Interferential (IFT) Therapy

Introduction & IFT Production :

The basic principle of Interferential Therapy (IFT) is to utilise the significant physiological effects of low frequency (@<250pps) electrical stimulation of nerves without the associated painful and somewhat unpleasant side effects sometimes associated with low frequency stimulation.

To produce low frequency effects at sufficient intensity and at sufficient depth, patients can experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the impedance of the skin being inversely proportional to the frequency of the stimulation. [The barrier presented by the skin to the passage of an electric current is more complex than just impedance, or resistance, but will be regarded as such for the purpose of this explanation] In other words, the lower the stimulation frequency, the greater the impedance to the passage of the current & so, more discomfort is experienced as the current is ‘pushed’ into the tissues against this barrier. The skin impedance at 50Hz is approximately 3200 whilst at 4000Hz it is reduced to approximately 40. The result of applying a higher frequency is that it will pass more easily through the skin, requiring less electrical energy input to reach the deeper tissues & 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 1KHz-100KHz) has yet to be established. It is unlikely to do nothing at all, but in terms of current practice, little is known of its physiological effects. It is not capable of direct stimulation of nerve in the common context of such stimulation, though some researchers are currently investigating this area.

Interferential therapy utilises two of these medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross & they literally interfere with each other – hence another term that has been used in the past but appears to be out of favour at the moment – Interference Current Therapy. This interaction gives rise to an interference current (or beat frequency) which has the characteristics of low frequency stimulation – in effect the interference mimics a low frequency stimulation.

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

By careful manipulation of the input currents it is possible to achieve any beat frequency that you might wish to use clinically. Modern machines usually offer frequencies of 1-150Hz, though some offer a choice of up to 250Hz or more. To a greater extent, the therapist does not have to concern themselves with the input frequencies, but simply with the appropriate beat frequency which is selected directly from the machine.

The magnitude of the low frequency interference current is (in theory) approximately equivalent to the sum of the input amplitudes. It is difficult to show categorically that this is the case in the tissues but it is reasonable to suggest that the resultant current will be stronger than either of the 2 input currents.

The use of 2 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 2 or 4 electrode systems. The key difference is that with a 4 pole application the interference is generated in the tissues and with a 2 pole treatment, the current is ‘pre modulated’ i.e. the interference is generated within the machine unit (Ozcan et al, 2004).

Whichever way it is generated, the treatment effect is generated from low frequency stimulation, primarily involving the peripheral nerves. There may indeed be significant effect on tissue other than nerves, but they have not as yet been unequivocally demonstrated. 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 & 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 whenever they have been tested or evaluated in the clinical environment

Note : Care needs to be taken when setting the sweep on a machine in that with some devices, the user sets the actual base and top frequencies (e.g. 10 and 25Hz) and with other machines the user sets the base frequency and then how much needs to be added for the sweep (e.g. 10 and 15Hz). Knowing which was round your machine works is critical to effective treatment.

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 6 seconds – though some machines offer 1 or 3 second options. In the example illustrated (A), the machine is set to sweep from 90 to 130Hz 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) 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. The figure (B) in adjacent diagram illustrates the effect of setting a 90 – 130Hz 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 is effectively a combination of these two (C).

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.

(A) Triangular sweep pattern

(B) Rectangular Sweep Pattern

(C) Trapezoidal Sweep Pattern

 

Physiological Effects & 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. It is rather a generic means of stimulation – the machine can be set up to act more like a TENS type device or can be set up to behave more like a muscle stimulator – by adjusting the stimulating (beat) frequency. It is often regarded (by patients) to be more acceptable as it generates less discomfort than some other forms of electrical stimulation.

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).

Selection of a wide frequency sweeps has been considered less efficient than a smaller selective range in that by treating with a frequency range of say 1-100Hz, the effective treatment frequencies can be covered, but only for a relatively small percentage of the total treatment time. Additionally, some parts of the range might be counterproductive for the primary aims of the treatment.

Clinical Application

The are 4 main clinical applications for which IFT appears to be used:               Pain relief               Muscle stimulation               Increased local blood flow               Reduction of oedema

In addition, claims are made for its role in stimulating healing and repair and for various specialised application – e.g. stress incontinence, though for the former examples (healing and repair) there is a dearth of quality research information available.

