An immediate reduction of hypoesthesia was observed in 5 of 6 patients with primary myofascial pain, and 4 of 24 patients with secondary myofascial pain (Figs. 1B and 2 right photographs, Table 3). The immediate reduction occurred more frequently in primary than secondary myofascial pain (P < 0.005).
Reversible tactile hypoesthesia was round or oval in shape and often showed a nondermatomal distribution (Figs. 1 and 2). The initial degree of hypoesthesia in the cutaneous area of the reversible hypoesthesia ranged from 5 to 8. Tactile hypoesthesia was felt by patients as something like a thin membrane layer between the cotton swab and the skin.
Myofascial trigger points were located in the muscles in the vicinity of ipsilateral cutaneous dermatomes to which the hypoesthetic areas belonged, or approximate myotomes that seemed to share the same spinal nerves (Figs. 1 and 2).
Ten of 11 patients who did not have sensory abnormalities underwent TPI. Nine of the 10 patients experienced more than a 50% decrease of pain in the NRS.
Reversible hypoesthesia with TPI therapy was found in 58.7% of patients with MTrP. Our findings indicate the relatively high prevalence of reversible hypoesthesia in patients with MTrP. We also observed that a reduction in the size or disappearance of such hypoesthesia occurred in parallel with a reduction of myofascial pain. The reduction in the size of the hypoesthesia was observed progressively during TPI therapy and occasionally immediately after TPI. These findings suggest a close pathophysiological relation between MTrP and tactile sensation. In painful conditions other than muscular pain, the phenomenon of pain-induced tactile hypoesthesia has been described previously.3,6,12,18,21,22,28 As for underlying mechanisms, several brain imaging studies have shown that the activity of the primary somatosensory cortex decreases along with a decline in touch sensation.4,19,26 Such inhibition of neuronal activity in the somatosensory cortex induced by ongoing afferent nociceptive input from MTrP may underlie the reversible tactile hypoesthesia in patients with MPS.
Tactile hypoesthesia associated with myofascial pain can mimic that of neuropathic pain.8,15,16 In our patients, a disappearance or reduction in the size of tactile hypoesthesia after TPI occurred less frequently in patients with secondary myofascial pain compared to those with primary myofascial pain. A lower frequency of reduction or disappearance of tactile hypoesthesia in patients with secondary MPS may be attributed to the existence of tactile hypoesthesia induced by nerve lesions in this group of patients. Topographically, remaining hypoesthetic areas observed after TPI therapy are thought of as hypoesthesia related to somatosensory nerve lesions that may be associated with neuropathic pain.
From a clinical point of view, we suggest that it is important to diagnose MTrP and treat it with TPI to distinguish neuropathic pain from myofascial pain. In addition, the mapping of hypoesthetic areas is useful for detecting MTrP because MTrP are located in the muscles in the vicinity of dermatomes to which the hypoesthetic areas belong.
This study had some limitations. One limitation is that we did not investigate the relationship between the area of reversible hypoesthesia and referred pain area. Numbness or hypoesthesia were reported to occur in the area of referred pain in patients with muscular pain2,7,16,17 as well as volunteers with experimentally induced muscular pain.14,15 Another limitation is that we were not able to investigate why some patients had tactile hypoesthesia and others did not, whereas 90% of patients in both groups experienced more than 50% pain relief by TPI.
In summary, although not always, reversible tactile hypoesthesia with TPI therapy often occurred in patients with MPS. Our present findings suggest that treating MTrP with TPI is important to distinguish tactile hypoesthesia associated with neuropathic pain from that with myofascial pain. Moreover, the mapping of tactile hypoesthesia on the skin seems useful for detecting MTrP because of their close topographical relation.
The authors have no conflict of interest to declare.
The authors thank the patients for permitting use of their photographs on this manuscript. The authors also thank Ms. Yuko Takeuchi for assistance with manuscript preparation.
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