The principle of Transcutaneous Electrical Nerve Stimulation is based on counter-irritation. TENS devices work as sound generators. Instead of being connected to a speaker, electrically conductive electrodes are pressed or glued onto the skin. With “higher frequencies” the higher tones are generated. Lower frequencies correspond to low notes. Manifested sensations beneath the electrodes – in the tissue or muscle – correspond to the pitch: High notes cause a fast twitch or tingling, deep sounds intense, slow twitch or rich tapping.
The high frequencies find an application in TENS treatment, which focuses on alleviation of pain. The medium to low frequencies are used in EMS (electrical muscle stimulation) for building and regeneration of muscle groups.
Gate shut, pain left out
In the 1960s, scientists Ronald Melzack and Patrick D. Whale developed the “Gate Control Theory” of pain. This assumes the existence of “pain gates” which, under certain circumstances, conduct a pain to the brain, but can also act as blockers. The central idea of the theory is that thick nerve fibres close these “pain-gates”, while the finer nerve fibres open them.
Without stimulation, the thin and thick fibres behave unresponsively and the “gate” – the so-called inhibitory interneuron – blocks the signal. It cannot be routed to the projection-nerve fibres associated with the brain. The result of the “closed” gate is the absence of pain sensation. In the case of a non-painful stimulation (eg tactile stimuli), the signal is routed via the thick nerve fibers and the gate is shut. For pain-signals, signal transduction goes via the fine nerve fibres. The “pain gate” opens and the signal gets to the brain: pain is perceived. The application of TENS stimulates the thick A-nerve fibres, closes the “gate”, and so relieves pain. Presumably TENS, just like acupuncture, leads to a release of endorphins. Depending on the disease, thirty to seventy-five percent of patients report improvement.
The four types of TENS
The most well-known for pain relief is conventional TENS – High Frequency and Low Intensity TENS (or simply called Hi-TENS). In this TENS application, frequencies between about 40 and 150 Hz and pulses with a duration of 10-150 microseconds are used, which are too short to invoke pain-conducting A-delta and C fibres. The intensity is adjusted so that the patient feels a mild to moderate tingling in the painful area; The treatment can, if necessary, be carried out for hours.
The electrodes are usually placed using a simple method: Stick it where it hurts. This is also called regional application. In addition, the electrodes can be placed onto trigger points or acupuncture points. For phantom pain, the contralateral side is treated. The objective of the application is the pain-free activation of segmental and probable descending pain-inhibitory mechanisms as Melzack and Wall described in 1965. Pain relief should occur immediately and usually lasts for as long as the current flows.
The High Intensity Low Frequency TENS has its conceptual roots in electro-acupuncture. The aim is to activate, with long pulses, the A-delta and C fibres and thereby bring about a segmental, descending and central pain-inhibition. This works best when, in the muscles in which pain arises, contractions are being triggered.
In order to do this, using frequencies between 2 and 4 Hz, single pulses with a phase duration of 200-400 microseconds are applied. The intensity is adjusted so that more or less significant contractions are triggered in the relevant muscles. For this purpose, in this application the electrodes are best placed on the motor point or nerve stimulation points.
The method is quite intense, due to beta-endorphin secretion, however also very effective. Pain relief occurs after about twenty minutes of treatment, so that a low-TENS application should last no less than 30 and up to a maximum of 45 minutes. Pain relief can last for hours or even days.
For patients in a chronic stress situation, in which the autonomic nervous system is ergotropically aligned, this method is contraindicated. The nervous system of these patients is not differentiated and every stimulus of these fibres can effect a further increase in their stress response.
Because the applied single pulses were felt by some patients to be uncomfortable, in 1976 Eriksson and Sjölund developed burst TENS. In this application, the muscle contractions are not caused by a single pulse but by tetanising pulse-bursts. Some, but certainly not all, patients perceive this as more pleasant. As with the low-TENS, pain relief by naloxone administration is suppressed, it is very likely therefore that beta-endorphin is involved.
