Trigger points (TPs) are zones in muscles and sometimes in other tissues that are tender when pressed and which may give rise to referred pain and other remote effects. The earliest recorded research on the referral of muscle pain was was carried out by Kellgren, following up the chance observation by Sir Thomas Lewis; his technique involved injecting 6% saline into various sites in volunteers and recording the pain felt. He found that injecting tendons or tough connective tissue produced local pain, whereas injecting muscle bellies produced pain referred in a constant pattern some distance away from the site of injection.
The most complete study of TPs has been made by J.G. Travell and D.G. Simons. Travell, an orthodox pain specialist in the USA who looked after President Kennedy, published a classic paper on the subject in 1992. She later collaborated with Simons in their major work on the subject, Myofascial Pain and Dysfunction: the Trigger Point Manual.
Melzack studied the relation between TPs and acupuncture points. He and his colleagues found that every TP recorded in the Western literature has a corresponding acupuncture point, and in 71 per cent of cases there was a close relationship between the patterns of pain associated with the two kinds of point.
TPs are a clinical phenomenon and little hard evidence exists to show what they actually are. Attempts to excise them for histological examination have generally been unsuccessful, partly because – at least at an early stage – they are very transient. There have been claims that they are localized areas of muscle spasm, or alternatively that they are localized areas of inflammation produced by the opening of small arteriovenous shunts shunts. It is also possible that the changes in the muscle are not really the primary event, but are secondary to altered patterns of function within the CNS.
Clinically, TPs may be latent TPs or active . An active TPs TP gives rise to referred pain and sometimes to other remote effects such as muscular weakness or autonomic changes. TPs can also be classified as primary TPs and secondary TPs (found in areas of referred pain). Satellite TPs TPs occur in synergist or antagonist muscles.
TPs have been described under many other names (fibrositis, fibromyalgia, muscular rheumatism). They underly many kinds of clinical problems. They appear to become active for many reasons: overuse, fatigue, chilling, maintenance of faulty posture, for example; the currently fashionable ‘repetitive strain injury’ is a classic example of a TP disorder. Once established, they may persist for many months or even indefinitely. Two common patterns of origin are seen. A sudden overload may cause a TP to develop in a muscle: for example, after digging the garden in spring. Alternatively, long-standing misuse of muscles, for example by faulty posture, may activate the TPs. An initial primary TP may give rise to secondary TPs, usually in a distal distribution. Dr Chann Gunn has proposed that partial denervation of a muscle, such as may occur in radiculopathy, causes hypersensitivity of the muscle concerned and consequent development of TPs.
TPs seem to increase in number up to middle age and then to decrease. There may be just a few of them or scores. If they are very numerous (almost always in women) treatment is seldom satisfactory, and this is probably a separate disorder. Several recent papers suggest that fibromyalgia should be distinguished from TPs brought on by unaccustomed activity and so forth. In fibromyalgia so defined, there are numerous TPs in many areas of the body, there is sleep disturbance, and usually a fair degree of associated depression. This disorder tends to do badly with acupuncture.
TPs are detected by means of palpation. The main technique consists in drawing the finger transversely across the muscle, or alternatively (for “strap” muscles such as the sternomastoid) by grasping the muscle between finger and thumb. Resistance may be felt in the muscle fibres, and the patient will experience pain, which can be quite severe (the ‘jump’ sign). A muscular twitch may occur, and there may be radiation of pain to the areas of referred pain. In general, referral of pain appears to occur from proximal to distal and from behind forwards. Learning to locate TPs is a very important part of acquiring skill in acupuncture.
Common sites for TPs are near the muscle attachment to tendon or bone, central within the belly, or at free borders. Because the TP may be very sensitive, examination must be done gently. It is important to compare the two sides of the body because it is increases in tenderness that are significant.
The actual TP may be fairly small – 3 to 6 mm in diameter, but there is often an associated band of taut muscle around it. In spite of a fair amount of research there is little agreement about the pathological nature of TPs. They are difficult to study scientifically, partly because they may disappear temporarily in response to pressure. (This propensity needs to be kept in mind in looking for them clinically.) There is stiffness and shortening of the affected muscles in association with active TPs but this is not usually noticed by patients.
TPs can be inactivated in various ways, including simple pressure (‘acupressure’) and acupuncture, as well as by injecting various substances (local anaesthetics, corticosteroids, or even saline). A great deal of modern acupuncture (perhaps 80 per cent) can be thought of as the detection and inactivation of TPs.