Clinical features of myofascial trigger points

Recent advances in the understanding of the nature of myofascial TrPs help to explain the strange combination of motor and sensory features that TrPs present clinically.

Prevalence

In Unselected and Control Groups. The prevalence of musculoskeletal pain identified as localized myofascial pain was quite high 37% of males and 65% of females] in a randomly selected Danish population of 1504 people aged 30, 40, 50, and 60 years (21). One study reported examination of a relatively unselected "normal" population to determine the prevalence of myofascial TrPs in the shoulder-girdle muscles only. In this study, Sola and associates (22) examined 100 male and 100 female Air Force airmen [mean age 19.5 years] who had been selected as healthy individuals for military service. The investigators found focal tenderness indicative of latent TrPs in shoulder-girdle muscles in 54% of the females and 45% of the males. Pain was referred from the TrP to its reference zone in 5% of these subjects. These subjects were not examined for taut bands. A recent study of 269 unselected female student nurses (23) showed a similar high prevalence of TrPs in masticatory muscles. A TrP was identified by palpating a taut band for spot tenderness of sufficient sensitivity to cause a pain reaction (24). No effort was made to distinguish active and latent TrPs, but a considerable number of TrPs were likely active because 28% of subjects were aware of pain in the temple area. In masticatory muscles, TrPs were found in 54% of right lateral pterygoid muscles, in 45% of right deep masseter, in 43% of right anterior temporalis, and in 40% of intraoral examinations of the right medial pterygoid muscle. Among the neck muscles, TrPs were identified in 35% of the right splenius capitis muscles and in 33% of right upper trapezius muscles. The insertion of the right upper trapezius was also tender in 42% of those muscles with TrPs. Enthesopathy of this muscle was common (23). Frohlich and Frohlich (25) examined 100 asymptomatic control subjects for latent TrPs in lumbogluteal muscles. They found latent TrPs in the following muscles: quadratus lumborum [45% of patients], gluteus medius [41a/o], iliopsoas [24%], gluteus minimus and piriformis [5%]. In Patient Groups. Individual reports of the prevalence of myofascial TrPs in patient populations are available and, together, indicate a high prevalence of this condition among individuals with a regional pain complaint. The reports that follow are summarized in Table 1. In an internal medicine group practice (3), 54 of 172 patients presented with a pain complaint. Sixteen [30%] of the pain patients met the criteria for myofascial TrPs. Four of these sixteen patients had pain duration of less than 1 month, three a duration for 1 to 6 months, and nine had pain duration of more than 6 months. A neurologist examining 96 subjects from a community pain medical center (2(,) found that 93% of them had at least part of their pain caused by myofascial TrPs, and in 74% myofascial TrPs were considered the primary cause of the pain. Among 283 consecutive admissions to a comprehensive pain center, a primary organic diagnosis of myofascial syndrome was assigned in 85% of cases (27). A neurosurgeon and a physiatrist made this diagnosis independently, based upon physical examination "as described by Simons and Travell (28)." Of 164 patients referred to a dental clinic for chronic head and neck pain of at least 6 months duration, 55% had the primary diagnosis of myofascial pain syndrome caused by active TrPs (29). Five lumbogluteal muscles of 97 patients complaining of pain in the locomotor system were examined in an orthopedic clinic (25). Forty-nine percent of the patients presented latent TrPs and 21% presented active TrPs in the piriformis muscle. The wide range in prevalence of myofascial pain caused by TrPs is likely due in part to differences in the patient populations examined and in the degree of chronicity. Also critical are differences in the criteria selected to make the diagnosis of myofascial TrPs and differences in the training of the examiner. Few of these studies gave a detailed description of the diagnostic examinations employed. This summary does not include papers that used the general definition of myofascial pain syndrome. Active myofascial TrPs are clearly very common and are a major source of musculoskeletal pain and dysfunction, but the poor agreement on appropriate diagnostic criteria must be resolved. This paper presents some useful guidelines. The question arises, "How commonly do TrPs occur with other conditions?" Data from six recent studies of four common diagnoses are summarized in Table 2. In every condition, myofascial TrPs made some contribution, often a major contribution, to the patients' pain.  Granges and Littlejohn (9) found that 68% of patients diagnosed as having fibromyalgia [FM] also had at least one TrP in the immediate vicinity of a designated tender point site; however, they found hat few of the positive tender point sites qualified as TrPs based on a tender spot in a taut band, or induced referred pain, or inducing pain recognized by the subject. The Cranges and Littlejohn study is a different kind of study than those of Finestone (30) and Gerwin (26). In the latter two studies, FM patients were examined for a TrP in any muscle that was indicated by the distribution of the patient's pain.

