and the ‘deeper’ heating modalities – shortwave diathermy (SWD), other RF applications and microwave diathermy (MWD). The use of the heating modalities has diminished in clinical practice over recent years and much as some of the interventions employed in the past may lack evidence, there are compelling reasons to keep heat therapies in the clinical repertoire and there are areas where the use of heat-based therapies is likely to re-emerge as a strongly evidence-based intervention. Examples of modern heat therapy applications include Michlovitz et al. (2004), Usuba et al. (2006), Mayer et al. (2006), and Leung and Cheung (2008).


The non-thermal modalities are grouped together on the basis that if delivered at sufficiently high levels, any of these modalities could produce a significant or even destructive heating effect in the tissues. If they are delivered at sufficiently low dose, they are considered to be ‘non-thermal’ in their primary effect. This is somewhat misleading in that any energy delivered to the tissues which is subsequently absorbed will achieve a heating effect. The non-thermal label is derived from the fact that there is no gross thermal change and the patient is not able to perceive a thermal effect. More properly, these should possibly be referred to as microthermal or subthermal modalities. Ultrasound, pulsed shortwave therapy (PSWT) and laser therapy fall most obviously into this group, and various forms of magnetic therapy which are gaining ground in the literature would also be best suited to this area, as would some of the developing low-power RF applications. The current clinical use of these modalities is primarily directed at enhancing the process of healing and tissue repair. Ultrasound presents a dilemma in that some practitioners use it as a modality which is employed with the deliberate intention of heating the tissues, although the evidence would support its ‘non-thermal’ use over and above its thermal application. In the context of this chapter, microcurrent therapy will be included in the electrical stimulation section (though it could fall into either group) and shockwave therapy will be included in the non-thermal group in that it is an energy delivery which is not intended as a thermal agent and is not an electrical stimulation modality – thus illustrating that any classification system fails at some point!



Electrical stimulation modalities


General principles of electrical stimulation


The general principles of electrical stimulation (ES) in the context of commonly employed electrotherapy modalities focusses on the use of electrical currents (usually in the form of discrete pulses) to initiate action potentials in nerves. This would be true for modalities like TENS, NMES in its various forms and interferential therapy (though this modality does not employ discrete pulses). There are other forms of intervention which employ alternative mechanisms – one of which is iontophoresis (which uses a direct or pulsed direct current to enhance the delivery of a chemical substance or drug through the skin) and microcurrent-type therapies which are delivered at a level that is insufficient to stimulate a nerve action potential but which do appear to have an effect on the repair responses in wounds and damaged tissues.



Nerve action potentials


Assuming that the majority of ES modalities work by means of nerve activation, a brief examination of how this is achieved would be beneficial. A nerve in its resting state is said to be ‘polarised’. When an action potential is transmitted along a nerve, the membrane at the point of the action potential is momentarily depolarised before returning to its normal state (repolarisation). Essentially, the employment of an electrical current or pulse is as a means of initiating an action potential along the course of the nerve. Once the action potential has been initiated by this exogenous (external to the body) signal, then it will continue along the nerve (whether sensory or motor) in the normal fashion: the electrical stimulator is simply an initiator of the activity. If the nerve is stimulated ten times a second, then there will be ten action potentials a second initiated. If stimulated 100 times a second, predictably, it will fire 100 times a second. There are some constraints to this relationship based on refractory periods and threshold potentials which are beyond the scope of this chapter but are usefully reviewed in most standard electrotherapy texts (Robertson et al. 2006; Watson 2008b).


The nerve being stimulated is largely unable to differentiate between different types of electrical stimulation. It is simply responding to an external stimulus and will fire accordingly. The main difference between stimulators is that they are set to have an optimal effect on particular nerve types such that a TENS machine will be the most efficient device to use to achieve stimulation of a sensory nerve and a NMES device will be optimal in stimulating motor nerves. TENS will, of course, stimulate both sensory and motor nerves; however, it is more efficient in having an effect on the sensory nerves. The same would be true for a NMES stimulator – the effect is not exclusively a motor one, it is just that the motor effect is dominant.


