Effects on muscle pain by intramuscular injection of granisetron in patients with fibromyalgia.

We have previously reported that the level of 5-HT in the masseter muscle is increased in patients with fibromyalgia as compared with healthy subjects and that high intramuscular level of 5-HT is associated with muscle pain. We have also reported that injection of the 5-HT(3) receptor antagonist granisetron (GRA) into the masseter muscle of healthy subjects reduced pain induced by 5-HT and abolished allodynia/hyperalgesia. The aim of this study was to investigate whether GRA can influence pain and allodynia/hyperalgesia of the masseter muscle in patients with fibromyalgia. Eighteen female patients who met the criteria of fibromyalgia according to the American College of Rheumatology participated in the study. They were examined regarding pain intensity and pressure pain threshold (PPT) over the masseter muscle. One milliliter of GRA (1 mg/ml) was injected into the masseter muscle on one side and 1 ml of isotonic saline on the other side in a randomized and double-blind manner. After the injections, the pain intensity and PPT were recorded during 30 min. The pain intensity increased after injection of saline and to a lower degree after injection of GRA. The PPT increased after injection of GRA, while no such change was observed after saline. The difference between GRA and saline was, however, not significant. Eight of the patients responded to the GRA injection by an increase of PPT during the experimental period that differed from saline. They also showed a tendency to a lower increase of pain intensity after injection of GRA when compared to saline. In conclusion, the results of this study do not prove that injection of the 5-HT(3)-antagonist GRA into the masseter muscle influences local pain and allodynia/hyperalgesia in patients with fibromyalgia.