Diagnostic Criteria

The diagnosis of myofascial TrPs depends on the history and on its confirmation by physical examination. There is poor agreement among authors as to the most appropriate diagnostic criteria. Numerous clinical features have been associated with myofascial TrPs, but only recently have interrater reliability studies been reported that give some guidelines. No satisfactory laboratory or imaging test is currently available for making the diagnosis of myofascial TrPs.

Clinical Features

Several clinical features are commonly associated with the diagnosis of myofascial TrPs (31); these include a confusing mixture of sensory and motor phenomena: History of spontaneous localized pain associated with acute overload or chronic overuse the muscle. The mildest symptoms are caused by latent TrPs that cause no pain but cause some degree of functional disability. More severe involvement results in pain related to the position of the muscle or muscular activity. The most severe level involves intermittent or continuous pain at rest (32).

The precise pattern of pain described by the patient is THE most valuable clue for finding where the TrP is located. Recognizing the pattern of pain as characteristic of a particular muscle tells the clinician where to look for the TrP or TrPs that are responsible for at least part of the patient's pain.

Palpable Band

A cord-like band of fibers is palpable in the involved muscle. This band helps to locate spot tenderness, but it may be inaccessible because of overlying muscles or thick [or tense] subcutaneous tissue. Its tendon attachment may evidence the spot tenderness of enthesopathy (23).

Spot Tenderness

This involves a VERY tender and VERY small spot which is found in a palpable band when the band is accessible to palpation. The sensitivity of this spot [the TrP] is increased by increasing the tension on the muscle fibers of the taut band.

Jump Sign

Pressure on the spot of tenderness causes the patient to physically react to the pain with a spontaneous exclamation or movement. This finding gives an indication of the degree of spot tenderness, but the results are strongly dependent on the amount of pressure exerted by the examiner (33).

Pain recognition

Digital pressure on a tender spot [the TrP] or needle injection of an active locus active locus induces at least some of the pain of which the patient complains, and the patient recognizes it as his or her pain. This finding by definition identifies an active TrP.

Twitch Response

The local twitch response is a transient contraction of the fibers of the taut band associated with a TrP; it can be elicited by vigorous snapping palpation of the taut band [when accessible] at the TrP or by needle penetration of an active locus in the TrP. The latter is an important phenomenon for assuring effective injection of a TrP. Snapping palpation is effective only in a taut band that is in a sufficiently superficial and accessible muscle; it also requires much skill.

Elicited Referred Pain and Tenderness

An active: TrP refers pain in a pattern characteristic of that muscle (31,34). The pain is often not located in the immediate vicinity of the TrP, but is referred to a distance. On initial examination, the patient is often surprised at the location and tenderness of the TrP. Eighty-five percent of the reported pain patterns (31,34) project distally (35). The deep tissue nature of the referred pain that is described by patients is substantiated by Vecchiet, et al. (36), whose study demonstrated the persistence of deep tenderness in the reference zone for a day or more after injection of hypertonic saline into a proximal limb muscle. Subcutaneous tissue was found to be more tender than the skin over both TrPs and FM tender Points (37).
Unfortunately, when the referred pain is elicited by the application of pressure to a tender location it is a non-specific finding (38). Whether one elicits only local pain, referred pain, or reaches pain tolerance depends upon the amount of pressure applied (33).

Elicited referred pain does not clearly distinguish latent TrPs from active TrPs; latent TrPs simply require more pressure (33).