Bearing these principles in mind, the commonly employed ES modalities will be considered in brief with their primary clinical applications described. There are many alternative and additional widespread uses of these modalities that are evidenced, but are beyond the scope of this overview chapter. More comprehensive considerations can be found in the references supplied, as well as in the major texts.



Transcutaneous electrical nerve stimulation (TENS)


TENS is a method of electrical stimulation which aims primarily to provide a degree of pain relief (symptomatic) by specifically exciting sensory nerves and thereby stimulating either the pain gate mechanism and/or the opioid system (Walsh 1997; Sluka and Walsh 2003; Johnson 2008). Strictly speaking, any form of electrical stimulation applied with surface electrodes that stimulates nerves can be referred to as TENS but in clinical practice the term is most commonly employed in the context identified above.


The different methods of applying TENS relate to these different physiological mechanisms. Success is not guaranteed with TENS. The percentage of patients who obtain pain relief will vary, but would typically be in the region of ≥70% for acute-type pains and ≥60% for more chronic pains. Both of these are significantly ‘better’ than the placebo effect.


The technique is non-invasive and has few side effects when compared with drug therapy. Modern TENS devices may be analogue (Figure 19.3a,b) or digital (Figure 19.3c,d) in design. Although their outputs are essentially the same, their control systems differ. TENS is normally included in the multimodal stimulators (Figure 19.3f). Most TENS applications are now made using self-adhesive, pre-gelled electrodes (Figure 19.3e) which have several advantages, including a lower allergy incidence, a reduced cross-infection risk, easier application and lower overall cost, although some practitioners retain the older, impregnated carbon rubber electrode systems. Specialist TENS variations include ‘maternity’ TENS (Figure 19.3g), which have a simple to operate ‘boost’ function.




Machine parameters


The main parameters (or settings) on a TENS machine are those that are influential in terms of sensory nerve stimulation, which is the primary aim of the modality. The location of these controls on typical analogue and digital TENS machines is illustrated in Figure 19.4.



The current intensity (A) (strength) will typically be in the range of 0–80 mA, though some machines may provide higher outputs. Although this is a small current, it is sufficient because the primary target for the therapy is the sensory nerves and so long as sufficient current is passed through the tissues to depolarise these nerves, the modality can be effective.


The pulse rate (B) will normally be variable from about 1 or 2 pulses per second (pps) up to 200 pps or more. To be clinically effective, it is suggested that the TENS machine should cover a range of 2–150 Hz.


In addition to the stimulation rate, the duration (or pulse width) of each pulse (C) may be varied from about 40 to 250 microseconds (µs). Recent evidence would suggest that this is possibly a less important control than the intensity and frequency. These are short duration pulses which are effective because sensory nerves have a relatively low threshold and will respond well to short duration, rapidly changing pulses. There is generally no need to apply a prolonged pulse in order to force the nerve to depolarise.


Most modern machines will offer a BURST mode (D) in which the pulses will be delivered in bursts or ‘trains’, usually at a rate of 2–3 bursts per second. Finally, a modulation mode (E) may be available which employs a method of making the pulse output less regular and therefore minimising the accommodation effects which are often encountered with this type of stimulation.


Machines most commonly offer a dual channel output, i.e. two pairs of electrodes can be stimulated simultaneously. In some circumstances this can be a distinct advantage, though it is interesting that most patients and therapists tend to use just a single channel application.


The pulses delivered by TENS stimulators vary between manufacturers, but tend to be asymmetrical biphasic modified square wave pulses. By employing biphasic pulses it means that there is usually no net direct current component, thus minimising any skin reactions owing to the build up of electrolytes under the electrodes.



