Interferential Current Therapy
Introduction
The basic principle of Interferential Therapy (IFT) is to utilise the strong physiological effects of low frequency (<250pps) electrical stimulation of nerves without the associated painful and somewhat unpleasant side effects sometimes associated with low frequency stim.
To produce low frequency effects at sufficient intensity at 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 interferntial stimulation. 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 electrical stimulation.
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. 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.
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 inteferential 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.
Whichever way it is generated, 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.
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.
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
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.
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 remain sa 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.