Restricted Range of Motion

Full stretch range of motion of the affected muscle is restricted by pain. This restriction is relieved by the release of the palpable taut bands through inactivation of associated TrPs. The importance of this finding is relatively muscle-specific because it varies considerably from muscle to muscle; therefore, it is more useful as a diagnostic criterion in some muscles than in others. When movement is markedly restricted, measurement of increase in range of motion becomes a useful objective measure of progress. Relatively inexpensive, accurate, and convenient electronic inclinometers are now available for measuring range of motion. The restricted range of motion of patients with active TrPs provides an objective diagnostic distinction from patients with FM who characteristically show joint hypermobility (39).

Muscle weakness

Clinically, the patient is unable to develop normal strength on static testing, as compared to testing of a contralateral uninvolved muscle. Static strength is measurable using a force meter. Dynamic testing of muscles with active TrPs is just beginning to be explored using surface electromyographic [EMG] techniques. The involved muscle may initially evidence fatigue, which is indicated by increased average amplitude of integrated EMG (40). Reduced mean spectral frequency and reduced forcefulness of movement look promising as additional measures of "initial fatigue" in pilot studies.
Interrater Reliability. Many of the features described above have not been tested for interrater reliability, but some have been. Four studies evaluated the reliability of myofascial TrP examinations. Results are summarized in Table 3. In 1992 Wolfe, et al. (12) reported a study, part of which involved the evaluation of 8 muscles in 8 patients by 4 physicians experienced in examining patients for TrPs. The muscles examined included the levator scapulae, supraspinatus, anterior scalene, upper trapezius, infraspinatus, pectoralis major, sternocleidomastoid, and the iliocostalis/longissimus muscles in the TlO-L1 region. The four examiners had no chance to agree on a technique for examining the upper body TrPs prior to this study. The physicians examined each muscle for 5 findings characteristic of TrPs [Table 3]. Since recent studies report interrater reliability results in terms of the kappa statistics, two co-authors of this study [Simons and Skootskyl analyzed the original data for the kappa statistic, which also corrects for chance agreement [Table 3]. In 1992 Nice, et al. (41) reported on the examination of three sites in the thoracolumbar paraspinal muscles of 50 patients with low back pain by 12 experienced full-time physical therapists who routinely treated patients with low back pain. "A practice session was held to allow the therapists to practice this method on each other until all physical therapists reported that they felt capable of using the method on patients" (41). In 1994, Nice et al. (42) reported on the examination of 2 muscles [quadratus lumborum and gluteus medius] in 6l patients with low back pain by 2 examiners picked from a pool of 1 physician in general practice and 4 medical students trained over a 3 month period by the physician. The average kappa values for the 6 examinations were essentially equal for the quadratus lumborum and gluteus medius muscles, indicating that those muscles were about equally difficult to examine. Currently, Gerwin, et al. (43) have reported a study in which 4 physicians examined 5 muscles in each of 10 subjects with myofascial TrPs. Following a three-hour training session,agreement among doctors was assessed statistically before proceeding with the study. Examination of the extensor digitorum communis and latissimus dorsi muscles was most reliable. Examination of the sternocleidomastoid and upper trapezius muscles was less reliable, and examination of the infraspinatus muscle was least reliable. Presumably it was the most difficult to examine. A number of inferences can be drawn from Table 3. The poor reliability in the first two studies (12,41) can be attributed to inadequate training of experienced examiners. The improved performance in the third study (42) reflects considerable training of inexperienced examiners. The more extensive training of experienced examiners in the fourth study (43) corresponds to a marked improvement in kappa values. Fricton (7), in a diagnostic study of masticatory myofascial pain, found that experienced raters were more reliable than inexperienced raters and concluded that findings by palpation are technique sensitive. Table 4 shows a comparison of the examinations summarized in Table 3. It can be seen in Table 4 that spot tenderness, jump sign, and pain recognition were the examinations most reliably performed. The examinations for a palpable band and for referred pain appear highly sensitive to the amount of training; the twitch-response appeared to be the most demanding off training and skill.

Value of Clinical Features as Diagnostic Criteria.