Mechanism of action


The type of stimulation delivered by the TENS unit aims to excite (stimulate) the sensory nerves and, by so doing, activate specific natural pain relief mechanisms. For convenience, if one considers that there are two primary pain relief mechanisms which can be activated – the pain gate mechanism and the endogenous opioid system – the variation in stimulation parameters used to activate these two systems will be briefly considered.


Pain relief by means of the pain gate mechanism primarily involves activation (excitation) of the Aβ sensory fibres, thus reducing the transmission of the noxious stimulus from the ‘c’ fibres through the spinal cord and on to the higher centres. The Aβ fibres appear to respond preferentially when stimulated at a relatively high rate (in the order of 80 or 90–130 Hz). It is difficult to find support for the concept that there is a single frequency that works best for every patient, but this range appears to cover the majority of individuals (Walsh 1997). This TENS mode is delivered with high frequency (traditional/normal) TENS.


An alternative approach is to stimulate the Aδ fibres which respond preferentially to a much lower rate of stimulation (in the order of 2–5 Hz), which will activate the opioid mechanisms and provide pain relief by causing the release of an endogenous opiate (encephalin) in the spinal cord which will reduce the activation of the noxious sensory pathways (Han et al. 1991; Walsh 1997; Sluka et al. 2006). This TENS mode is delivered with low-frequency (acupuncture (AcuTENS) ) TENS.


A third possibility is to stimulate both nerve types at the same time by employing a burst mode stimulation. In this instance, the higher frequency stimulation output (typically at about 100 Hz) is interrupted (or burst) at the rate of about 2–3 bursts per second. When the machine is ‘on’, it will deliver pulses at the 100 Hz rate, thereby activating the Aβ fibres and the pain gate mechanism, but by virtue of the rate of the burst, each burst will produce excitation in the Aδ fibres, therefore stimulating the opioid mechanisms. For some patients this is by far the most effective approach to pain relief, though as a sensation, numerous patients find it less acceptable than the other forms of TENS.




Acupuncture TENS (lo-TENS, AcuTENS)


When using AcuTENS, TENS is used at a lower stimulation frequency (2–5 Hz) with longer duration pulses (200–250 µs). The intensity employed will usually need to be greater than with the traditional TENS – a definite, strong sensation but still one that is not painful (see below). A minimally useful stimulation of 30 minutes should be delivered. It takes some time for the opioid levels to build up with this type of TENS and hence the onset of pain relief may be slower than with the traditional mode. Once sufficient opioid has been released, however, it will keep on working after cessation of the stimulation. Many patients find that stimulation at this low frequency at intervals throughout the day is an effective strategy. The ‘carry over’ effect may last for several hours in the clinical setting, though timeframes of rather more limited duration have been demonstrated by Chesterton et al. (2002).



Brief intense TENS


Brief intense TENS is a mode that can be employed to achieve rapid pain relief, but some patients may find the strength of the stimulation too intense and will not tolerate it for sufficient duration to make the treatment worthwhile. The pulse frequency applied is high (in the 90–130 Hz band) and the pulse width is also high (≥200 µs). The current is delivered at, or close to, the tolerance level for the patient such that they would not want the machine turned up any higher. In this way, the energy delivery to the patients is relatively high when compared with the other approaches. It is suggested that 15–30 minutes at this stimulation level is the most that would normally be used. Pain relief onset is rapid and marked if the patient can cope with the stimulation intensity (Walsh 1997; Sluka and Walsh 2003).




Frequency selection


With all of the above mode guides, it is probably inappropriate to identify very specific frequencies that need to be applied to achieve a particular effect. If there was a single frequency that worked for everybody, it would be much easier, but the research does not support this concept. The patient (or the therapist) needs to identify the most effective frequency for their pain and manipulation of the stimulation frequency dial or button is the best way to achieve this. Patients who are told to leave the dials alone are less likely to achieve optimal effects. Frequency ranges within which the ‘ideal’ is likely to be found are those identified above. Some TENS devices do not enable control of specific pulse rate and duration settings, but instead offer ‘automatic’ programmes that effectively deliver some of each of the effective modes. This can be considered advantageous (that the patient has less to worry about on the machine) but others consider it a broad brush approach and the amount of time spent at the optimal setting for the patient is minimal; hence, it may, in fact, be ineffective. There is currently no published evidence that supports the ‘bit of everything’ approach in the clinical environment.



