TENS - transcutaneous electrical nerve stimulation
TENS stands for transcutaneous (through the skin) electrical nerve stimulation and is a treatment that uses low voltage electrical currents to relieve pain.
The gentle electrical current is passed through electrodes which are taped to the skin near the site of the pain. The current comes from a small battery-operated machine. Because the machines are quite portable and easy to use, people can use them at home.
How does TENS relieve pain?
It is not certain how TENS relieves pain. One suggestion involves the 'gate control' theory of pain. To understand this, we first have to have a basic understanding of how we feel pain. Put simply, we feel pain when pain receptors in our body are stimulated and a pain signal is sent along certain types of nerve fibres (A-delta and C) to the spinal cord. In the spinal cord the nerve fibres encounter a gate-like mechanism that opens and closes to allow or prevent signals from then travelling on to the brain. If the so-called ‘gate’ is open, the pain signal travels on to the brain, where the pain is registered and so we perceive the pain.
It is thought that the electrical current from a TENS machine stimulates a different type of nerve fibre (called A-beta) that carries signals relating to touch. The signals from the A-beta fibres travel to the spinal cord where they temporarily block the transmission of pain sensations to the brain – this is analogous to the ‘gate’ being closed - and so the body does not experience pain. This theory is known as the ‘gate control’ theory of pain (see box).
TENS is also thought to increase release of endorphins - the body’s own natural painkilling substances, which are produced in the brain in response to pain or stress.
|Gate control theory of pain|
The gate control theory of pain was put forward by Ronald Melzack and Patrick Wall in 1965. They proposed that there was a ‘gate’ mechanism in the central nervous system that opened to allow pain messages through to the brain and closed to prevent them getting through.
When we feel pain, such as when we touch a hot stove, sensory receptors in our skin send a message via nerve fibres (A-delta fibres and C fibres) to the spinal cord and brainstem and then onto the brain where the sensation of pain is registered, the information is processed and the pain is perceived.
The gate theory says that as these pain messages come into the spinal cord and the central nervous system (before they even get to the brain), they can be amplified, turned down or even blocked out. There are many accounts of how people injured on the battlefield or in sports games don’t feel any pain from their injuries until afterwards. This has to do with the brain being busy doing other things and shutting the gate until it can pay attention to the messages.
Large diameter nerve fibres (A-beta fibres) responsible for transmitting signals of touch to the brain have the ability to close the pain gate and so block signals from other smaller diameter nerve fibres which transmit pain.
An example of this would be when a child falls over and hurts their knee — if they rub the knee, the signal from that sensation of touch temporarily blocks the pain signal travelling from the injured knee to the brain.
How effective is TENS at relieving pain?
Some people find that they only gain relief while the TENS unit is operating; for others the pain relief continues after they’ve stopped their TENS treatment. But, while some people find that TENS is effective at relieving their pain, the overall clinical evidence supporting its effectiveness is not so clear. Not enough good studies have been done to draw firm conclusions. Some people find that TENS does not help their pain at all.
What types of pain do people use TENS for?
TENS is used by people to treat a variety of acute and chronic pain conditions, including low back pain, sciatica, osteoarthritis, rheumatoid arthritis, fibromyalgia, tennis elbow, neck pain, knee pain, period pain, neuropathic pain (pain caused by nerve or spinal cord damage, such as phantom limb pain and multiple sclerosis pain), cancer pain and the pain of childbirth. The electrodes must be placed near to the source of the pain, for example for back pain they should be placed around the centre of the painful area.
You should be aware though that there is no strong scientific evidence in favour of using TENS for pain conditions and more conclusive scientific studies are needed to support its use.
What does TENS feel like?
You will feel a tingling or buzzing feeling on your skin where the electrodes are placed. Some people may find this sensation unpleasant.
Are there any side effects from using TENS?
TENS appears to be free of any side effects except for occasional irritation where the electrodes are applied to the skin. Some people may find the sensation of TENS too unpleasant and won’t be able to use the machines.
Can anyone use a TENS machine?
The short answer is no – TENS is not suitable for everyone.
You should NOT use TENS:
- if you have a pacemaker fitted or you have a severe heart disorder;
- if you have epilepsy and you are using the machine alone;
- if you are in the first trimester of pregnancy (at least not around the abdomen);
- over the front of your neck or over your face;
- over an area of broken skin; and
- if you have a cochlear implant hearing device.
Always check with your doctor if you are thinking of using TENS or any other form of self-treatment.
Where can I access a TENS machine?
Physiotherapists use TENS machines – both in hospitals and in private practice - and they are available for loan or hire from some hospitals. However, as they may not be widely available for hire, people who gain pain relief from TENS often find it convenient to buy their own machine for long-term use.
TENS machines can be bought from the many manufacturers who advertise their products on the internet, or from medical equipment suppliers, and from some pharmacies. Some health funds will rebate some or all of the cost of the machine.
Although you can buy a TENS machine direct from a manufacturer or distributor, ideally it should be demonstrated by a trained healthcare professional so that you get the most benefit. This person can show you the best place to apply the electrodes and the best settings for you.
TENS machines usually have an adjustment for the intensity and also the frequency (high/low) of the electrical current. Conventional TENS is high frequency. High-frequency TENS is the signal that causes the pain gate to close. Low-frequency TENS is said to be more effective than high frequency for stimulating relief of endorphins. Some machines have three modes: a constant mode (high frequency, low intensity); a burst mode (low frequency, high intensity); and a modulation mode (cycling through variable frequency and pulse).
TENS and acupuncture
TENS is also sometimes used to provide a non-needle form of acupuncture. In this method, the TENS electrodes are applied to the skin over traditional acupuncture points rather than acupuncture needles being inserted.
PENS (percutaneous electrical nerve stimulation)
PENS is a similar therapy to TENS, but, with PENS, the electrical stimulation is passed through the skin into the soft tissue using probes similar to acupuncture needles.
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