Robert Egert, MD

Introduction

What is it ?
- TENS (Transcutaneous Electrical Nerve Stimulation) is used to improve pain relief in a number of pain conditions (including back pain), allowing reductions in drug consumption and ease of movement. A small box (signal generator) about the size of a cigarette packet, which sends an electrical signal via a lead(s) to one or two pairs of skin electrodes placed over the painful area. For those with an interest in physics, most TENS machines are capable of producing signals with a current range of 0-80mA, a pulse width between 40-250 us, and pulse frequencies between 1-250 Hz. Modern TENS electrodes are self-adhesive and come in a variety of shapes and sizes to suit most areas of the body. Older electrodes are made of rubber, need to have some conductive gel placed between the electrode and the skin, and are held in place by adhesive tape.
How does it work ?
- TENS is thought to work by closing the gate on pain i.e. preventing onward transmission of pain signals from the dorsal horn of the spinal cord to the ascending sensory nerves (Spinothalamic tract), which connect to the pain centers in the brain. The TENS signals arrive at the dorsal horn via large sensory A/fibres (L above) at the same time as pain signals arriving via small sensory C/fibres (S above). If the large A/fibre input is sufficiently strong, the C/fibre input is inhibited, producing pain relief (See Pain Mechanisms).
How long can I keep it on for ?
- Generally as long as you can tolerate it for. Hi-TENS will only work for as long as you have the machine switched on, whereas Lo-TENS analgesia will last longer due to endorphin release in the spinal cord (see below). I would not recommend using it during sleep or while driving, as the knobs can be accidentally switched to maximum.
Are there are points in the body where TENS should not be used ?
- The electrodes should not be placed over the front of the neck, as the electrical signals can stimulate the carotid sinus which controls heart rate.
- They should also not be used over the heart, especially in those with pacemakers, for fear of produce rhythm problems.
- Don't place the electrodes on areas of skin that have been affected by severe nerve damage. Mild to moderate nerve impairment may be OK.
Can TENS be used during pregnancy ?
- The manufacturers suggest that TENS should not be used during pregnancy. There are indemnity problems related to miscarriage in the first trimester, and premature labour in the last trimester. TENS is regularly used in labour with one pair of electrodes either side of the spine at T10/11 (autonomic nerve supply to the uterus and sensory supply to the lower abdomen), and the other pair of electrodes either side of the back of the sacral bone (S2/3/4 - autonomic supply to the uterus and sensory supply to the birth canal).

TENS Settings

There are two commonly used TENS setting - Hi-TENS and Lo-TENS.

	Traditional (Hi-TENS)	Acupuncture (Lo-TENS)
Use	General Pain Relief	Muscle Pain
Frequency Range	90 - 130 Hz	2 - 5 cycles Hz
Pulse Width	starting at 100 us	200-250 us
Sensory Nerves Stimulated	A-beta fibres	A-delta fibres
Mechanism of action	Gate theory	Spinal Endorphin Release
Stimulation Intensity	Present but not uncomfortable	Strong Sensation
Reversed by Morphine Antagonist	No	Yes
Duration of analgesia	Short	Long

Two other forms of TENS are:-
- Brief Intense TENS is used for rapid pain relief, and employs 90 - 130 Hz with 200 us pulse width settings. The current delivery is so high with these settings that most patients can only tolerate 15 - 30 minutes of treatment.
- Burst Mode TENS uses the traditional settings as described above, but with the burst mode switched on. The machine delivers the traditional output which is interrupted by bursts at a frequency of 2 -3 Hz. These settings stimulate A-delta and A-beta sensory nerve fibres, "closing the gate" and releasing endorphins to give maximum pain relief.

Electrode Placement

Where do I place the electrodes ?
- As a general rule the electrodes should be placed in the same Dermatome as the pain, so that the TENS effect is acting on the correct part of the dorsal horn in the spinal cord. With one pair of electrodes treatment is limited to the area around the pain. With two pairs of electrodes, one pair can be placed close to the pain, and the other pair can be placed on the spine posteriorly at the correct segmental level for the pain (see dermatomes above).
- Example 1 - Left sided knee pain.
  - Using one pair of electrodes
    - Option 1 - Place the pair of electrodes around the knee where you feel the pain in the knee. Both are in the anterior L3 dermatome. Be careful not to allow them to touch each other.
    - Option 2 - Place one electrode on the knee where you feel the pain (anterior L3), and place the other on the left side of the lumbar spine at the L3 level about 1" from the midline (posterior L3).
  - Using two pairs of electrodes
    - Option 1 - Place one pair of electrodes just above the pain in the knee, and place the other pair just below the pain (all 4 electrodes anterior L3). Be careful not to allow any of them to touch each other.
    - Option 2 - Place one pair of electrodes where you feel the pain in the knee (anterior L3), and the other pair on the left hand side of the lumbar spine at the L3 level 1" from the midline (posterior L3).
- Example 2 - Low Back Pain with left sciatica in the L5 dermatome
  - Using one pair of electrodes - place one electrode where the sciatic pain is in the leg (anterior L5), and the other where you feel the back pain (left posterior L5 close to the spine).
  - Using two pairs of electrodes - place one pair of electrodes along the L5 dermatome in the leg (anterior L5), and place the other pair of electrodes (one above the other) where you feel the back pain (left posterior L5 close to the spine).

