In outline, there seem to be five types of mechanism involved in pain modulation by acupuncture.
Another way of thinking about chronic pain is as a form of faulty learning.
Not all kinds of memory are as useful as is acquired immunity. Sometimes it's just the opposite—think of autoimmune disease. Chronic pain is due to unwanted memory that has some resemblance to autoimmunity. It can be thought of a form of faulty learning, in which case pain relief by acupuncture is produced by erasure of the unwanted memory. The mechanism by which this happens is still not entirely clear but it may be connected with a phenomenon known as LTP.
Acupuncture may be a means of counteracting LTP in cases of chronic pain.
Acupuncture is normally done in conscious patients. (There is not much evidence to show that it works in patients who are unconscious.) Pain can be modulated by attention and expectation.
For example, Benedetti and colleagues induced pain by the injection of capsaicin into several body areas simultaneously while appling a placebo cream to just one of these areas. This produced analgesia only in the area to which the cream was applied; the effect was blocked by naloxone.
Most important, this specific effect is mediated by endogenous opioids, indicating that placebo-activated opioids do not act on the entire body but only on the part where expectancy is directed. This suggests that a highly organized and somatotopic network of endogenous opioids links expectation, attention, and body schema. [emphasis added] [Benedetti F, Arduino C, Amanzio M. Somatotopic activation of opioid systems by target-directed expectations of analgesia. The Journal of Neuroscience 1999;19(9):3639-3648.
The important thing to keep in mind is that there is more to acupuncture than the needles. An acupuncture session usually involves a preliminary examination for myofascial trigger points, for example—in other words touching the patient. There may also be the sight of the needles being inserted. And of course there is the information aspect—what the patient understands about what is happening. All these features, as well as others that neither the acupuncturist nor the patient may be fully aware of, influence the outcome. This means that acupuncture should be thought of as a form of multisensory analgesia.
This is a relatively new idea although the first example, the gate theory of pain, which gave rise to treatments such as transcutaneous electrical nerve stimulation (TENS), dates from the 1960s. But we now know that other sensory inputs can also modulate pain,including stimulation of the cochlear vestibular apparatus and vision. These observations have led to the concept of multisensory analgesia, whose relevance to acupuncture seems likely to be far-reaching.
It is difficult if not impossible to isolate the contribution of the needles from that of the other elements of acupuncture in the clinical setting. Still, there is no doubt that needle stimulation has particular effects that are not produced, or not so reliably produced by other kinds of sensory stimulation. Moreover, not all needle stimulation is of equal effect; for example, periosteal (bone) needling and subcutaneous needling are not equivalent.
What this amounts to is that there is nothing magic about the needles. Acupuncture is a form of manual therapy that has much in common with osteopathy, chiropractic, and physiotherapy, all of which involve touching the patient and have effects which are often quite similar to acupuncture effects. But needles can produce responses that are difficult or sometimes impossible to produce in other ways.
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