Masticatory muscle pain biomarkers

Abstract

This chapter focuses on the potential role of biomarkers for masticatory muscle pain, i.e., myalgia. To date, no biomarkers have been identified that can be used clinically for diagnosis or treatment of myalgia of jaw muscles. There is evidence from microdialysis studies that intramuscular levels of glutamate and serotonin are elevated in patients with chronic myalgia, including myalgia of jaw muscles. High muscle levels of glutamate and serotonin correlate to pain intensity and mechanical allodynia, and both glutamate and serotonin have been shown to induce pain and mechanical hyperalgesia when injected into jaw muscles. This pain, consequently, can be blocked with specific receptor antagonists, indicating that glutamate and serotonin may be promising biomarker candidates. However, muscle levels of glutamate and serotonin do not correlate to plasma levels, which is a disadvantage since measuring intramuscular biomarker levels with currently available techniques is too complicated to be clinically useful. Nerve growth factor (NGF) has also been shown to cause long-lasting hyperalgesia, albeit with no pain, when injected into jaw muscles, but muscle biopsies did not show any differences in NGF levels between patients with jaw myalgia and pain-free controls. Additionally, muscle levels of prostaglandins, bradykinin, or substance P, commonly characterized pain mediators, do not seem to be elevated in myalgic jaw muscles. Because pain mediation and peripheral sensitization are complex events that involve many substances, future research should focus on investigating intramuscular profiles of multiple biomarkers. This, in turn, is possible with newly developed methods, such as proteomics and metabolomics.