An early attempt to identify the most appropriate diagnostic criteria under different circumstances (44) suffered from the absence of interrater reliability studies. Since no studies have been reported that evaluate the discriminating power of the clinical characteristics of TrPs, including the history, one can only estimate their likely diagnostic value [Table 4] based <,n information now available.

The finding of a palpable taut band alone is ambiguous because it is often seen in normal subjects (12,42).
The value of spot tenderness alone is limited because of ambiguity with regard to tender points of FM and other focal painful conditions. However, the presence of spot tenderness in a palpable band would likely provide good discrimination if the examiners were skillful at detecting the taut band. If quantification of spot tenderness is desired, properly administered algometry should be superior to manual testing for the jump sign.

Pain recognition is a reasonably reliable test in trained hands and has much clinical significance.

Referred pain, per se, is likely to have little discriminating power. Although the manually elicited twitch response is the most demanding of operator skill, it has the potential of providing the best discrimination of any single measure when the taut band and TrP are in an accessible location.

Natural Course

Only one longitudinal study is known that examined the muscles for taut bands and for evidence of TrP tenderness in a normal population to learn the incidence of TrPs. Fricton,et a1.(45) examined 269 female nursing students initially, again at 18 months and at 36 months for evidence of masticatory myofascial pain attributable to TrPs. They found an annual incidence of 8% with 5% developing masticatory myofascial pain only, and with 3% developing mixed myofascial pain and disk displacement in the temporomandibular joint. Additional studies will give us a more complete picture of what the natural course of myofascial TrPs may be. Figure 1 outlines a proposed natural course of myofascial pain caused by TrPs, based on information now available. The common presence of taut bands in pain-free individuals (12,42) suggests that taut bands are a necessary precursor to the development of TrPs. Some individuals appear to be genetically more vulnerable to the development of taut bands than others. Apparently, because of stressful life events and abnormal muscle stress (46) combined with genetic predisposition, a latent TrP develops in a taut band. This TrP, with further mechanical stress or other aggravating [perpetuating factors, can develop into an active TrP. The active TrP may recover spontaneously, may persist without progression, or, in the presence of perpetuating factors, the individual may develop additional TrPs and a chronic myofascial pain syndrome. Although in the past it had been assumed that TrPs caused a taut band (31,35), it now appears more likely that a taut band is a necessary precursor to the development of a TrP. In one study (12) taut bands were found to occur with nearly equal frequency in control subjects, myofascial pain patients, and FM patients. This indicates that neither myofascial pain nor FM significantly influences the number of taut bands present. A more recent study (42) reported that both examiners [100% agreement] found taut bands present in 6 of 63 [nearly 10%] of normal control subjects. These bands were the only suggestion of TrPs found in these normal subjects, who were free of spot tenderness. In 1989, Pellegrino, et a1.(47) reported clinical signs and symptoms that they identified as primary FM, but the description fit a diagnosis of myofascial TrPs much better than it fit FM (48). They described "abnormal, palpable muscle consistency years before acquiring clinical symptoms in teenage twins" (47). There are no studies which indicate that palpable bands are a diagnostic criterion for tender points of FM, but there is much clinical experience (31) and experimental evidence (49,50) that taut bands are an integral part of the TrP phenomenon. This study of 17 families (47) suggests that a proclivity to develop taut bands is an inherited characteristic and that those who are more prone to develop taut bands are also more likely to develop TrPs. In addition to general agreement among clinicians (2,4,7,8), the only documented evidence that muscle overload can initiate TrPs or convert a latent TrP into an active one is the study by Fricton, et a1.(46). Specifically, the course of an untreated latent TrP has not been studied. An incidence study of masticatory myofascial pain suggests that a latent TrP probably persists, with occasional increase in activity sufficient to cause symptoms become an active TrP]. An active TrP sometimes regresses without treatment to a latent TrP (45). Latent TrPs have been known to persist for many years, painlessly restricting range of motion, and then respond immediately to spray-and-stretch therapy. After an individual develops an active TrP, especially in the absence of any perpetuating factor, continuing normal gentle daily activity and avoiding muscle overload often permit spontaneous regression from an active TrP to a latent one in a few days to a few weeks. The presence of perpetuating factors assures persistence of an active TrP and sets the stage for the development of secondary TrPs, additional symptoms, and chronicity with progressive functional disability and psychological distress (31,34). The presence of perpetuating factors is one of the most common, and often one of the most important, factors in the management of patients with chronic myofascial TrPs (6,8,34). Recently, Gerwin demonstrated that iron insufficiency is a risk factor for myofascial pain caused by TrPs (51).

