Uncovering the Biochemical Milieu of Myofascial Trigger Points Using In Vivo Microdialysis

Abstract

Objectives: To 1. confirm our previous findings demonstrating the unique biochemical milieu of substances associated with pain and inflammation in an active myofascial trigger point [TrP] in the upper trapezius muscle, and 2. determine whether these findings are unique to the upper trapezius when compared with a remote uninvolved site in the upper medial gastrocnemius muscle.

Methods: We developed and successfully utilized a novel microanalytical system, employing a minimally invasive 32-gauge needle, capable of the in vivo collection of small volumes [∼ 0.5 μ l] and subnanogram levels [< 75 kDa] of solutes from muscle tissue. The needle also serves as a surrogate acupuncture needle during routine treatment of TrPs. This system provides us the unprecedented ability to safely explore and measure the local biochemical milieu of TrPs before, during, and after a local twitch response [LTR]. We previously found elevated levels of inflammatory mediators, neuropeptides, catecholamines, and cytokines at a standardized location in the upper trapezius muscle of subjects with active TrPs compared with those with latent TrPs and those without TrPs. Furthermore, the local biochemical milieu does appear to change after a LTR. There were nine healthy subjects, all of whom had no calf pain or calf TrPs and were divided into three groups based on the following findings in the trapezius: Active [painful TrP present, three subjects], Latent [nonpainful TrP present, three subjects], and Normal [no pain, no TrP present, three subjects]. Samples were obtained continuously with the microdialysis needle at regular intervals at two standardized anatomical locations: first in the upper trapezius [at acupuncture point GB-21, which is equivalent to region TP1] and then in the upper medial gastrocnemius at acupuncture point LV-7. The main outcome measures were levels of protons, bradykinin, substance P, calcitonin gene-related peptide [CGRP], serotonin, norepinephrine, tumor necrosis factor-alpha, interleukin-1 beta [IL-1β], IL-6, and IL-8.

Results: At needle insertion in the Active group, levels of all measured analytes were higher in the trapezius than the gastrocnemius [P < 0.05]. This difference was also found in the Latent group for levels of tumor necrosis factor-alpha, substance P and calcitonin gene-related peptide [P < 0.005], and in the Normal group for levels of bradykinin only [P < 0.05]. In the gastrocnemius, all analytes except bradykinin were higher in the Active group than either the Latent and Normal groups [P < 0.01].

Conclusions: Significant differences in analyte levels were demonstrated between the trapezius [at GB-21] and gastrocnemius [at LV-7] in the Active group, suggesting that the vicinity of the active TrP exhibits a unique biochemical milieu of substances associated with pain and inflammation. Gastrocnemius analyte levels were higher in the Active group than either the Latent or Normal groups. This suggests that analyte abnormalities may not be limited to local areas of active TrPs. The application of this microanalytical system to nonarticular musculoskeletal disorders may help elucidate the pathogenesis, persistence, and amplification of myofascial pain syndrome and create a model for chronic soft tissue pain syndromes. Improved treatment strategies and techniques may evolve from understanding the local pathophysiology and the underlying cellular and molecular mechanisms of myofascial pain.

KEYWORDS:

• Microdialysis
• myofascial pain
• rehabilitation
• trigger points