As IFT acts primarily on the excitable (nerve) tissues, 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, thought the direct research evidence for the use of IFT in this role is limited. Logically one could use the higher frequencies (90-130Hz) to stimulate the pain gate mechanisms & thereby mask the pain symptoms. Alternatively, stimulation with lower frequencies (2-5Hz) can be used to activate the opioid mechanisms, again providing a degree of relief. These two different modes of action can be explained physiologically & will have different latent periods & varying duration of effect. It remains possible that relief of pain may be achieved by stimulation of the reticular formation at frequencies of 10-25Hz or by blocking C fibre

transmission at >50Hz. Although both of these latter mechanisms have been proposed (theoretically) with IFT, neither have been categorically demonstrated.

A good number of recent studies (e.g. Hurley et al 2004, Johnson and Tabasam 2003, Walker et al 2006, McManus et al 2006, Jorge et al 2006) 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. 1Hz) will result in a series of twitches, whist stimulation at 50Hz will result in a tetanic contraction. There is limited evidence at present for the ‘strengthening’ effect of IFT (though this evidence exists for some other forms of electrical stimulation), though the paper 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 can not 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, maybe 10 and 25Hz. Stimulation below 10Hz results in a series of coarse twitches which may be of clinical benefit, though it has yet to be unequivocally demonstrated with IFT. Stimulation at higher frequencies than that needed to bring about a partial tetany (usually around 20 or 25Hz) can generate a strong tetanic contraction, which might be considered beneficial to assist patient appreciation of the required muscle work, but again, in terms of IFT intervention, it has yet to be demonstrated that this contraction level is needed over and above a partial tetany.

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 – the contractions are forced via the motor nerve – and short stimulation periods with adequate rest might be 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, but more comfortable than Faradism.

Blood flow :

There is very little, if any quality evidence demonstrating a direct effect if IFT on local blood flow changes. Most of the work that has been done involves laboratory experimentation on 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 experimentation by Noble et al (2000) demonstrated vascular changes at 10–20Hz, though was unable to clearly identify the mechanism for this change. The stimulation was applied via suction electrodes, and the outcome could therefore be as a result of the suction rather than the stimulation, though this is largely negated by virtue of the fact that other stimulation frequencies were also delivered with the suction electrodes without the blood flow changes. The most likely mechanism 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 or 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, and thus the 10-20 or 10-25Hz frequency sweep options appears to be the most likely beneficial option.

Oedema :

IFT has been claimed to be effective as a treatment to promote the reabsorption of oedema in the tissues. Again, the evidence is very limited in this respect and the physiological mechanism by which is could be achieved as a direct effect of the 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 that will result (see above) 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, though the patients did the circumferential knee measures (rather than the therapist) and circumferential knee measurement is not an especially reliable method for identifying oedema as such. The Christie and Willoughby study (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 most effective.

Other Clinical Applications :

In addition to the 4 key areas identified above, there are several other specialist application for which IFT has been employed. These include stimulation as part of the management of incontinence and pelvic floor training (e.g. Parkkinen et al, 2004), constipation in children (Chase et al, 2005), Fibromyalgia (Almedia et al, 2003; Raimundo et al, 2004) and trigger point intervention (Hou, 2002; Jenson et al, 2002). Enhancement of fracture healing has also been investigated with mixed results (e.g. Ganne, 1988)

Treatment Parameters:

Stimulation can be applied using pad electrodes and sponge covers (which when wet provide a reasonable conductive path), though 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 make the IFT application easier in the view of many practitioners. The suction electrode application method has been in use for several years, and whilst it is useful, especially for larger body areas like the shoulder girdle, trunk, hip, knee, it does not appear to provide any therapeutic advantage over pad electrodes (in other words, the suction component of the treatment does not appear to have a measurable therapeutic effect). Care should be taken with regards maintenance of electrodes, electrode covers and associated infection risks (Lambert et al 2000).