Stimulation intensity


It is not possible to describe the treatment current strength in terms of how many (milli)amps should be applied. The most effective intensity management appears to be related to what the patient feels during the stimulation and this may vary from session to session, though will tend to be fairly consistent for any individual patient. As a general guide, it appears to be effective to go for a ‘definitely there but not painful’ level for the normal (high) TENS and a ‘strong but not painful’ level for the acupuncture (low) mode (Sluka and Walsh 2003). Figure 19.5 illustrates these settings on a ‘subjective’ scale. Some evidence (e.g. Bjordal et al. 2003; Aarskog et al. 2007) suggests that stronger stimulation might be more effective for clinical pain states.




Electrode placement


In order to get the maximal benefit from the modality, target the stimulus at the appropriate spinal cord level (appropriate to the pain). Placing the electrodes either side of the lesion – or painful areas – is the most common mechanism employed to achieve this. There are many alternatives that have been researched and found to be effective – most of which are based on the appropriate nerve root level/spinal cord segment:



It is beyond the scope of this chapter to detail specific electrode combinations for specific clinical problems and, in any case, would probably be inappropriate to do so. The TENS literature covers electrode placement in some detail and the interested reader is referred to useful specific texts in this context (Walsh 1997; Johnson 2008).


If the pain source is vague, diffuse or particularly extensive both channels can be employed simultaneously. A two-channel application can also be effective for the management of a local plus a referred pain combination, with one channel used for each component. Most standard machines do not allow different stimulation parameters to be set for each channel, though there are some devices that will allow this possibility, for example channel A on low frequency, opioid setting and channel B on higher frequency pain gate setting.


Numerous systematic and Cochrane Reviews have been published relating to the application of TENS for several different clinical pain groups (e.g. Rutjes et al. 2009; Walsh et al. 2009). Many of these come to an ‘inconclusive’ conclusion, though this may be related to dose-related issues (therapeutic windows) (Johnson and Martinson 2007; Watson 2010) and methodological limitations of the research rather than the failure of TENS to have a significant effect.



Interferential therapy (IFT)


The basic principle of IFT is to utilise the strong physiological effects of low frequency (≅<250 pps) electrical stimulation of muscle and nerve tissues without the associated pain encountered with low frequency stimulation (Watson 2000; Palmer and Martin 2002).


IFT is delivered using either dedicated main s-powered interferential devices (Figure 19.6), portable (battery-powered) devices (Figure 19.7) or multimode units that include IFT stimulation among several other treatment modes (Figure 19.8).





To produce low frequency effects at sufficient intensity at depth, patients may experience considerable discomfort in the superficial tissues (i.e. the skin). This is a result of the compound impedance of the skin being inversely proportional to the frequency of the stimulation. The result of applying this higher frequency is that it will pass more easily through the skin, requiring less electrical energy input to reach the deeper tissues and giving rise to less discomfort.


The effects of tissue stimulation with these ‘medium frequency’ currents (medium frequency in electromedical terms is usually considered to be 1 kHz–100 kHz) is not fully understood and while it is likely to have an effect, little detail is currently known though it is assumed not to directly stimulate nerve. Ward (2009) recently reviewed the key issues with medium frequency currents.


Interferential therapy utilises two of these medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross and they literally interfere with each other. This interference gives rise to an interference (beat frequency) which has the characteristics of low frequency stimulation. In effect the interference mimics a low frequency stimulation in the cross over (interference) zone.


The exact frequency of the resultant interference (or beat frequency) can be controlled by the input frequencies. If, for example, one current was at 4000 Hz and its companion current at 3900 Hz, the resultant beat frequency would be at 100 Hz carried on a medium frequency 3950 Hz amplitude modulated current (Figure 19.9).



