Capsaicin-induced muscle hyperalgesia in the exercised and non-exercised human masseter muscle

Aims: Strong jaw muscle exercises such as tooth grinding in sleep bruxism a re frequently believed to be a predisposing factor in myogenous types of te mporomandibular disorders. However, it is not known wither tooth grinding i n sleep bruxism is associated with increased sensitivity to intramuscular s timuli. This study therefore compared the hyperalgesic effects of an intram uscular injection of capsaicin into the right masseter wit and without a pr ior experimental tooth-grinding exercise. Methods: Ten healthy men particip ated in 2 randomized sessions (exercises, non-exercise session) separated b y 1 week. In the exercise session, 0.1 mL capsaicin (100 mu g/mL) was injec ted into the right masseter immediately after 45 minutes of experimental to oth grinding. In the non-exercise session. the exact same paradigm was used , except that the experimental tooth grinding was omitted. The perceived in tensity of pain evoked by intramuscular capsaicin was scored on a 100-mm vi sual analog scale (VAS). Pain detection thresholds (PDT) to pressure stimul i and maximal voluntary occlusal force (MVOF) were measured before capsaici n injection; 5, 15, and 45 minutes after the injection; and once a day for the following 3 days. Results: Injections of capsaicin into an exercised or non-exercised masseter did not cause significant differences in peak pain intensity on the VAS (57 +/- 6 mm in exercised masseter vs. 53 +/- 6 mm in non-exercised masseter; P = 0.464). The PDTs in the exercised masseter were significantly decreased for up to 1 day after the capsaicin injection (P l ess than or equal to 0.038), whereas PDTs in the non-exercised masseter wer e decreased for only 5 minutes (P = 0.017). The MVOF on the right side was decreased 5 minutes after the capsaicin injection in both sessions (P < 0.0 10). The MVOF on the left side was significantly reduced for up to 15 minut es after the capsaicin injection in the exercise session only (P < 0.019). Conclusion: Increased sensitivity to percutaneous pressure stimuli probably reflects a post-exercise muscle soreness following tooth grinding, whereas intramuscular sensitivity to noxious chemical stimuli immediately followin g exercise seems to be unchanged.