Since the first modern, patient-wearable TENS machine was patented in the United States in 1974, Transcutaneous Electrical Nerve Stimulation has been a hugely popular way of reducing pain in a variety of circumstances. Yet, although the concept seems relatively new, the idea of using electrical stimulation to control pain has actually been around for at least two thousand years.
It was pioneered in antiquity by the ancient Greeks, who used the Torpedo or electric ray to numb the pain of childbirth and operations, albeit that they almost certainly did not understand the principles underlying the therapy. Rays are capable of producing an electric discharge – ranging from as little as 8 volts up to 220 volts according to the species – in order to stun prey and for defence.
The electrogenic properties of electric rays were also known in Rome as early as the first century AD, when Scribonius Largus, court physician to the Roman emperor Claudius, reported that pain could be relieved by standing on a Torpedo at the seashore. He advocated this treatment for gout and headaches in his work Compositiones Medicae.
Such fish were considered to be magical creatures by the ancients; their ability to numb fishermen without seeming to touch them reinforced their occult qualities before the concept of electricity was understood. The genus Torpedo lends its name to the maritime weapon and derives from the Latin ‘torpere’, which means ‘to be stiffened’ or ‘paralysed’, underlining the effect on anybody who handled or stepped on a living electric ray.
As the centuries progressed, attention turned to man-made electrical devices for pain relief. From the 16th to the 18th century, various electrostatic apparatuses were used to treat headaches, a famous proponent of this treatment being Benjamin Franklin. In the 19th century, further devices were invented, including a low-powered gadget developed by Gaiffe in Paris, which may be seen as the modern precursor to today’s TENS.
In 1919, Charles Willie Kent developed the Electreat in Illinois, for use as a pain control and cancer ‘cure’. Costing $1, the Electreat was marketed under the slogan ‘Shooting Electricity in forces the Pain out’, and some 250,000 were sold over the following 25 years.
TENS should not be used:
- Over the eyes due to the risk of increasing 'intraocular pressure' (or pressure on the fluid in the eye) or close to the mouth.
- On the front of the neck due to the risk of an acute hypotension (through a vasovagal reflex) or even a laryngospasm
- Through the chest using an anterior and posterior electrode position, 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 specialised units.
- On broken skin areas or wounds (although it can be placed around wounds) or areas of infection.
- Over a tumour - in vitro experiments have found that electricity promotes cell growth.
- Directly over the spinal column.
- In patients with pacemakers or implantable cardioverter-defibrillators (ICDs) and only cautiously for pregnant women or patients with epilepsy.
- On areas of numb skin or where there is decreased sensation - treatment is likely to be less effective where there's nerve damage.