By careful manipulation of the input currents it is possible to achieve any beat frequency that is needed clinically. Modern machines usually offer beat frequencies of 1–150 Hz, though some offer a choice of up to 250 Hz or more. Some machines offer a range of ‘carrier’ frequencies, i.e. other than 4000 Hz. The evidence would suggest that the higher the carrier frequency, the less discomfort will be experienced by the patient and therefore where there is a choice, the higher carrier frequency should be employed.


The use of two-pole IFT stimulation is made possible by electronic manipulation of the currents – the interference occurs within the machine instead of in the tissues. There is no known physiological difference between the effects of IFT produced with two- or four-electrode systems; in fact, the pre-modulated currents can be considered superior in clinical effectiveness terms (e.g. Ozcan et al. 2004). The key difference is that with a four-pole application the interference is generated in the tissues and with a two-pole treatment, the current is ‘pre-modulated’, i.e. the interference is generated within the machine unit.


Whichever generation system is employed, the treatment effect is generated from low frequency stimulation, primarily involving the peripheral nerves. Low frequency nerve stimulation is physiologically effective (as with TENS and NMES) and this is the key to IFT intervention.



Frequency sweep


Nerves will accommodate to a constant signal and a sweep (or gradually changing frequency) is often used to overcome this problem. The principle of using the sweep is that the machine is set to automatically vary the effective stimulation frequency using either pre-set or user-set sweep ranges. The sweep range employed should be appropriate to the desired physiological effects (see below). It has been repeatedly demonstrated that ‘wide’ sweep ranges are ineffective in the clinical environment. The clinical advantage of the sweep treatment application, beyond that of minimising the accommodation effects, are that a range of treatment frequencies can be automatically applied (Watson 2000).



The pattern of the sweep makes a significant difference to the stimulation received by the patient. Most machines offer several sweep patterns, though there is very limited ‘evidence’ to justify some of these options. In the classic ‘triangular’ sweep pattern, the machine gradually changes from the base to the top frequency, usually over a time period of six seconds, though some machines also offer one- or three-second options. In the example illustrated (Figure 19.10), the machine is set to sweep from 90 to 130 Hz, employing a triangular sweep pattern. All frequencies between the base and top frequencies are delivered in equal proportion.



Other patterns of sweep can be produced on many machines, for example a rectangular (or step-like) sweep. This produces a very different stimulation pattern in that the base and top frequencies are set, but the machine then ‘switches’ between these two specific frequencies rather than gradually changing from one to the other. Figure 19.11 illustrates the effect of setting a 90–130 Hz rectangular sweep.



There is a clear difference between these examples, even though the same ‘numbers’ are set: one will deliver a full range of stimulation frequencies between the set frequency levels and the other will switch from one frequency to the other. There are numerous other variations on this theme and the ‘trapeziodal’ sweep (Figure 19.12) is effectively a combination of these two.



The only sweep pattern for which ‘evidence’ appears to exist is the triangular sweep. The others are perfectly safe to use, but whether they are clinically effective or not remains to be shown.



Physiological effects and clinical applications


It has been suggested that IFT works in a ‘special way’ because it is ‘interferential’ as opposed to ‘normal’ stimulation. The evidence for this special effect is lacking and it is most likely that IFT is just another means by which peripheral nerves can be stimulated. Many regard it as more acceptable than other forms of electrical stimulation as it generates less (skin) discomfort (e.g Shanahan et al. 2006).


The clinical application of IFT therapy is based on peripheral nerve stimulation (frequency) data, though it is important to note that much of this information has been generated from research with other modalities, and its transfer to IFT is assumed, rather than proven. There is a lack of IFT-specific research compared with other modalities (e.g. TENS, NMES).