Whichever electrode system is employed, electrode positioning should ensure adequate coverage of the area for stimulation. Using larger electrodes will minimise patient discomfort whilst small, closely spaced electrodes increase the risk of superficial tissue irritation and possible damage / skin burn.

The bipolar (2 pole) application method is perfectly acceptable, and there is no physiological difference in treatment outcome despite several anecdotal stories to the contrary. Recent research evidence supports the benefit of 2 pole application (e.g. Ozcan et al 2004).

Treatment times vary widely according to the usual clinical parameters of acute/chronic conditions & the type of physiological effect desired. In acute conditions, shorter treatment times of 5-10 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 initially adopted especially with the acute case in case 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.

References :

Adedoyin, R. A., et al. (2002). "Effect of interferential current stimulation in management of osteo-arthritic knee pain." Physiotherapy 88(8): 493-9.

Almeida, T. F., et al. (2003). "The effect of combined therapy (ultrasound and interferential current) on pain and sleep in fibromyalgia." Pain 104(3): 665-72.

Alves-Guerreiro, J.et al. (2001). "The effect of three electrotherapeutic modalities upon peripheral nerve conduction and mechanical pain threshold." Clinical Physiology 21(6): 704-711.

Bircan, C. et al. (2002). "Efficacy of two forms of electrical stimulation in increasing quadriceps strength: a randomized controlled trial." Clin Rehabil 16(2): 194-9.

Chase, J., et al. (2005). "Pilot study using transcutaneous electrical stimulation (interferential current) to treat chronic treatment-resistant constipation and soiling in children." J Gastroenterol Hepatol 20(7): 1054-61.

Christie, A. D. and G. L. Willoughby (1990). "The effect of interferential therapy on swelling following open reduction and internal fixation of ankle fractures." Physiotherapy Theory and Practice 6: 3-7.

Ganne, J.-M. (1988). "Stimulation of bone healing with interferential therapy." Australian Journal of Physiotherapy 34(1): 9-20.

Hou, C. R.et al. (2002). "Immediate effects of various physical therapeutic modalities on cervical myofascial pain and trigger-point sensitivity." Arch Phys Med Rehabil 83(10): 1406-14.

Hurley, D. A., et al. (2001). "Interferential therapy electrode placement technique in acute low back pain: a preliminary investigation." Arch Phys Med Rehabil 82(4): 485-93.

Hurley, D. A. et al. (2004). "A randomized clinical trial of manipulative therapy and interferential therapy for acute low back pain." Spine 29(20): 2207-16.

Jarit, G. J. et al. (2003). "The effects of home interferential therapy on post-operative pain, edema, and range of motion of the knee." Clin J Sport Med 13(1): 16-20.

Jenson, M. G. (2002). "Reviewing approaches to trigger point decompression." Physician Assistant 26(12): 37-41.

Johnson, M. I. and G. Tabasam (2003). "An investigation into the analgesic effects of different frequencies of the amplitude-modulated wave of interferential current therapy on cold-induced pain in normal subjects." Arch Phys Med Rehabil 84(9): 1387-94.

Johnson, M. I. and G. Tabasam (2003). "A single-blind investigation into the hypoalgesic effects of different swing patterns of interferential currents on cold-induced pain in healthy volunteers." Arch Phys Med Rehabil 84(3): 350-7.

Jorge, S. et al. (2006). "Interferential therapy produces antinociception during application in various models of inflammatory pain." Phys Ther 86(6): 800-8.

Lambert, I. et al. (2000). "Interferential therapy machines as possible vehicles for cross infection." Journal of Hospital Infection 44: 59-64.

McManus, F. J. et al. (2006). "The analgesic effects of interferential therapy on two experimental pain models: cold and mechanically induced pain." Physiotherapy 92(2): 95-102.

Noble, J. G. et al. (2000). "The effect of interferential therapy upon cutaneous blood flow in humans." Clin Physiol 20(1): 2-7.

Ozcan, J. et al. (2004). "A comparison of true and premodulated interferential currents." Arch Phys Med Rehabil 85(3): 409-15.

Palmer, S. T. et al. (2004). "Effects of electric stimulation on C and A delta fiber-mediated thermal perception thresholds." Arch Phys Med Rehabil 85: 119-128.

