Uncoveringthebiochemicalmilieuofmyofascialtriggerpointsusinginvivomicrodialysis:Anapplicationofmusclepainconceptstomyofascialpainsyndrome

Myofascial pain syndrome (MPS) is one of the most common causes of non-articular musculoskeletal pain, yet its underlying biology has remained poorly understood. The condition is characterized by myofascial trigger points (MTrPs)—tender, hyperirritable nodules within tight bands of skeletal muscle that can cause local pain, referred pain, and muscle dysfunction. Active MTrPs produce spontaneous pain, while latent MTrPs are only painful when pressed. Despite how widespread MPS is, clinicians have historically lacked objective measures to distinguish these states or explain why some trigger points hurt while others do not.

Understanding the biochemical differences between active, latent, and normal muscle could transform how we diagnose and treat this prevalent condition.

Dr. Shah and colleagues developed a novel microanalytical technique using in vivo microdialysis to sample the biochemical environment of human skeletal muscle. They used a tiny 30-gauge needle—similar in size to an acupuncture needle—that could collect minute fluid samples (about 0.5 microliters) while simultaneously eliciting a local twitch response (LTR), the involuntary muscle contraction that occurs during effective dry needling. In their first study, they recruited nine subjects and classified them into three groups: healthy individuals with no neck pain or trigger points; individuals with latent MTrPs (trigger points present but not spontaneously painful); and individuals with active MTrPs (painful trigger points with neck pain).

All subjects had samples taken from a standardized point in the upper trapezius muscle. Samples were collected before needle movement, during needle advancement and LTR, and after the LTR, for a total of 15 minutes. The researchers then analyzed the fluid for pH and concentrations of substance P (SP), calcitonin gene-related peptide (CGRP), bradykinin (BK), serotonin (5-HT), norepinephrine (NE), tumor necrosis factor-alpha (TNF-a), and interleukin-1 beta (IL-1b). In a second study, they expanded this approach to nine additional subjects, sampling not only from the upper trapezius but also from the upper medial gastrocnemius—a distant, uninvolved muscle—to test whether biochemical abnormalities were localized or widespread.

The results revealed striking biochemical differences between the three groups. In the first study, concentrations of SP, CGRP, BK, 5-HT, NE, TNF-a, and IL-1b were all significantly higher in the active MTrP group compared to both the latent and normal groups (p<0.01). Additionally, pH was significantly lower in active MTrPs, indicating a more acidic environment (p<0.03). There were no significant differences between the latent and normal groups.

After eliciting a local twitch response, SP and CGRP levels in the active group dropped significantly compared to pre-LTR levels (p<0.02), suggesting that the mechanical intervention had a measurable biochemical effect. The second study confirmed these trapezius findings and added important new information: IL-6 and IL-8 were also significantly elevated in active MTrPs (p<0.002). When comparing the trapezius to the remote gastrocnemius muscle, the active group showed elevated biochemical levels even at this distant, uninvolved site—though concentrations were lower than at the trapezius. For example, bradykinin, substance P, and norepinephrine were all significantly lower in the gastrocnemius than peak trapezius values.

Notably, pH was similarly acidic in both muscles for the active group. The temporal response to needle insertion also differed: the trapezius showed sharp peak concentrations around 5 minutes, while the gastrocnemius showed no such peaks, possibly reflecting differences in muscle function and fiber composition.

This research provides the first direct evidence that active myofascial trigger points have a distinct biochemical signature characterized by elevated inflammatory mediators, neuropeptides, catecholamines, and cytokines, combined with local acidity. The finding that these substances decrease after a local twitch response offers a biological explanation for why dry needling can provide immediate pain relief—it may literally "wash out" pain-producing chemicals. Perhaps most importantly, the discovery of elevated biochemicals in distant, uninvolved muscles suggests that active MTrPs may be associated with a systemic or centrally-mediated sensitization process, not merely a local problem. This challenges the purely local model of MTrP pathophysiology and points toward central nervous system involvement.

For clinicians, these findings validate the clinical distinction between active and latent trigger points with objective biomarkers. For patients, it means that chronic myofascial pain has a real biological basis—it's not "just in your head." The microdialysis technique itself opens doors for future research to track how trigger points develop over time, whether they can resolve spontaneously, and how different treatments alter the biochemical environment. Ultimately, understanding these biochemical cascades could lead to more targeted therapies that address the specific inflammatory and neurosensory mechanisms driving myofascial pain.