TENS is a non-invasive, safe nerve stimulation treatment intended to reduce pain, both acute and chronic. There is a small electrical device that delivers electrical impulses across the skin. The device is usually about the size of a personal stereo, although lightweight versions not much bigger than a credit card are available.
The device is connected by wires to sticky pad electrodes, which are placed on the skin in the area of the pain. This allows a small, low-intensity electric charge to be passed across the area.
How do 'TENS' devices work? Transcutaneous means 'through the skin'. TENS devices deliver small electrical pulses to the body via electrodes placed on the skin. TENS devices are thought to affect the way pain signals are sent to the brain. Pain signals reach the brain via nerves and the spinal cord. If pain signals can be blocked then the brain will receive fewer signals from the source of the pain. We may then feel less pain.
TENS devices are thought to work in two ways:
- When the device is set on a high pulse rate (90-130 Hz) it is thought to trigger the 'pain gate' to close. The 'pain gate' is a theory that says that when certain non pain-carrying nerves are stimulated then these can interfere with ('close') signals from nerves that transmit pain. The TENS device is thought to stimulate certain non pain-carrying nerves. This may then interfere or block pain messages sent to the brain. This is the normal method of use.
- When the device is set on a low pulse rate (2-5 Hz) it stimulates the body to make its own pain easing chemicals called endorphins. These act a bit like morphine to block pain signals.
The device is usually used for 15 to 20 minutes, several times a day, and is controlled by the user rather than a health professional. Pain relief may be rapid and last for days.
Unlike many pain-relieving drugs, TENS isn't addictive and seems to have few side-effects. Most people can use a TENS device but it's not suitable for:
- Those with pacemakers and certain other types of heart disease.
- Unknown causes of pain.
- Unsupervised in pregnancy (other than in labour).
TENS is widely used around the world for a variety of painful conditions (for example, there are an estimated 450,000 users annually in Canadian state hospitals, while in Scandinavia TENS is well known as a treatment for period pain). In the UK, the use of TENS is increasing in many fields and, whilst originally used by women in labour, it's now marketed for most sorts of pain, especially musculo-skeletal.
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.
Many people confuse TENS with Electro 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.
While controversy exists as to its effectiveness in the treatment of chronic pain, a number of systematic reviews or meta-analyses have confirmed its effectiveness for postoperative pain, osteoarthritis, and chronic musculoskeletal pain.
Conversely, results from the Bone and Joint Decade 2000-2010 Task Force on Neck Pain, reported in early 2008, showed no clinically significant benefit to TENS for the treatment of neck pain when compared to sham treatment.
A January 2010, systematic literature search published in the journal Neurology, advised against recommending TENS for chronic low back pain since evidence is controversial for that type of pain. However, other reputable neurologists wrote in the same issue of the journal: "Absence of evidence is not evidence of absence," and although the research on TENS may be thin, "there seems to be considerable empirical evidence that, at least in some patients, TENS is useful." Recent clinical studies and meta-analysis suggest that having an adequate intensity of stimulation is necessary to achieve pain relief with TENS.
In palliative care and pain medicine, TENS units are used in an attempt to temporarily alleviate neuropathic pain (pain due to nerve damage). Some patients benefit from this approach, while others do not, depending on individual differences, and pain threshold. Use of TENS causes cell death at the site of application and should not be used long term. A significant number of TENS device brands have been targeted for use for labour 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."
TENS electrodes should never be placed:
- Over the eyes due to the risk of increasing intraocular pressure
- On the front of the neck due to the risk of a acute hypotension (through a vasovagal reflex) or even a laryngospasm
- 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.
- Over a tumour/malignancy (based on in vitro experiments where electricity promotes cell growth)
- Directly over the spinal column
TENS should not be used across an artificial cardiac pacemaker (or other indwelling stimulator, including across its leads) due to risk of interference and failure of the implanted device. Serious accidents have been recorded in cases when this principle was not observed.
On areas of numb skin/decreased sensation TENS should be used with caution because it's likely less effective 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.