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(Thompson, 1998)

TENS is similar to electro-acupuncture in a number of ways, the principal difference being that the electricity is transmitted through the skin via conducting pads instead of via needles. Another difference is that low- intensity, high-frequency stimulation is most commonly used (40–150 Hz, 10–30 mA). However, some machines offer a ‘burst’ facility, with interrupted trains of fast (100 Hz) stimulation to give an effective pulse frequency which may be as low as 2 Hz; this causes tetanic twitching in the stimulated muscles. Because habituation occurs with

TENS, as it does with electrical stimulation of needles, there have been attempts to design more sophisticated machines to vary the pulse pattern either rhythmically or randomly.

The TENS apparatus consists essentially of the stimulator, which is battery-operated and is about the size of a cigarette packet, together with the connecting leads and the electrodes or pads. The minimum controls are one to regulate the amplitude of the current and another to regulate the frequency. Some machines allow the pulse width (the duration of each pulse) to be altered but in most it is fixed. Many manufacturers now produce TENS machines and patients can buy them by mail order quite easily.

It is important to make sure that patients give TENS a proper trial and understand clearly how to use the machine. They must be able to feel the stimulation, which causes a tingling sensation. The nervous system adapts quickly to the stimulus and therefore the amplitude of the current has to be increased after a few minutes’ use. Initially patients should use the machine for at least an hour three times a day; alternatively, they can use it all day. Pain relief generally occurs soon after the machine is switched on; if it doesn’t, the position of the electrodes or the setting of the controls should be altered. Trial and error are essential for success. Patients also need to understand that relief is often felt only while the stimulation is happening; if there is a carry-over into the post-stimulation time this is a bonus but it doesn’t always occur.

TENS is generally safe, the main problem being skin irritation at the sites of the electrodes. This can be minimized by making sure that the area of skin to which the electrodes are applied is kept dry and free from grease and cosmetics; this applies both to the carbon rubber electrodes, which require electrode jelly, and the self-adhesive type. Like electro- acupuncture, TENS should not be used in patients with demand pacemakers. It should also not be applied to the front of the neck, to avoid the theoretical possibility of stimulating the nerves of the carotid sinus, causing hypotension, or the larynx, causing laryngeal spasm.

Choosing where to place the electrodes is important. The commonest method is to place them on either side of the painful area; another idea is to place them over a main nerve leading to the area. A third recommendation is to place them over the spinal cord, two or three segments above the painful segment, but this is less often effective.

TENS is used to treat both acute and chronic pain; success has also been claimed for its use in labour pain. TENS can relieve some kinds of pain that are otherwise difficult to treat: for example, neuropathic pain (post-herpetic neuralgia) and pain due to brachial plexus avulsion and spinal cord trauma. However, the response to TENS is not always sustained over time; this may be due to an increase in the pain level or to a change in the patient’s response to TENS. The original response may have been a placebo effect, or tolerance to TENS may develop through various mechanisms.

144 Acupuncture in Practice

The relation between acupuncture and TENS is somewhat complicated. Some patients will respond to acupuncture but not TENS; some will respond to TENS but not acupuncture; some will respond to both. It is often worth trying TENS in patients who do not improve with acupuncture or who do so for only short periods. In some painful disorders, such as post-herpetic neuralgia, TENS is much more likely to succeed than is acupuncture. The unpredictability of response must however always be kept in mind.

A woman had had a lumbar puncture performed at another hospital. The procedure had been technically difficult, and in the process a nerve root in the cauda equina had been damaged, resulting in severe pain in the leg that persisted for many months. I thought she would respond to TENS so got her into hospital for an extended trial of this treatment. After a week she was no better. On the morning she was to be discharged I tried acupuncture at LR3 on the painful side, with no real expectation that it would help, but to my surprise it produced immediate relief. After a few more treatments at intervals she was practically pain-free.

Non-pain use of TENS

I have found that TENS will relieve spasm in some patients with multiple sclerosis. For example, if the electrodes are placed over the femoral nerve in a patient whose leg is in spasm, it may become flaccid immediately. This is not always desirable, since it can make it impossible for the patient to stand up; but it is at least of theoretical interest and can on occasion be useful in practice. However, it never seems to work in patients who have had a spinal cord transection due to trauma.