The are four main clinical applications for which IFT appears to be used:



In addition, claims are made for its role in stimulating healing and repair, though they are not specifically covered in this section. As IFT acts primarily on nerve, the strongest effects are likely to be those which are a direct result of such stimulation (i.e. pain relief and muscle stimulation). The other effects are more likely to be secondary consequences of these.



Pain relief


Electrical stimulation for pain relief has widespread clinical use, though the direct research evidence for the use of IFT in this role is limited. Logically one could use the higher frequencies (90–130 Hz) to stimulate the pain gate mechanisms and thereby mask the pain symptoms. Alternatively, stimulation with lower frequencies (2–5 Hz) can be used to activate the opioid mechanisms, again providing a degree of relief. These two different modes of action can be explained physiologically and will have different latent periods and varying duration of effect (same as for TENS). It remains possible that pain relief may be achieved by stimulation of the reticular formation at frequencies of 10–25 Hz or by blocking C fibre transmission at >50 Hz. Although both of these latter mechanisms have been proposed with IFT, neither have been categorically demonstrated (Palmer and Martin 2008).


A good number of studies (e.g. Johnson and Tabasam 2003; Hurley et al. 2004; McManus et al. 2006; Jorge et al. 2006; Walker et al. 2006; Lau et al. 2008; Fuentes et al. 2010) provide substantive evidence for a pain relief effect of IFT.



Muscle stimulation


Stimulation of the motor nerves can be achieved with a wide range of frequencies. Clearly, stimulation at low frequency (e.g. 1 Hz) will result in a series of twitches, while stimulation at 50 Hz will result in a tetanic contraction. There is limited evidence at present for the ‘strengthening’ effect of IFT (evidence does exist for some other forms of electrical stimulation), though the article by Bircan et al. (2002) suggests that it might be a possibility. On the basis of the current evidence, the contraction brought about by IFT is no ‘better’ than would be achieved by active exercise, though there are clinical circumstances where assisted contraction is beneficial. For example, to assist the patient to appreciate the muscle work required (similar to surged Faradism used previously, but much less uncomfortable). For patients who cannot generate useful voluntary contraction, IFT may be beneficial as it would be for those who, for whatever reason, find active exercise difficult. There is no evidence that has demonstrated a significant benefit of IFT over active exercise. The choice of treatment parameters will depend on the desired effect. The most effective motor nerve stimulation range with IFT appears to lie between approximately 10 and 20 Hz (maybe between 10 and 25 Hz).


Caution should be exercised when employing IFT as a means to generate clinical levels of muscle contraction in that the muscle will continue to work for the duration of the stimulation period (assuming sufficient current strength is applied). It is possible to continue to stimulate the muscle beyond its point of fatigue and short stimulation periods with adequate rest is probably a preferable option. Some IFT devices are capable of generating a ‘surged’ stimulation mode which might be advantageous in that fatigue would be minimised – this surged intervention would be similar to, but more comfortable than, Faradism.



Blood flow


There is very little, if any, quality evidence demonstrating a direct effect of IFT on local blood flow changes. Most of the work that has been done involves laboratory experimentation on animals or asymptomatic subjects, and most blood flow measurements are superficial, i.e. skin blood flow. Whether IFT is actually capable of generating a change (increase) in blood flow at depth remains questionable. The elegant study by Noble et al. (2000) demonstrated vascular changes at 10–20 Hz, though they were unable to identify clearly the mechanism for this change. The stimulation was applied via suction electrodes and the outcome could, therefore, be a result of the suction rather than the electrical stimulation, though this is largely negated by virtue of the fact that other stimulation frequencies were also delivered with the suction electrodes without significant flow changes. The most likely mechanism therefore is via muscle stimulation effects (IFT causing muscle contraction which brings about a local metabolic and thus vascular change). The possibility that the IFT is acting as an inhibitor of sympathetic activity remains a theoretical possibility rather than an established mechanism.


Based on current available evidence, the most likely option for IFT use as a means to increase local blood flow remains via the muscle stimulation mode; thus, the 10–20 or 10–25 Hz frequency sweep appears to be preferable.