Treatment

Current literature calls attention to three new treatment issues: new approaches to TrP injection, the importance of eliciting a local twitch response when doing TrP injections, and the use of botulinum A toxin [Botox] for injecting TrPs.
New Approaches to injection of trigger points. Hong (ll) described a new technique for injecting TrPs and stated its rationale. Instead of the usual relatively slow methodical search for the sensitive spot in a TrP using a relatively large 21-gauge needle (31), he used a 25-or 27-gauge, 1 =-inch needle, peppering the TrP region with multiple fast-in, fast-out strokes (withdrawing the needle from the muscle but NOT through the skin). Based on the new concept that a TrP consists of multiple minute active loci [see next section] (11,49,50), this is a logical procedure. Hong's method has the advantage that the smaller needles will cause less muscle-fiber trauma. Also, when the needle elicits a local twitch response, the fast-out stroke withdraws the needle from the taut band before the contracting muscle can damage itself by pulling against the needle. In addition, Hong (ll) illustrates a new method of holding the barrel of the syringe between the thumb and fingers, using the index finger to press the plunger. This allows the clinician to stabilize his or her forearm against the patient, greatly reducing the danger of unwanted needle penetration due to unexpected movement by the patient. Fischer (52) indicated that the obnoxious [sometimes intolerable] pain experienced during TrP injection, as well as much of the post-injection soreness, can be avoided by local analgesic block of the TrP site before injection. This may be particularly helpful when injecting the TrPs of FM patients (53). Whether preemptive analgesia also blocks the local twitch response would be of considerable practice and theoretical interest. The effectiveness of preemptive analgesia has been convincingly demonstrated in both human and animal studies (54,55,56). Eliciting a Local Twitch Response. Hong (57) reported that with either lidocaine injection or dry needling of TrPs, the patients experienced almost complete relief of pain immediately after injection, if local twitch responses were elicited. On the other hand, they experienced only minimal relief if no such response occurred during: injection. When searching for TrP active loci in the course of experiments (58-60), we found that rapid advancement of the EMG needle through the TrP region was much more likely to elicit local twitch responses than was the very slow advancement technique required to locate spontaneous electrical activity [SEA]. Local twitch responses often occurred in close spatial relation to an active locus of SEA. Hong, et a1.(53) presented experimental evidence to substantiate the clinical impression (,1) that injection of myofascial TrPs substantially benefits FM patients. The authors also showed that the relief from TrP pain that the FM patients experienced was considerably less than the relief obtained by myofascial TrP patients without FM. Injecting Botulinum A Toxin. Recently, several authors have reported the successful use of botulinum A toxin [Botox] for the treatment of myofascial pain caused by TrPs (62,63,64). In a small double-blind, placebo-controlled study, four patients experienced 30% reduction of pain following Botox injection but not following saline injection (63). Results were statistically significant. The effectiveness of Botox injection is of great theoretical interest, but it has limited therapeutic application. In therapeutic doses, Botox paralyzes muscles by blocking release of acetylcholine from motor nerve terminals at the neuromuscular junction (65 ). This eventually causes degeneration of that neuromuscular junction. If the denervated muscle fiber survives, it normally becomes reinnervated in about 3-6 months. The fact that Botox quickly inactivates myofascial TrPs is a strong indicator that the myofascial TrP mechanism is intimately associated with the neuromuscular junction. However, because of its destructiveness, Botox should be used only when other more conservative approaches have failed. Generally, the basic reason why injection [or other treatment] provides only temporary relief is because perpetuating factors have not been adequately addressed (6). With use of Botox in the presence of persistent perpetuating factors, the TrP is likely to recur.

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