Transcutaneous Electrical Nerve Stimulation (TENS)

Transcutaneous electrical nerve stimulation (TENS) is a therapy that uses low voltage electrical current to provide pain relief.

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.

Transcutaneous Electrical Nerve Stimulation (TENS)

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 frequency HF (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 low frequency LF (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 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.

Impedence of Current Flow

The quantity of current that flow in the tissues depend on impedance of that pathway.The impedance include oh-mic resistance and inductive resistance. The inductive resistance negligible in the tissues . Generally,watery tissue as blood,muscle and nerve has low ohmic resistance :bone and fat has higher and epidermis has the highest of all.

Electrode Placement Technique

this is determined by the target muscle or muscle group either single or in relation to other muscles

• Unilateral : Unilateral placement causes inflammation of one limb or half of muscle pair.
• Bilateral : It allow the stimulation of both limbs or both halves of a muscle Pair
• Uni-polar : Only one of two essential leads and the electrode connected to it are placed over the target area affected by the stimulation This electrode is called treatment electrode.
• Quadripolar : Involve the use of two sets of electrode each originating from its own channel. It may be considered the concurrent application of two bipolar circuit. This technique could be used with stimulation of agonist and antagonist. Also could be used in crossed pattern or for large flat area as back.

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).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.