Oedema


IFT has been claimed to be effective as a treatment to promote the reabsorption of oedema in the tissues. Again, the published, as opposed to the anecdotal, evidence is very limited in this respect and the physiological mechanism by which it could be achieved as a direct effect of IFT remains to be established. The preferable clinical option in the light of the available evidence is to use the IFT to bring about local muscle contraction(s) which, combined with the local vascular changes, could be effective in encouraging the reabsorption of tissue fluid. The use of suction electrodes may be beneficial, but also remains unproven in this respect.


A study by Jarit et al. (2003) demonstrated a change in oedema following knee surgery in an IFT group; however, a study by Christie and Willoughby (1990) failed to demonstrate a significant benefit on ankle oedema following fracture and surgery. The treatment parameters employed are unlikely to be effective given the information now available. If IFT has a capacity to influence oedema, the current evidence and physiological knowledge would suggest that a combination of pain relief (allowing more movement), muscle stimulation (above) and enhanced local blood flow (above) is the most likely combination to be effective and thus 10–20 Hz stimulation around the largest local muscle group is probably the most effective approach.



Treatment parameters


Stimulation can be applied using several different electrode systems (Figures 19.13 and 19.14). Pad electrodes and sponge covers are commonly employed; electroconductive gel is an effective alternative. The sponges should be thoroughly wet to ensure even current distribution. Self-adhesive pad electrodes are also available (similar to the newer TENS electrodes) and in the view of many practitioners make IFT application easier. The suction electrode application method has been in use for several years and while it is useful, especially for larger body areas like the shoulder girdle, trunk, hip and knee, it does not appear to provide any therapeutic advantage over pad electrodes. Care should be taken with regard to the maintenance of electrodes, electrode covers and associated infection risks (Lambert et al. 2000).




Electrode positioning should ensure adequate coverage of the area for stimulation. In some circumstances, a bipolar method is preferable if a longitudinal zone requires stimulation rather than an isolated tissue area. Placement of the electrodes should be such that a crossover effect is achieved in the desired area (when using the four-pole application).


Treatment times vary widely according to the usual clinical parameters of acute/chronic conditions and the type of physiological effect desired. In acute conditions, shorter treatment times of ten minutes may be sufficient to achieve the effect. In other circumstances, it may be necessary to stimulate the tissues for 20–30 minutes. It is suggested that short treatment times are adopted initially, especially with acute cases in the event of symptom exacerbation. These can be progressed if the aim has not been achieved and no untoward side effects have been produced. There is no research evidence to support the continuous progression of a treatment dose in order to increase or maintain its effect.



Muscle stimulation modalities


Historically, motor nerve stimulation in order to generate muscle contraction was a widely employed modality, most commonly in the form of Faradism. More recently, this intervention has fallen somewhat ‘out of favour’. There are, in fact, still circumstances where it could be of significant benefit, but in clinical practice using IFT (preferably in a surged mode) is probably a more effective means to the same end.


There has been a significant increase in the use of portable or mains-powered muscle stimulators which are effectively the modern replacement for Faradism. Figure 19.15 illustrates examples of dedicated battery-powered and multimodal mains-powered devices.



There is a growing range of chronic (meaning ‘not short-term’) electrical stimulation devices that are aimed at stimulating the motor nerve and hence bringing about muscle activity/contraction. This form of therapy goes by several names, but the most commonly applied are neuromuscular electrical stimulation (NMES), chronic NMES and neuromuscular stimulation (NMS). NMES is probably the preferred generic term.


