Intramuscular Injections of NGF Do Not Cause Increased Pain Referral

Objectives:
It was hypothesized that repeated injections of nerve growth factor (NGF) into the masseter and temporalis muscles would mimic the peripheral and central nervous system aspects of myofascial pain and contribute to an increased prevalence of referred pain among the studied individuals.
Methods: The study was performed in a randomized double-blind, placebo-controlled manner. 16 healthy participants were injected on 2 days, 1 day apart, with NGF into the masseter and anterior-temporalis muscles and isotonic saline on the contralateral side. Assessments were performed before and 3 hours after injections on days 0 and 2 and on days 7, 14 and 21. Mechanical sensitivity (MS) was assessed by application of 3 different forces (0.5, 1 and 2kg) for 5 seconds to 15 different sites on each of the muscles. NGF injection side, and order of the assessed sites was randomized using randomization.com.
Results: The main findings were: 1) intramuscular injections of NGF in both the masseter and temporalis muscles caused a significant increase in MS scores when compared to the saline side (P<0.001, Four-way ANOVA); 2) NGF injections in the masseter but not the temporalis muscle caused a significant time-dependent increase in MS scores (P<0.001, Three-way ANOVA); 3) of the 16 healthy participants in this study, 3 of them (18.8%) experienced referred pain at baseline MS assessment; 4) there was no significant difference between the NGF side and the saline side in regards to prevalence of referred pain (P=0.48, McNemar’s test).
Conclusions: These results show that referred pain can be elicited in healthy individuals without muscle pain through palpation of the masseter and temporalis muscles. Furthermore, increase in MS scores caused by intramuscular NGF injections does not cause an increase in referred pain when compared to intramuscular saline injections. These observations could have implications in understanding the pathophysiology of referred pain from jaw-muscles.