MULTIDISCIPLINARY PAIN ABSTRACTS: 16

The present double-blind, placebo-controlled study was designed to test quantitatively the effect and of nerve growth factor (NGF) injected into the masseter muscle. Pressure pain thresholds (PPT) and pressure tolerance thresholds (PTOL) were used as indices of mechanical allodynia and hyperalgesia in the jaw-closing muscles. In addition, perceived pain intensity was assessed by the subjects on a 0 to 10 numerical rating scale (NRS) with the jaw at rest and in relation to various oral functions (chewing, yawning, talking, swallowing, drinking and smiling). Repeated measures analysis of variance (ANOVA) was used to test for significant effects. Injection of NGF into the masseter muscle was associated with significantly reduced PPT for 7 days and PTOL for 1 day. Buffered isotonic saline injections into the masseter muscle also significantly lowered the PPT after 1 day and isotonic saline had no significant effect on PTOL. In contrast, assessment of PPT and PTOL in the noninjected temporalis muscles demonstrated a significant increase after 14 to 28 days, which may have reflected an adaptation to the test procedure. NRS scores of chewing and yawning were significantly increased for 7 days following NGF injection. Systemic adverse effects were noted in one subject who reported fever and slight discomfort about 8 hours after the NGF injection. In conclusion, this is the first study to show that injection of NGF into the human masseter muscle causes local signs of mechanical allodynia and hyperalgesia that persist for at least 7 days as well as pain during strenuous jaw movement. The present pain model is safe and may be used to gain further insight into the neurobiological mechanisms of muscle pain and sensitization.