Nerve growth factor-induced muscle hyperalgesia facilitates ischaemic contraction-evoked pain

Immediate curative effects of exercise therapy in patients with myalgia of the masticatory muscles

BACKGROUND

Exercise therapy is occasionally considered as an initial treatment for temporomandibular disorders. However, pain can be exacerbated during exercise therapy.

OBJECTIVE

To investigate the immediate curative effects of exercise therapy in patients with masticatory muscle myalgia.

METHODS

Fifty-nine patients with masticatory muscle myalgia were included. Therapists performed exercise therapy (stretched the painful masseter and/or cervical muscles along the direction of muscle contraction) in 10 rounds of traction, each lasting 10 seconds. The patient's pain-free maximum mouth opening distance and degree of pain (VAS value) before and immediately after exercise therapy were compared using the Wilcoxon signed rank test. Mann-Whitney U test was used for the subgroup comparisons.

RESULTS

Mouth opening increased from 41 (IQR 38-43) to 46 (IQR 43-48) mm, and pain alleviation from 48 (IQR 31-56) to 21 (IQR 10-56) immediately following exercise therapy (p < 0.001 for both). None of the patients experienced pain exacerbation or reduction in mouth opening post exercise. No difference in mouth opening distance changes according to sex, painful side, painful site, and therapist were observed (p > 0.05 for all). Pain reduction was greater in patients with unilateral pain (26, IQR 12-39) than those with bilateral (13, IQR 5-25) (p = 0.019). There were no differences in the change in the degree of pain according to sex, painful site, and therapist (p > 0.05 for all).

CONCLUSION

Exercise therapy immediately enlarged the mouth opening distance and reduced myalgia; therefore, it could be helpful in managing masticatory muscle myalgia.

A systematic review of porcine models in translational pain research

Translating basic pain research from rodents to humans has proven to be a challenging task. Efforts have been made to develop preclinical large animal models of pain, such as the pig. However, no consistent overview and comparison of pig models of pain are currently available. Therefore, in this review, our primary aim was to identify the available pig models in pain research and compare these models in terms of intensity and duration. First, we systematically searched Proquest, Scopus and Web of Science and compared the duration for which the pigs were significantly sensitized as well as the intensity of mechanical sensitization. We searched models within the specific field of pain and adjacent fields in which pain induction or assessment is relevant, such as pig production. Second, we compared assessment methodologies in surrogate pain models in humans and pigs to identify areas of overlap and possible improvement. Based on the literature search, 23 types of porcine pain models were identified; 13 of which could be compared quantitatively. The induced sensitization lasted from hours to months and intensities ranged from insignificant to the maximum attainable. We also found a near to complete overlap of assessment methodologies between human and pig models within the area of peripheral neurophysiology, which allows for direct comparison of results obtained in the two species. In spite of this overlap, further development of pain assessment methodologies is still needed. We suggest that central nervous system electrophysiology, such as electroencephalography, electrocorticography or intracortical recordings, may pave the way for future objective pain assessment.

Repeated Injections of Low-Dose Nerve Growth Factor (NGF) in Healthy Humans Maintain Muscle Pain and Facilitate Ischemic Contraction-Evoked Pain

OBJECTIVE

Nerve growth factor (NGF) is essential for generating and potentiating pain responses. This double-blinded crossover study assessed NGF-evoked pain in healthy humans after repeated NGF injections in the tibialis anterior (TA) muscle compared with control injections of isotonic saline.

SUBJECTS

Twenty healthy subjects participated in two experimental phases; each consisted of seven sessions over 21 days.

METHODS

At day 0, day 2, and day 4, a low-dose NGF (1 µg) was injected. Data on daily self-reported muscle pain (using a Likert scale) were collected. Data on pressure pain thresholds (PPTs), pain evoked by nonischemic and ischemic muscle contractions (using a numerical rating scale [NRS]), pressure pain detection (PDT), and pain tolerance thresholds (PTTs) to cuff algometry were recorded before day 0 and at 1, 2, 4, 7, 10, and 21 days after the first injection. Temporal summation of pain (TSP) and conditioned pain modulation (CPM) were recorded to assess central pain mechanisms.

RESULTS

Likert scores remained elevated for 9 days after NGF injection (P<0.05). PPTs at the TA muscle were decreased at day 1 until day 7 after NGF injection compared with day 0 (P=0.05). In subjects presenting with NGF-induced muscle hyperalgesia, pain NRS scores evoked by nonischemic contractions were higher after NGF injection at day 4 and day 7 (P<0.04) compared with the control condition. At all time points, higher pain NRS scores were found with ischemic compared with nonischemic contractions (P<0.05). The pain NRS after ischemic contractions was elevated following prolonged NGF hyperalgesia at day 7 compared with the control condition and day 0 (P<0.04). The PDT, PTT, TSP, and CPM remained unchanged during the period of NGF-induced hyperalgesia.

CONCLUSIONS

Repeated low-dose NGF injections maintain muscle pain and potentiate pain evoked by ischemic contractions during prolonged NGF hyperalgesia.