Disposable Electrode Care

Disposable self-adhesive TENS electrodes are very convenient but more expensive than the older style ones.
If you follow the following suggestions, you should see 2 weeks service from them even with continuous use:-
- Always place the electrodes on non-hairy, clean, dry skin. Do not place them on skin that has hand cream or body oil on it. If the area is hairy then shaving first ensures proper skin contact. Poor skin contact increases the current needed to stimulate the area, and causes the batteries to discharge faster than usual.
- Prevent the electrodes from drying out. After use dampen the side of the electrode that was in contact with the skin with a drop of tap water. Then place the electrodes back in the "freezer bag" that they came in, sealing the bag properly, and making sure all the air is expelled first. This will help to stop the electrodes from drying out between use. Again a dry electrode reduces TENS effectiveness, increases the current required, and drains batteries faster. There is a tendency for the electrodes to dry slightly during use because body temperature is at 37 deg C. Sometimes it is worth dampening them slightly during a long treatment session to improve conductivity.
- Between uses keep the electrodes cool. I would suggest placing them in the fridge (not the freezer), again to reduce the chances of them drying out. Don't keep them next to any heat source like a radiator or sunny window.

Trouble Shooting

I've connected everything up, switched on the machine, and nothing happens.
- Is the battery dead ? - Most machines run on a 9 volt PP3 sized battery. try inserting another one to check
- Does the battery fit properly in the battery compartment ? - Not all batteries are the same length. Try using the battery type that the manufacturer recommended when you bought the machine. If your battery is too short in length, try inserting a small piece of cardboard at the end opposite to where the contacts are, so that an electrical connection is made again.
- Has the TENS machine been immersed in water ? - The commonest scenarios are that the machine fell in the toilet or the hand basin accidentally. If this is the case applying gentle heat to the machine may encourage it to work when it is dry again. Try putting it on a towel in an airing cupboard for 24 hours, before trying it again. DO NOT expose the machine to direct heat like a coal fire or a radiator as this may cause the battery to explode. If all else fails, you may need to purchase another from your supplier !!
- Are the leads broken ? - The common place for lead failure is at the 90 degree bend where the lead plugs into the TENS machine. If you have two leads, try the other one, otherwise complete all the other checks. You may have to buy another lead.
- Have the electrodes dried out ? - When you place the electrodes on your skin they should feel slightly wet, and when you pull them off they should still feel sticky. If neither of these is the case, then replace them. The whole electrode should have contact with the skin, and there should be no curled edges. Some people make their disposable electrodes last for longer by taping around the edges with micropore tape, which can be bought at your local chemist's.
My machine works initially for a short time, but after a while the stimulus declines or stops completely.
- Is the battery nearly dead ? - Try changing it for another one.
- Have the leads fallen off ? - Are the electrodes moving under your clothes so that contact with the skin is lost. Experiment with the way that the TENS leads hang when they are in place. It's probably better to have the leads hanging down vertically in most positions in the body. Try securing the electrodes with micropore tape around the edges.
- Are the electrodes past their best ? - If the electrodes fell dry or have lost their stickiness, replace them with a fresh pair.
- Have you become tolerant to the signal strength ? - As you become more confident with using your TENS machine, you may find that you can tolerate higher signal strengths. Try nudging up the power output to see whether this is the case.
The only way I can feel any stimulus is with the power output on maximum
- Is the battery nearly dead ? - Try changing it for another one.
- Are the electrodes past their best ? - If the electrodes fell dry or have lost their stickiness, replace them with a fresh pair.
- Is the lead partially broken ? - Try wiggling the lead, and if the signal suddenly jumps on, then you may have a partially damaged lead. Try changing it for another one.
- Do you have normal skin sensation in the area ? - Don't use TENS in an area where there has been nerve damage.
I have developed a red itchy rash where my electrodes have been
- Try varying the position of the electrodes, and the length of time they have in contact with the skin.
- Try using different manufacturers electrodes (which may have different chemicals in the adhesive.
- Try reverting back to old fashioned rubber electrodes, gel and tape. Use micropore for the least chance of skin sensitivity.
- If all these steps fail to help, then you have an allergy problem with no other solution than to give up using TENS - sorry.
If you have any other queries about TENS, please Contact Me via e/mail.