Transcutaneous electrical nerve stimulation (TENS) is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes. TENS, by definition, covers the complete range of transcutaneously applied currents used for nerve excitation although the term is often used with a more restrictive intent, namely to describe the kind of pulses produced by portable stimulators used to treat pain. The unit is usually connected to the skin using two or more electrodes. A typical battery-operated TENS unit is able to modulate pulse width, frequency and intensity. Generally TENS is applied at high frequency (>50 Hz) with an intensity below motor contraction (sensory intensity) or low frequency (<10 Hz) with an intensity that produces motor contraction. The benefit of TENS for pain management is controversial.
TENS devices available to the domestic market are used as a non-invasive nerve stimulation intended to reduce both acute and chronic pain. One review from 2007 felt that the evidence supports a benefit in chronic musculoskeletal pain while another review (from the Cochrane Collaboration in 2008) deemed the evidence of poor quality and thus no conclusions were possible regarding chronic pain. Results from a task force on neck pain in 2008 found no clinically significant benefit to TENS for the treatment of neck pain when compared to a placebo treatment. A 2010 review did not find evidence to support the use of TENS for chronic low back pain. There is tentative evidence that it may be useful for painful diabetic neuropathy.
In principle, an adequate intensity of stimulation is necessary to achieve pain relief with TENS. An analysis of treatment fidelity (meaning that the delivery of TENS in a trial was in accordance with current clinical advice, such as using "a strong but comfortable sensation" and suitable, frequent treatment durations) showed that higher fidelity trials tended to have a positive outcome.
A few studies have shown objective evidence that TENS may modulate or suppress pain signals in the brain. One used evoked cortical potentials to show that electric stimulation of peripheral A-beta sensory fibers reliably suppressed A-delta fiber nociceptive processing. Two other studies used functional magnetic resonance imaging (fMRI): one showed that high-frequency TENS produced a decrease in pain-related cortical activations in patients with carpal tunnel syndrome, while the other showed that low-frequency TENS decreased shoulder impingement pain and modulated pain-induced activation in the brain.
A significant number of TENS machine brands have been targeted for use for labor pain, although a 1997 report of a study done by the University of Oxford said that TENS "has been shown not to be effective in postoperative and labour pain." Use is documented in the attached references: in obstetric care, particularly in labor;
Electrical stimulation for pain control was used in ancient Rome, 63 A.D. It was reported by Scribonius Largus that pain was relieved by standing on an electrical fish at the seashore. In the 16th through the 18th century various electrostatic devices were used for headache and other pains. Benjamin Franklin was a proponent of this method for pain relief. In the 19th century a device called the electreat, along with numerous other devices were used for pain control and cancer cures. Only the electreat survived into the 20th century, but was not portable, and had limited control of the stimulus.. Development of modern TENS unit is generally credited to C. Norman Shealy.
The first modern, patient-wearable TENS was patented in the United States in 1974. It was initially used for testing the tolerance of chronic pain patients to electrical stimulation before implantation of electrodes in the spinal cord dorsal column. The electrodes were attached to an implanted receiver, which received its power from an antenna worn on the surface of the skin. Although intended only for testing tolerance to electrical stimulation, many of the patients said they received so much relief from the TENS itself that they never returned for the implant.
A number of companies began manufacturing TENS units after the commercial success of the Medtronic device became known. The neurological division of Medtronic, founded by Don Maurer, Ed Schuck and Dr. Charles Ray, developed a number of applications for implanted electrical stimulation devices for treatment of epilepsy, Parkinson's disease, and other disorders of the nervous system.
Today many people confuse TENS with electrical muscle stimulation (EMS). EMS and TENS devices look similar, with both using long electric lead wires and electrodes. TENS is for blocking pain, where EMS is for stimulating muscles.
There are several anatomical locations where TENS electrodes are contraindicated:
Through the chest using an anterior and posterior electrode positions, or other transthoracic applications understood as "across a thoracic diameter"; this does not preclude coplanar applications
Internally, except for specific applications of dental, vaginal, and anal stimulation that employ specialized TENS units
On broken skin areas or wounds, although it can be placed around wounds.
TENS used across an artificial cardiac pacemaker (or other indwelling stimulator, including across its leads) may cause interference and failure of the implanted device. Serious accidents have been recorded in cases when this principle was not observed. A 2009 review in this area suggests that electrotherapy, including TENS, "are best avoided" in patients with pacemakers or implantable cardioverter-defibrillators (ICDs). They add that "there is no consensus and it may be possible to safely deliver these modalities in a proper setting with device and patient monitoring", and recommend further research. The review found several reports of ICDs administering inappropriate treatment due to interference with TENS devices, but notes that the reports on pacemakers are mixed: some non-programmable pacemakers were inhibited by TENS, but others were unaffected or auto-reprogrammed.
The use of TENS is likely to be less effective on areas of numb skin/decreased sensation due to nerve damage. It may also cause skin irritation due to the inability to feel currents until they are too high. There's an unknown level of risk when placing electrodes over an infection (possible spreading due to muscle contractions), but cross contamination with the electrodes themselves is of greater concern. TENS should also be used with caution in people with epilepsy or pregnant women; do not use over area of the uterus as the effects of electrical stimulation over the developing fetus are not known.
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^Haldeman, S.; Carroll, L.; Cassidy, J. D.; Schubert, J.; Nygren, Å.; Bone Joint Decade 2000-2010 Task Force on Neck Pain Its Associated Disorders (2008). "The Bone and Joint Decade 2000–2010 Task Force on Neck Pain and Its Associated Disorders". Spine33 (4 Suppl): S5–S7. doi:10.1097/BRS.0b013e3181643f40. PMID18204400. edit
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