Parkkinen, A., et al. (2004). "Physiotherapy for female stress urinary incontinence: individual therapy at the outpatient clinic versus home-based pelvic floor training: a 5-year follow-up study." Neurourol Urodyn 23(7): 643-8.

Philipp, A., et al. (2000). "Interferential current is effective in palmar psoriaris: an open prospective trial." Eur J Dermatol 10(3): 195-8.

Raimundo, A. K. S., et al. (2004). "Comparative study of the analgesic effect between frequencies of interferential current in the fibromyalgia [Portuguese]." Fisioterapia em Movimento 17(4): 65-72.

Roche, P. et al. (2002). "Modification of induced ischaemic pain by placebo electrotherapy." Physiotherapy Theory and Practice 18: 131-139.

Shanahan, C. et al. (2006). "Comparison of the analgesic efficacy of interferential therapy and transcutaneous electrical nerve stimulation." Physiotherapy. 92(4): 247-53.

Sontag, W. (2000). "Modulation of cytokine production by interferential current in differentiated HL-60 cells." Bioelectromagnetics 21(3): 238-44.

Stephenson, R. and E. Walker (2003). "The analgesic effects of interferential (IF) current on cold-pressor pain in healthy subjects: a single blind trial of three IF currents against sham IF and control." Physiotherapy Theory and Practice 19: 99-107.

Tabasam, G. and M. I. Johnson (2006). "The use of interferential therapy for pain management by physiotherapists... including commentary by Poitras S." International Journal of Therapy and Rehabilitation 13(8): 357-64.

Walker, U. A. et al. (2006). "Analgesic and disease modifying effects of interferential current in psoriatic arthritis." Rheumatol Int: 1-4.

Ward, A. R. and W. G. Oliver (2007). "Comparison of the hypoalgesic efficacy of low-frequency and burst-modulated kilohertz frequency currents." Phys Ther 87(8): 1056-63.

Ward, A. R. et al. (2002). "Optimal frequencies for electric stimulation using medium-frequency alternating current." Arch Phys Med Rehabil 83(7): 1024-7.

Watson, T. (2000). "The role of electrotherapy in contemporary physiotherapy practice." Man Ther 5(3): 132-41.

Interferential Contraindications :

Patients who do not comprehend the physiotherapist’s instructions or are unable to co-operate should not be treated

Patients with Pacemakers – some pacemakers are relatively immune to interference from electrical stimulation whilst others can demonstrate serious adverse behaviour. It is suggested that as a general rule, if the patient has a pacemaker, it is best to avoid all electrical stimulation, but like TENS, if it is a treatment that is needed. The stimulation should be tried in a carefully controlled environment where appropriate equipment is available to correct any pacing problems should they arise.

Patients who are taking anticoagulation therapy or have a history of pulmonary embolism or deep vein thrombosis should not be treated with the vacuum electrode applications

Similarly, patients whose skin may be easily damaged or bruised

Application over :

The trunk or pelvis during pregnancy (though this MAY be modified in time in line with the TENS advice. At the present time, it is suggested that it is best avoided in these regions

Active or suspected malignancy except in hospice/palliative/terminal care

The eyes

The anterior aspect of the neck

The carotid sinuses

Dermatological conditions e.g. dermatitis, broken skin

Danger of haemorrhage or current tissue bleeding (e.g. recent soft tissue injury)

Avoid active epiphyseal regions in children

Transthoracic electrode application is considered to be ‘risky’ by many authorities

Interferential Precautions :

Care should be taken to maintain the suction at a level below that which causes damage / discomfort to the patient

If there is abnormal skin sensation, electrodes should be positioned in a site other than this area to ensure effective stimulation

Patients who have (marked) abnormal circulation

For patients who have febrile conditions, the outcome of the first treatment should be monitored

Patients who have epilepsy, advanced cardiovascular conditions or cardiac arrhythmias should be treated at the discretion of the physiotherapist in consultation with the appropriate medical practitioner

Treatment which involves placement of electrodes over the anterior chest wall

Interferential Treatment Record :

• Electrode number (2 pole, 4 pole) and positions

• Frequency applied

• Sweep settings employed (if applicable)

• Current intensity applied (or patient reported sensation)

• Treatment duration

Laser Therapy

I have tried to update these pages, and there is some more work to do yet, but in the meantime, there are some useful links on this page and a link to my own laser pages (as far as I have got to date).

