The significance of Ge and colleagues' study is tempered by concerns with the validity of MTPs [1
]. There is no widely agreed-upon definition of MTPs. Ge and colleagues used the Travell and Simons' criteria, as noted by Bennett. Tough and colleagues, however, found 19 different diagnostic criteria for MTP pain in an extensive literature review [12
]. Most of those studies cited the work by Travell and Simons yet failed to apply their diagnostic criteria. The systematic review by Lucas and colleagues concluded: 'On the basis of the limited number of studies available, and significant problems with their design, reporting, statistical integrity, and clinical applicability, physical examination cannot currently be recommended as a reliable test for the diagnosis of trigger points' [13
There is significant interobserver variability in the MTP examination. For example, four rheumatologists, including Bennett and myself, and four experts on MFP syndrome performed trigger point and tender point examinations on three groups of subjects (seven patients with FM, eight patients with MFP, and eight healthy persons) while blinded as regards diagnosis [14
]. Active MTPs were found in 18% of patients with FM and MFP, but latent trigger points were rare in all groups. Taut muscle bands and muscle twitches were common (50% and 30%, respectively) and were noted equally in all three diagnostic groups. There were significant problems with interobserver reliability for taut bands, muscle twitch and active trigger points. The interexaminer reproducibility of the MTP examination varies even among experts but improves with a standardized technique and experience [15
]. Palpation of taut bands and muscle-snapping techniques are especially prone to interobserver variability.
MFP experts point to electrophysiologic evidence of muscle pathology. Ge and colleagues report that EMG evidence of spontaneous electrical activity is the only electrophysiological method to document the existence of MTP, and they therefore used this technique [1
]. In their study, the EMG was performed after the manual examination, the needle was 'redirected twice if the first insertion failed to find the spontaneous electrical activity' and the needle electrode length varied with different muscles. Some investigators have been unable to find characteristic spontaneous EMG activity in MTPs [17
]. Other techniques said to demonstrate abnormalities in the MTP, such as microdialysis, magnetic resonance elastography and specialized ultrasound, are not widely available and the results have not been duplicated.
Although MFP is considered a localized muscle pain disorder, there is considerable clinical overlap with FM. Two studies reported that 25 to 42% of subjects with chronic cervical MFP met diagnostic criteria for FM [18
], and two reports found that 75 to 80% of FM patients met the criteria for MFP [19
There is strong evidence that abnormal central pain processing, characteristic of FM, is also prominent in MFP. Similar somatosensory pain profiles are found in both FM and MFP [21
], and women with MFP had bilateral widespread mechanical pain sensitivity [22
]. Bennett mentioned above that sustained mechanical stimulation of latent MTPs induced central sensitization in healthy subjects [14
]. What makes that different from mechanical pressure on tender points inducing central pain? Both Bennett and Ge and colleagues mention that proinflammatory mediators have been reported in MTPs. Similar observations have been found in FM. De Stefano and colleagues found evidence for elevated substance-P immunoreactivity in both MFP and FM [23
MFP is postulated to be typically self-limited whereas FM is postulated as chronic. FM patients are said to have greater co-morbidity and other somatic symptoms, such as fatigue and sleep and mood disturbances. This hypothesis, however, has not been carefully evaluated. MFP experts claim that localized therapy, particularly trigger point injections, are very effective for MTPs but not for tender points. Unfortunately, there are no randomized, controlled studies to document this belief. The uncontrolled studies of multiple different injection techniques, different injectable agents, dry needling and physical modalities attest to lack of universal success. A large, multicenter prospective study comparing subjects who meet criteria for FM, for MFP and for both conditions would be necessary.
Finally, there is no convincing evidence that the MTP can be clinically or pathophysiolgically distinguished from a FM tender point. No study has matched painful muscles containing only tender points with those containing only trigger points. Since trigger points always have a tender point, such a study seems impossible.
Just like fibrositis and fibositic nodules have become historical curiosities, MTPs will eventually be discounted as discrete pathologic abnormalities in the muscle. MFP will be brought into the realm of central pain disorders, including chronic headaches, irritable bowel syndrome, temporomandibular dysfunction and FM. The likelihood that MFP will spread to FM will be attributed to central factors, such as generalized pain tolerance, co-morbid illness and psychosocial factors. Identifying and treating any peripheral pain is a noble pursuit in the management of central pain disorders, such as FM. However, it is unlikely that the MTP is a specific peripheral pain phenomenon.