It is suggested, with a growing body of evidence, that gains in strength, endurance capacity and function can be achieved with these types of stimulation. Much of the early work has been conducted in laboratory studies and also with athletes rather than typical patient groups. There are recent articles, however, that have demonstrated significant clinical benefit with patient groups, including strengthening of peripheral musculature (Talbot et al. 2003; Callaghan and Oldham 2004; Stevens et al. 2004; Lyons et al. 2005), work with shoulder problems in stroke patients (Chantraine et al. 1999; Ada and Foongchomcheay 2002), chronic obstructive pulmonary disease (COPD)/cardiac patients (Neder et al. 2002; Zanotti et al. 2003; Vivodtzev et al. 2006) and various forms of incontinence (Indrekvam et al. 2001; Amaro et al. 2003; Barroso et al. 2004). Useful reviews of this field of intervention are included in McDonaugh (2008), Robertson et al. (2006) and Lake (1992).


A further whole branch of neuromuscular-based stimulation is the so called functional electrical stimulation (FES). In this branch of electrotherapy, the explicit intent of the stimulation is to achieve controlled muscle activation in order to facilitate some aspect of functional activity. The range of applications is growing swiftly, especially with the recent advances in computer-controlled stimulators. One of the early, and most successful areas, is the dropped foot stimulator (Taylor et al. 1999; Sheffler et al. 2006) which assists patients – most often following a stroke – to achieve ankle dorsiflexion during the swing phase of gait utilising stimulation of the anterior tibial nerve. Modern devices incorporate a range of foot switches or pressure detectors which enable stimulation to be active at exactly the right part of the gait cycle (Figure 19.16). Other forms of FES include standing and gross walking activity with paraplegic patients which is also making significant gains with computerised control systems.




Microcurrent therapy


Microcurrent therapy is becoming increasingly employed in the clinical environment as a new and emerging modality (though, in fact, it is neither new nor emerging – it has strong published evidence dating back several decades).


It is interesting in that it appears to break all the classification ‘rules’ that were identified earlier in the chapter. It is not delivered with the intention of stimulating nerves, but rather plays to the bioelectric environment of the tissues, and therefore has a primary effect in terms of tissue repair (Watson 2006a; Poltawski and Watson 2009). The general characteristics of this type of therapy are that they utilise a direct current (pulsed or continuous) delivered at a very low amplitude (literally in the microamperage (millionths of an Amp) range) which is usually subsensory from the patient perspective. This type of therapy has already been shown to be effective in several clinical areas – most notably fracture repair (Simonis et al. 2003; Ciombor and Aaron 2005) and healing of open wounds (Watson 1996; Evans et al. 2001; Watson 2008a) though soft tissue repair research is now evolving and showing strong future potential. The use of this therapy in tissue injury treatment has been reviewed recently (Poltawski and Watson 2009).


Microcurrent therapy devices appear to be most effectively employed when used for hours a day (rather than minutes a week in the clinic) and therefore home-based therapy with small, inexpensive, portable devices (Figure 19.17) is a likely way forward. Microcurrent therapy can, of course, be delivered using mains-powered and multimodal devices (e.g. Chatanooga Intelect and Gymna 400) and it has yet to be clearly determined whether the small portable battery-powered home-based machines or the clinic-based ones will become the most widely employed.




Other forms of electrical stimulation


There are probably more ‘new’ electrical stimulation-type devices that come out in a year than in any other area of electrotherapy. Many of them are actually based on one of the three main areas identified in previous sections (TENS, IFT or NMES), though there are, of course, machines that do, indeed, deliver a ‘different’ form of stimulation and others that are simply a variation on a theme or a combination of themes.


In addition, combination therapy involves the simultaneous delivery of ultrasound and IFT, thereby achieving the effects of both modalities, though there is no evidence that any additional effects will occur by using this combination approach. Ultrasound is sometimes combined with other electrical stimulation modes (e.g. diadynamic currents, TENS).


It is not possible in a general chapter such as this to identify all the possible combinations and variations. The interested reader might try investigating the major texts in the area (Robertson et al. 2006; Watson 2008b; Belanger 2010). Try the current literature (though many of these ‘new’ devices are yet to have specific research published with regard to their efficacy) or utilise web-based resources (though as with any web-based material, one needs to be both critical and selective with regards the information accepted).

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Jan 7, 2017 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Electrotherapy
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