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 is most commonly delivered from small, hand held, battery powered devices. They can be purchased 'over the counter' in many (but not all) countries. In some locations, they need to be 'prescribed' by a therapist, doctor or other healthcare practitioner. Most multi-modal clinic based stimulators include TENS as an option, though its use in the clinic is less well supported than its use as a home based, patient delivered therapy. Examples of typical TENS units are illustrated below.
It is interesting that in therapy practice, the majority of practitioners consider TENS as a treatment options in circumstances when a patient is experiencing CHRONIC pain. This is not a problem as there is a significant evidence base to support this mode of application. There is however, a significant and growing body of evidence that supports the use of TENS as a valid and effective intervention in a ACUTE pain conditions. Examples would include : Desantana et al (2009); Sbruzzi et al (2012); Silva et al (2012); Solak et al (2007) and Unterrainer et al (2010).
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.
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 (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 200 microsec.
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 80 - 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, though some authors consider a wider range of 2 - 10Hz), 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.
Traditional TENS (Hi TENS, Normal TENS)
Usually uses stimulation at a relatively high frequency (80 - 130Hz) and employ a relatively narrow (short duration) pulses though as mentioned above, there is less support for manipulation of the pulse width in the current research literature. Most patients seem to find best effect at around 200 microsec. 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)
Uses a lower frequency stimulation (2-5Hz) with wider (longer) pulses (200-250 microsec). 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 80-130Hz band) and the pulse duration (width) is also high (200 microsec pluse). 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.
The diagram below shows stimulation (at around 100Hz) delivered in BURST mode (at 2-3Hz) such that both the A Beta (due to 100Hz) and the A delta (due to 2-3Hz) are stimulated
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.
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 treatment 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. There is a growing body of evidence that suggests that a ‘strong’ sensation, whichever mode is being used, might achieve better clinical effects.
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 :
Stimulation of appropriate nerve root(s)
Stimulate the peripheral nerve (best if proximal to the pain area)
Stimulate motor point (innervated by the same root level)
Stimulate trigger point(s) or acupuncture point(s)
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.
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Walsh, D. (1997). TENS: Clinical Applications and Related Theory. Edinburgh, Churchill Livingstone.
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