I would hope to expand on this data as soon as I can, but there is a LONG list of jobs to do, and not enough time to do it all straight away I am afraid

Keep watching these pages, and you never know, some new stuff might get here soon!

Laser Therapy - Summary Pages (Tim Watson)

LINKS :

World Association for Laser Therapy (WALT)

This is an important and very informative site wih links to many other laser sites of significance. The site includes :

WALT Laser Therapy - Recommended Doses

Useful section of the WALT site where 2005 dose recommendations are summarised

Laser World

The Swedish Medical Laser Society site - loads of links to the literature, current research, news and associated items of interest

Professor Tiina I. Karu

Professor Karu has been publishing laser research material and excellent (if a bit tricky in places) texts for 20 years. Useful resource site for her current cork and biography / publications

Internet Photochemistry and Photobiology

The site aims to promote the use of the internet in the communication of research and education in all areas of Photochemistry and Photobiology.

ASLMS : American Society of Laser Medicine and Surgery

This website may have a surgical / medical emphasis, but worth exploring none the less

Australian Medical Laser Association

The Australian Medical Laser Association's primary responsibility is to promote and facilitate the advancement of laser medicine, photobiomodulation, and allied sciences in Australia.

American Society for Photobiology

The ASP promotes research in photobiology, integration of different photobiology disciplines, dissemination of photobiology knowledge, and provides information on photobiological aspects of national and international issues.

The Photomedicine Society

The Photomedicine Society was established in 1992 to promote a greater understanding of the scientific and medical aspects of light in health and disease.

European Society for Photobiology

The primary aims of the Society are to co-ordinate and promote all aspects of photobiology in Europe in a way which will optimise the achievements of European photobiology across a range of scientific, technological and medical arenas

Books :

Therapeutic Lasers: Theory and Practice : G D Baxter : Churchill Livingstone : 1994

Although I think that the UK edition is currently out of print, this is still a great text (if you can get hold of a copy). There is an American edition out at the moment, and a quick search around on Amazon should find you a new or used copy.

 

 

Laser Therapy - Clinical Practice and Scientific Background : Jan Tuner and Lars Hode : Prima Books : 2002

This is a great (but somewhat expensive) text covering many aspects of laser therapy and a whole load of references (something like 1400). It too is out of print (I think) at the moment, but there are copies around if you go a searching

 

 

The Laser Therapy Handbook : Jan Tuner and Lars Hode : Prima Books : 2004

A smaller, much less expensive, but still very useful laser therapy text - looks like a black/white paperback edition of the main book (above). It IS still available

Transcutaneous Electrical Nerve Stimulation (TENS)

TENS is a method of electrical stimulation which primarily aims to provide a degree of symptomatic pain relief by exciting sensory nerves and thereby stimulating either the pain gate mechanism and/or the opioid system. The different methods of applying TENS relate to these different physiological mechanisms. The effectiveness of TENS varies with the clinical pain being treated, but research would suggest that when used ‘well’ it provides significantly greater pain relief than a placebo intervention. There is an extensive research base for TENS in both the clinical and laboratory settings and whilst this summary does not provide a full review of the literature, the key papers are referenced. It is worth noting that the term TENS could represent the use of ANY electrical stimulation using skin surface electrodes which has the intention of stimulating nerves. In the clinical context, it is most commonly assumed to refer to the use of electrical stimulation with the specific

intention of providing symptomatic pain relief. If you do a literature search on the term TENS, do not be surprised if you come across a whole lot of ‘other’ types of stimulation which technically fall into this grouping.

