Intramuscular pH in Myofascial Pain Syndrome of the Masticatory Muscles

Abstract

Objective: Myofascial pain syndrome [MPS] of the masticatory muscles is characterized by regional muscle pain and the presence of hypersensitive trigger points. The pathophysiological mechanisms that underlie MPS are poorly understood. Sustained contracture of muscle fibers may produce ischemia and localized acidosis that can excite nociceptors. The objective of this study was to compare pH in the most and the least painful areas of the masseter muscle in subjects with and without MPS of the masticatory muscles.

Methods: Symptoms and signs of MPS, pressure-pain thresholds [PPT], and intramuscular pH were determined in 14 female subjects who met the criteria for MPS according to the Research Diagnostic Criteria for Temporomandibular Disorders and 14 female age-matched control subjects.

Results: Subjects with MPS reported more clinical symptoms and more clinical signs of MPS than did healthy normal control [HNC] subjects. Subjects with MPS had significantly lower PPTs at the most painful site [97.4 ± 9.5 kPa] than did HNC subjects at either the most [143.0 ± 17.7 kPa] or the least [161.1 ± 15.5 kPa] painful sites. However, there were no significant differences in intramuscular pH between the least painful [7.40 ± 0.17] and the most painful [7.22 ± 0.08] sites in subjects with MPS or in HNC subjects [least painful site, 7.30 ± 0.09; most painful site, 7.25 ± 0.09]. Moreover, there was no significant correlation between PPT and intramuscular pH.

Conclusion: These results suggest that intramuscular pH does not contribute to pain in the masseter muscle associated with MPS of the masticatory muscles.

KEYWORDS:

• Myofascial pain syndrome
• intramuscular pH
• pressure-pain threshold
• masseter muscle
• myalgia