Intramuscular injection of granisetron into the masseter muscle increases the pressure pain threshold in healthy participants and patients with localized myalgia

Objectives: The aims of this study were to experimentally investigate whether an intramuscular injection of the 5-HT3 antagonist granisetron into the masseter muscle increases the mechanical pain threshold in healthy participants and reduces masseter Muscle pain or allodynia in patients with craniofacial myalgia. Methods: Eighteen patients with bilateral localized myalgia of the masseter muscle and 24 healthy participants participated in this randomized, double-blind study, in which granisetron was injected on one side and isotonic saline on the other side. Pain (Visual Analog Scale) and pressure pain threshold (PPT) were recorded before and during 30 Minutes after injections and the changes from baseline were analyzed with analysis of variance. Results: In both groups, the PPT increased after injection of granisetron whereas it decreased after saline. The difference between substances was significant (patients: P = 0.016; healthy participants: P = 0.029). In the healthy participants there was also a significant time effect (P < 0.001) and an interaction between time and Substance (P = 0.022). The post-hoc test showed that the difference between substances was significant 0 to 15 Minutes after injections (Bonferroni t tost; P < 0.05). The pain intensity from the masseter muscle did not differ between substances, but there was a sianificant time effect (P < 0.001) and an interaction between time and Substance (P < 0.001). The post-hoc test showed significantly lower pain intensity on the granisetron side 0 to 2 minutes after injections (Bonferroni t test; P < 0.05). Conclusions: This study indicates that intramuscular injection of granisetron into the masseter Muscle increases the PPT in healthy participants and in patients with craniofacial myalgia.