TENS as a treatment technique is non invasive and has few side effects when compared with drug therapy. The most common complaint is an allergic type skin reaction (about 2-3% of patients) and this is almost always due to the material of the electrodes, the conductive gel or the tape employed to hold the electrodes in place. Most TENS applications are now made using self adhesive, pre gelled electrodes which have several advantages including reduced cross infection risk, ease of application, lower allergy incidence rates and lower overall cost. Digital TENS machines are becoming more widely available and extra features (like automated frequency sweeps and more complex stimulation patterns) are emerging, though there remains little clinical evidence for enhanced efficacy at the present time. Some of these devices do offer pre-programmed and/or automated treatment settings.

Machine parameters:

Before attempting to describe how TENS can be employed to achieve pain relief, the main treatment variables which are available on modern machines will be outlined. The location of these controls on a typical (analogue) TENS machine is illustrated in the diagram below.

The current intensity (A) (strength) will typically be in the range of 0 - 80 mA, though some machines may provide outputs up to 100mA. 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 machine will deliver discrete ‘pulses’ of electrical energy, and the rate of delivery of these pulses (the pulse rate or frequency (B) will normally be variable from about 1 or 2 pulses per second (pps) up to 200 or 250 pps (sometimes the term Hertz or Hz is used

here). To be clinically effective, it is suggested that the TENS machine should cover a range from about 2 – 150 pps (or Hz).

 In addition to the stimulation rate, the duration (or width) of each pulse (C) may be varied from about 40 to 250 micro seconds (ms). (a micro second is a millionth of a second). Recent evidence would suggest that this is possibly a less important control that the intensity or the frequency and the most effective setting in the clinical environment is probably around 200ms.

The reason that such short duration pulses can be used to achieve these effects is that the targets are the sensory nerves which tend to have relatively low thresholds ( i.e. they are quite easy to excite) and that they will respond to a rapid change of electrical state. There is generally no need to apply a prolonged pulse in order to force a sensory nerve to depolarise, therefore stimulation for less than a millisecond is sufficient.

In addition, most modern machines will offer a BURST mode (D) in which the pulses will be allowed out 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. Both the burst and modulation modes will be discussed in more detail in the following sections.

Most machines offer a dual channel output - i.e. two pairs of electrodes can be used 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. Widespread and diffuse pain presentations can be usefully treated with a 4 electrode (2 channel) system, as can a combined treatment for local and referred pain (see later).

The pulses delivered by TENS stimulators vary between manufacturers, but tend to be asymmetrical biphasic modified square wave pulses. The biphasic nature of the pulse means that there is usually no net DC component (often described in the manufacturers blurb as ‘zero net DC’), thus minimising any skin reactions due 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 involves activation (excitation) of the A beta (Aβ) sensory fibres, and by doing so, reduces the transmission of the noxious stimulus from the ‘c’ fibres, through the spinal cord and hence on to the higher centres. The Aβ fibres appear to appreciate being stimulated at a relatively high rate (in the order of 90 - 130 Hz or pps). 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. Clinically it is important to enable the patient to find their optimal treatment frequency – which will almost certainly vary between individuals. Setting the machine and telling the patient that this is the ‘right’ setting is almost certainly not going to be the maximally effective treatment, though of course, some pain relief may well be achieved.

An alternative approach is to stimulate the A delta (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. In a similar way to the pain gate physiology, it is unlikely that there is a single (magic) frequency in this range that works best for everybody – patients should be encouraged to explore the options where possible.

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 100Hz) 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 100Hz 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 some other forms of TENS as there is more of a ‘grabbing’, ‘clawing’ type sensation and usually more by way of muscle twitching than with the high or low frequency modes.

TENS Modes

Traditional  TENS (Hi TENS, Normal TENS)

Usually uses stimulation at a relatively high frequency (90 - 130Hz) and employ a relatively narrow (short duration) pulses (start at about 100ms) though as mentioned above, there is less support for manipulation of the pulse width in the current research literature. The stimulation is delivered at ‘normal’ intensity - definitely there but not uncomfortable. 30 minutes is probably the minimal effective time, but it can be delivered for as long as needed. The main pain relief is achieved during the stimulation, with a limited ‘carry over’ effect – i.e. pain relief after the machine has been switched off.  

