Transcutaneous electrical nerve stimulation (TENS)
Amanda Howarth RGN, B Med Sci, MSc (Pain)
Lecturer, CARER Department, School of Nursing and Midwifery, The University of Sheffield
Way Ahead 2005;9(2):4-5
Transcutaneous electrical nerve stimulation (TENS) is the electrical stimulation of peripheral nerves for analgesic effects. A TENS machine is a small battery operated unit that people can use to help them manage long-standing pain. It is totally portable and should not restrict a person's activity in any way except when showering, swimming and in some instances, when driving. Most physiotherapy departments, pain clinics and some GP practices have TENS machines that health care professionals trained in their use can lend out to people.
In theory TENS broadly works in two ways. Firstly it exploits Melzack and Wall's gate control theory of pain. TENS stimulation can produce pre-synaptic inhibition of noxious information in the afferent C fibres. Similar effects could be produced by vibration, rubbing or massage. Secondly TENS may also excite higher centres causing the release of endogenous opioids. These opioids have a descending inhibitory effect at the dorsal horn binding to receptors on nociceptive afferent neurones, so inhibiting the release of substance P.
TENS in multiple sclerosis
Historically MS has been regarded as having a painless pathology. More recent evidence strongly refutes this idea. Over 50% of people with MS present with pain syndromes associated with, if not directly caused by, their pathology. It is important to be aware that pain in MS can be intrinsic, ie directly due to the demyelinating lesion, but it can also be extrinsic in origin. Extrinsic problems, for example low back pain, can affect people with MS just like anyone else.
Acute pain syndromes such as trigeminal neuralgia, painful tonic seizures and Lhermitte's sign are probably not the types of pain to be using TENS for. It is more appropriate to use TENS for chronic pain syndromes. There are three main types/locations of chronic pain experienced by people with MS:
- dysasthetic extremity pain - the pain that is described as burning, tingling or shooting in nature;
- back pain - often caused by reduced mobility, poor posture and the use of wheelchairs and other aids;
- painful leg spasms - TENS may be useful where antispasmodics are not tolerated.
A TENS machine has two settings, the settings used being determined by the type of pain a person is experiencing:
1. Pulse frequency (rate)
The pulse frequency (sometimes called pulse rate) - is the number of pulses per second, or Hertz (Hz) of current delivered. The frequency range typically available in TENS units is 0-250Hz.
2. Pulse duration (width)
The pulse duration or width is the length of time each pulse lasts, usually in microseconds (mcsecs). The common clinically useful range of adjustment is 100-200mcsecs.
The two electrodes that attach to the TENS machine are called the anode and the cathode. The anode is usually black, positively charged, and sometimes represented by a + (plus) sign. The cathode is usually red, negatively charged, and sometimes represented by a - (minus) sign. The electrical current travels from the anode to the cathode, so the sensation is usually stronger under the anode.
TENS machines should have a continuous mode and a burst mode. In the continuous mode it is thought that the gate control theory of pain works. The frequency should be set at 80-100Hz and the pulse width at 200mcsec (see figure 1). In this mode the machine can be used for long periods of time (minimum 60 minutes).
In the burst mode it is thought the body produces endorphins. The frequency should be 2Hz and the pulse width 200mcsec (see figure 1). The treatment time with this mode is around 45 minutes, 4-5 times a day.
Fig 1. Constant and burst mode settings
TENS can be applied in four different ways:
- directly to the painful area;
- over peripheral nerves;
- over spinal nerve roots;
- specifically to motor, trigger and acupuncture points.
Someone who has been trained in the use of TENS can assess a patient, decide on the probable cause and location of their pain and then choose the most appropriate mode and method of application.
Traditionally black carbon rubber pads, gel and tape have been used with TENS machines. However, these are messy and may be difficult for a person to manage at home. They can also leave the patient susceptible to burns, skin irritation or allergies to the tape. If this is the only option, hypoallergenic tape such as Micropore should be used, as well as an electro-conductive gel manufactured for this purpose - not KY Jelly. Wherever possible self-adhesive pads should be used. They can be purchased for around £5 for a pack of four (cheaper if bought in bulk) and can be used over and over again until they are no longer sticky. If looked after they can last around a month.
The distance between the electrodes is important. If they are too close together the current will 'short' and bypass the person, but if they are too far away from each other stimulation may be lost. Ideally pads should be a 'pad's width' apart.
What is best practice?
If a health care professional feels a patient may benefit from a TENS machine, they should be referred to someone who has had training in the use of TENS. It is useful to have a referral protocol to identify whether a patient is suitable for TENS:
- individuals must be motivated to help themselves. If they are expecting a cure or do not want to achieve some level of pain relief then it is inappropriate;
- the individual or a carer must be able to apply the pads to the required area;
- the individual or a carer must be able to manage and understand the controls on the machine;
- TENS should ideally be used as an adjunct not as a sole treatment.
Individuals should be given a trial of TENS under the supervision of a trained health care professional and they should ideally be offered the opportunity to take the machine home and use it for a few weeks to determine whether it is going to be of any benefit to them. In doing so it avoids people wasting their money on something that may not be of use to them. Individuals can then be expected to purchase their own machines - a dual channel machine with two pairs of pads that can be used to treat either two separate areas or one large area can be bought for around £30.
Advice about where to buy a machine can be obtained from the clinic or professional who has lent them the machine in the first instance. However, TENS machines must be approved under the medical devices directives 93'42/EEC before they can be sold to the public. Therefore, a legally approved unit will be labelled with a CE mark, which is a four-figure number. This confirms the safety of the unit.
Driving - individuals need to inform their insurance company if they are going to drive with the machine on as it may invalidate their insurance.
Electrode sites - do not use around the anterior cervical spine. This risks stimulating the carotid sinus, which could cause cardiac problems (usually hypotension). Neither should stimulation be attempted over the eyes due to the delicate nature of this organ.
Skin condition - electrodes should not be placed over broken skin, sores or areas of acute eczema and psoriasis.
Dermal reactions (electrical irritation) - if the intensity of the machine is very high some people may find that their skin burns.
Contact allergy - an allergy may develop to the pads, adhesive or tape used. If this occurs alternatives should be found.
Pre-existing lymphoedema - TENS is thought to have an effect on the circulatory system. Therefore it could adversely affect the lymphatic circulation causing further fluid retention. Issues of skin integrity should also be considered in this group of people.
Caffeine intake - a high intake of coffee (above four cups a day) is thought to reduce the effectiveness of TENS.
Contraindications to using TENS
- on-demand pacemakers;
- percutaneous central venous catheter (PCVP);
- 1st trimester of pregnancy;
- undiagnosed pain;
- a confused patient;
- not to be used in the shower.
TENS is a useful tool in helping in the management of pain in multiple sclerosis. However, it does not get rid of the pain or the underlying problem permanently. It is something that needs to be used on a long-term basis. People need to be shown how to use the machine correctly to give them the optimum chance of it being successful. This should be done by a trained health care professional.