Transcutaneous electrical nerve stimulation (TENS) uses electric current to stimulate the nerves for therapeutic purposes. TENS therapy is a non-invasive, safe treatment that has been used to reduce musculoskeletal pain, both acute and chronic. Decreased pain occurs during use and the therapeutic effect may last for minutes to a couple hours after treatment. The unit is usually connected to the skin using two or more electrodes. A typical battery-operated TENS therapy unit is able to modulate pulse width, frequency and intensity. Home units are relatively affordable, easy to operate, and are very portable.
While controversy exists as to its effectiveness in the treatment of chronic pain, several systematic reviews or meta-analyses indicate its effectiveness for postoperative pain, osteoarthritis, and chronic musculoskeletal conditions. In general, electrical nerve stimulation is probably best used in conjunction with other treatment strategies (i.e. therapeutic exercise, manual therapies, anti-inflammatory and pain medications, injection therapy, lifestyle changes, etc.) and is less effective when used as the primary treatment. Some companies report that electrical current helps to promote healing of damaged tissues which reduces chronic pain. However, this has not been proven scientifically.
Gate Theory of Pain
Scientific studies suggest that TENS units work by blocking the transmission of pain signals to the brain as outlined by the Gate Theory of Pain. The Gate Theory was first described by Melzack and Wall in 1965. They proposed that there is a gating mechanism within the spinal cord. Small nerve fibers are pain receptors and large nerve fibers are normal receptors. Both synapse or give electrical information on projection cells which travel up to the brain. There are inhibitory interneurons within the spinal cord that are turned on as a default setting. This means that a stimulus must overcome this baseline activity to let the brain know there is pain. The inhibitory interneurons prevent the brain from receiving a pain signal and are turned on where there is no information being sent or when normal large nerve fibers are stimulated. When painful small fibers are stimulated they relay pain and cause the inhibitory neuron to turn off. This lets the brain know there is pain. Thus, the inhibitory neuron acts like a gate and only opens when there is perceived painful stimulus. Descending pathways from the brain close the gate by inhibiting the projector neurons and diminishing pain perception.
Distracting stimulus, such as rubbing, massage, heat, ice or TENS stimulation activate the large nerve fibers so that pain is not perceived. Before using a TENS unit you should always consult your physician as there are certain situations for which treatment with electrical stimulation is contraindicated, such as someone with a pacemaker or defibrillator, over areas of tumor/malignancy, and directly over the spinal column in a patient with a prior laminectomy surgery.