Acupuncture TENS (Lo TENS, AcuTENS)

Use a lower frequency stimulation (2-5Hz) with wider (longer) pulses (200-250ms). The intensity employed will usually need to be greater than with the traditional TENS - still not

at the patients threshold, but quite a definite, strong sensation. As previously, something like 30 minutes will need to be delivered as a minimally effective dose. 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, though the duration of this carry over will vary between patients.

Brief Intense TENS :

This is a TENS mode that can be employed to achieve a 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 90-130Hz band) and the pulse duration (width) is also high (200ms plus). 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.

Burst Mode TENS :

 

As described above, the machine is set to deliver traditional TENS, but the Burst mode is switched in, therefore interrupting the stimulation outflow at rate of 2 - 3 bursts / second. The stimulation intensity will need to be relatively high, though not as high as the brief intense TENS – more like the Lo TENS. It is proposed that the application of BURST mode TENS can effectively stimulate both the PAIN GATE and the OPIOID mechanisms simultaneously.

Modulation Mode TENS :

In modulation mode, the machine delivers a less regular pattern of TENS stimulation in an attempt to reduce or minimise the accommodation effects of regular, patterned stimulation. Machines offer different methods of varying the stimulation pattern – some vary the frequency, some vary the intensity and some vary the pulse duration, and some machines offer a choice between these methods, though the research evidence to date does not favour one variation method over another. This potentially most useful for patients who use TENS for hours a day, if for no other reason than accommodation occurs at a slower rate and therefore less intensity adjustment may be required.

 

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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. Patients (or the therapist) need 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

Stimulation Intensity :

As identified above, it is not possible to describe treat

ment current strength in terms of how many microamps. 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. 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 (lo) mode.

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 pain 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 :

a) Stimulation of appropriate nerve root(s)

b) Stimulate the peripheral nerve (best if proximal to the pain area)

c) Stimulate motor point (innervated by the same root level)

d) Stimulate trigger point(s) or acupuncture point(s)

e) Stimulate the appropriate dermatome, myotome or sclerotome

If the pain source is vague, diffuse or particularly extensive, one can employ both channels simultaneously. A 2 channel application can also be effective for the management of a local + a referred pain combination – one channel used for each component. The low frequency (Acupuncture like) TENS can be effectively applied to the contralateral side of the body

CONTRAINDICATIONS

Patients who do not comprehend the physiotherapist’s instructions or who are unable to co-operate

It has been widely cited that application of the electrodes over the trunk, abdomen or pelvis during pregnancy is contraindicated BUT a recent review  suggests that although not an ideal (first line) treatment option, application of TENS around the trunk during pregnancy can be safely applied, and no detrimental effects have been reported in the literature (see www.electrotherapy,org for publication details)

DOWNLOAD TENS Guidelines During PREGNANCY document

TENS during labour for pain relief is both safe and effective

Patients with a Pacemaker should not be routinely treated with TENS though under carefully controlled conditions it can be safely applied. It is suggested that routine application of TENS for a patient with a pacemaker or any other implanted electronic device should be considered a contraindication.

Patients who have an allergic response to the electrodes, gel or tape

Electrode placement over dermatological lesions e.g. dermatitis, eczema

Application over the anterior aspect of the neck or carotid sinus

PRECAUTIONS

If there is abnormal skin sensation, the electrodes should preferably be positioned elsewhere to ensure effective stimulation

Electrodes should not be placed over the eyes

Patients who have epilepsy should be treated at the discretion of the therapist in consultation with the appropriate medical practitioner as there have been anecdotal reports of adverse outcomes, most especially (but not exclusively) associated with treatments to the neck and upper thoracic areas

Avoid active epiphyseal regions in children (though there is no direct evidence of adverse effect)

The use of abdominal electrodes during labour may interfere with foetal monitoring equipment and is therefore best avoided

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REFERENCES

Key papers/articles/texts

Johnson, M. (2008). TENS In : Electrotherapy: Evidence Based Practice. Ed. Watson. T. Elsevier

Robertson, V. et al (2007). Electrotherapy Explained.  Elsevier.

Walsh, D. (1997). TENS: Clinical Applications and Related Theory. Edinburgh, Churchill Livingstone.

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