This is a brief introduction to transcutaneous electrical nerve stimulation (TENS). This article explores the history, basic function, uses, and safety aspects of the TENS unit, a small battery-operated device that uses low-voltage current to relieve pain.
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Electrical stimulation for pain reduction is not a novel idea. Scribonius Largus, court physician to Julius Caesar, documented its use in ancient Rome as early as 63 A.D. in the form of standing on an electrical fish. It was not until the 16th-20th century that people began to use electrostatic devices for pain relief, a method which gained the support of academics like Benjamin Franklin. Clyde Norman Shealy, MD, PhD, an American neurosurgeon and pioneer in pain medicine, is mainly credited for the development of the modern TENS unit. The first wearable TENS unit was patented in the US in 1974.
The modern day transcutaneous electrical nerve stimulation (TENS) unit is a portable device that uses an electric current to stimulate nerves, muscles, and tendons. It is intended to reduce both acute and chronic pain. These devices are able to manipulate frequency, pulse width, and intensity of the electric current. It is usually placed on the skin or attached to acupuncture needles using two or more electrodes (a conductor through which electricity enters or leaves an object, substance, or region). An adequate intensity of electrical stimulation is necessary to achieve pain relief with TENS. 
A few studies have shown that TENS may modulate or suppress pain signals in the brain. The currently proposed mechanisms of the TENS unit's ability to neuromodulate are as follows:
- Restoration of impulses from the periphery of the body to the brain or spinal cord
- Internal pain control via opioid peptides: endorphins, enkephalins, and dynorphins
- Inhibitory signals originating from the spinal cord
- Direct inhibition of an abnormally excited nerve
The TENS unit is commonly used in physical therapy in Western culture, and in acupuncture in Eastern culture. However, the approaches taken by a physical therapist and an acupuncturist differ in how the device is physically connected to the patient. Physical therapists usually use TENS units that have rubber electrodes that conduct through the skin, while acupuncturists connect the TENS unit to acupuncture needles, producing a stronger effect. Duration of use ranges from fifteen minutes to several hours a day. The use of a TENS unit should be discussed with a provider prior to application, as duration and frequency may vary based on the condition being treated.
In Chinese medicine, the TENS unit is either placed directly on the skin at acupoints or attached to acupuncture needles. It can be used for relaxation and to build up Qi, the Chinese term for energy and energy flow, at a low frequency (below 4 Hz). To open a blockage in a meridian (energy channel within the body) and let Qi flow freely, the frequency is usually set above 10 Hz.
Research on the effectiveness of TENS for a wide range of medical conditions yielded varying results from unfavorable to very favorable. Existing clinical reports address the different uses of TENS (based on the Western approach), its efficacy, as well as some controversies. As of now, the use of TENS is considered a favorable pain-relief option, with efficacy varying by study and medical condition. Some of the conditions for which TENS has been studied include:
- Musculoskeletal pain, such as joint pain from osteoarthritis, rheumatoid arthritis, and acute post operative pain: The evidence of TENS for low back pain and myofascial pain is controversial. A 2007 review showed evidence supporting TENS in chronic musculoskeletal pain. Another review from the Cochrane Collaboration in 2008 deemed the evidence evaluating TENS for low back pain to be poor quality and no definite conclusions could be drawn [2,5]. Evidence from another study in 2008 found no benefit in using TENS for neck pain when compared to a placebo treatment . A 2010 review showed evidence that argued against the efficacy of TENS in chronic low back pain.
- Neurogenic pain, trigeminal neuralgia, postherpetic neuralgia, atypical facial pain, pain after spinal cord injury (SCI) [4,7]: A recent study conducted in 2014 suggests that postherpetic neuralgia cannot be completely prevented. However, as a single therapy, TENS was found to be the most successful among the tested treatments in reducing the incidence of subacute herpetic neuralgia.