Opiate resistance is like electrical resistance
Patients who do not respond to opiates such as tramadol will only benefit little or not at all from low-and burst-TENS. Endorphins which are responsible for pain reduction and tramadol dock onto the same μ-opioid receptors. When these receptors, as can occur in neuropathic pain, become reduced in their expression, this method is less effective or even without effect. In instances where patients who suffer pain, and who have been treated unsuccessfully with antidepressant drugs belonging to the selective serotonin-reuptake inhibitors (SSRIs) class, the impact of treatment using low-and burst-TENS may be disappointing as well. The reason for this is that descending pain-inhibition depends partly on serotonin production. If this has been disturbed, the inhibiting-system is not available or only exists at a reduced level. In these instances and with the above-mentioned patients, the use of high-frequency low-intensity TENS can still lead to success.
The High Frequency High Intensitiy TENS is a very intense, short-acting TENS application which finds its use in sports physiotherapy and, for example, in trigger point herapy. The effect is probably based on the inhibition of conduction mechanisms of the membrane of the relevant area’s innervating nerves. Frequencies from 60 to 100 Hz are used and phase durations between 100 and 300 microseconds. The intensity sits at the limit of pain tolerance or even significantly higher. The electrodes are placed on the enervating nerve of the pain region or, in the case of trigger point treatment, directly on the respective point. The treatment takes about 15 minutes, pain relief continues for just about as long.
Possible indications for TENS
• Headaches (migraines, tension headaches)
• Neurologically-related pain (eg sciatica, shingles)
• Phantom pain, pain from scars
• Pain in childbirth
• Pain in circulatory disorders
• Pain during or after cancer
• Pain of the musculoskeletal system
• Pain after an accident
Good for neck-cracking
A Spanish, randomized study by Escortell-Mayor et al. demonstrated that TENS and manual therapy for neck pain have a comparably similar effect. Six months later, however, the remnant effects were ascertainable with only one third of the subjects.
Better in neuropathy
The National Guidelines for Diabetic Neuropathy indicate a detailed interest in TENS. In a single-blind, placebo-controlled, randomised study by Kumar et al. a significantly better pain reduction with TENS as compared to sham treatment in 23 patients with painful diabetic polyneuropathy was documented.
A systematic literature search up until 2009 identified two clinical studies of high-frequency muscle stimulation (HTEMS) in painful diabetic neuropathy. In all studies an improvement in pain symptoms was documented. The Commission for Care Guidelines also emphasises, however, that the numbers of patients in some studies are too small to be able to make a final assessment.
In an observational study of the Western German Diabetes and Health Centre WDGZ (Prof. Dr. Stephan Martin) in 2007, 414 diabetes patients kept a TENS domestic unit for treatment at home. In 88.4 percent of cases the polyneuropathic discomforts improved, sleep disturbances equally were alleviated through its use.
Good results in children
According to the recommendations of the Working Group of the GSSP (German Society for the Study of Pain) on pain management in children, TENS is considered by therapists to be, in treating some types of headaches, of value. The effectiveness of TENS is valued more highly in tension-headache treatment than in treatment of migraine. The advantage of this method lies particularly in its long-term use on combination headaches, whereby independence from the therapist (and from drugs) essentially means an improved sense of self-control, and power over oneself, for the child. A reduction of at least 50% of seizures can be expected for tension headaches after 1-3 months for about 4 in 5 of the children, says a study of Pothman’s.
Controversial results for back pain
A working group of the American Academy of Neurology holds the view that TENS is not sufficiently able to alleviate chronic back pain. With neuropathic pain the body of data is more supportive, according to the working group led by Richard Dubinsky from the Kansas University Medical Center. As a side effect of TENS treatment, mild skin irritation under the electrodes may occur, which normally disappears after a pause of several days. Some patients are allergic to certain adhesive-electrodes. The duration of phase, phase interval and burst frequencies should be able to be adjusted on devices for home treatment, through which a varied range of program variants can be made available.