- Visceral pain, urge incontinence, and diabetic neuropathy (per an American Academy of Neurology literature study) : In this particular study, researchers performed a systematic literature search of Medline and the Cochrane Library from their respective inceptions to April The literature search demonstrated that TENS is likely effective in treating painful diabetic neuropathy. In another study, TENS improved incontinence symptoms, enhanced quality of life, and decreased adverse effects; hence, it is recommended in treating poststroke urge incontinence (UI) .
If used appropriately, a TENS unit has the potential to provide pain relief and is associated with minimal side effects and is considered low risk.
There are several areas of the body where the use of TENS electrodes are contraindicated:
- Over the eyes (risk for increasing intraocular pressure)
- On the head
- Directly over the spinal column
- Over a tumor
- Over broken skin or open wounds; however, electrodes can be placed around the wounds.
- On the front of the neck (risk for causing acute low blood pressure)
- On the chest either through electrodes placed anteriorly or posteriorly
- Over an artificial cardiac pacemaker or implantable cardioverter-defibrillators (ICDs)
- The TENS unit is thought to be less effective on areas of decreased sensation/numbness due to nerve damage.
- TENS has the potential to cause skin damage at high currents.
- TENS is to be used with caution in pregnant women (avoid placing electrodes on the uterus and surrounding areas) and people with epilepsy.
The use of transcutaneous electrical nerve stimulation (TENS) has existed for many centuries. However, it has only been studied in a few medical conditions. This demonstrates a need for more studies to verify the efficacy of TENS, and its use remains controversial at this time. Despite the paucity of scientific research, many who have tried TENS units subjectively report pain relief.
Although TENS units are widely available for purchase from commercial vendors, one should consult the advice of a healthcare provider prior to use. The individual vendor for a TENS unit may provide instructions for safe use of their device along with the product; however, no official guidelines currently exist. TENS units may range in price from thirty to several hundred dollars.
- Johnson M, Martinson M. Efficacy of electrical nerve stimulation for chronic musculoskeletal pain: a meta-analysis of randomized controlled trials. Pain. 2007;130:157–165. [PubMed]
- Nnoaham, K. E.; Kumbang, J. (2008). "Transcutaneous electrical nerve stimulation (TENS) for chronic pain". In Nnoaham, Kelechi E. "Cochrane Database of Systematic Reviews". The Cochrane database of systematic reviews(3): CD003222.
- 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.
- Dubinsky, R. M.; Miyasaki, J. (2009). "Assessment: Efficacy of transcutaneous electric nerve stimulation in the treatment of pain in neurologic disorders (an evidence-based review): Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology". Neurology74(2): 173–176.
- Khadilkar, A.; Odebiyi, D. O.; Brosseau, L.; Wells, G. A. (2008). "Transcutaneous electrical nerve stimulation (TENS) versus placebo for chronic low-back pain". In Brosseau, Lucie. "Cochrane Database of Systematic Reviews". The Cochrane database of systematic reviews(4): CD003008.
- Sluka KA. The Neurobiology of pain and foundations for electrical stimulation. In: Robinson AJ, Snyder-Mackler L, editors. Clinical Electrophysiology. Lippincott Williams & Wilkins; Philadelphia: 2008. pp. 107–149.7.
- Stepanovic A, Kolsek M, Kersnik J, Erculj V. Prevention of post-herpetic neuralgia using transcutaneous electrical nerve stimulation. Wien Klin Wochenschr. 2014 Dec 4.
- Guo ZF, Liu Y, Hu GH, Liu H, Xu YF, Transcutaneous electrical nerve stimulation in the treatment of patients with poststroke urinary incontinence. Clin Interv Aging. 2014 May 23;9:851-6. doi: 10.2147/CIA.S61084. eCollection 2014.
By Felicia C. Yu, MD
UCLA Center for East-West Medicine, East-West Primary Care Fellow
Rosana Chan, MPH, UCLA Fielding School of Public Health
UCLA Center for East-West Medicine, Administrator
Vivianne Chang, Human Biology and Society B.S., UCLA 2014
UCLA Center for East-West Medicine, Administrative Assistant