Masseter muscle pain and its relation to pain mediators in fibromyalgia and local myalgia

The aim of this investigation was to study chronic jaw muscle pain in fibromyalgia, local myalgia and healthy individuals in relation to the pain mediators PGE2, LTB4 and NPY and to study the effects of glucocorticoid administration and repetitive isometric contraction. A questionnaire showed presence of symptoms from the temporomandibular region in almost all fibromyalgia patients and the local pain was correlated to the general pain but on a lower level. The clinical part comprised patients with fibromyalgia and local myalgia in the orofacial region as well as healthy individuals. The subjects were examined for local spontaneous pain, palpatory tenderness, pressure pain threshold and tolerance levels of the superficial masseter muscle, bite force, intramuscular temperature and mobility of the mandible. Both patient groups showed low values of pain threshold as well as intramuscular temperature and maximum voluntary mouth opening compared to the healthy individuals. The number of tender mandibular muscles was higher than the pressure pain threshold lower in fibromyalgia than in local mylgia. This result and the high frequency of temporomandibular disorders in the fibromyalgia patients indicates that involvment of the temporomandibular system is common in fibromyalgia. Microdialysis was used to sample PGE2 and LTB4 from the superficial masseter muscle in patients with fibromyalgia and local myalgia as well as in healthy individuals and venous blood was collected for analysis of plasma levels of these substances. Intramuscular PGE2 and LTB4 were found in all groups. LTB4 levels increased upon the trauma created by microdialysis in patients with fibromyalgia and local myalgia, but not in the healthy individuals. PGE2 levels were related to muscular pain in the fibromyalgia patients and was negatively correlated to pressure pain threshold in the healthy individuals. Masseter muscle pain therefore seems to be partly of peripheral inflammatory origin. To evaluate the effects of glucocorticoid administration into the painful masseter muscle microdialysis and clinical examination was performed two weeks before and after its local administration in fibromyalgia and local myalgia. Both groups showed a similar response regarding clinical parameters but only the patients with local myalgia reported a positive subjective treatment result. In the patients with fibromyalgia a reduction of masseter muscle level of PGE2 was associated with a decrease of spontaneous pain, while the masseter muscle level of LTB4 increased in both patient groups after the intramuscular glucocorticoid administration. In an experimental microdialysis study the intramuscular levels of NPY and its relation to development of pain was determined during rest and during repetitive isometric contraction of the superficial masseter and trapezius muscles in healthy females. The NPY level in the masseter muscle at rest was higher than the corresponding levels of the trapezius muscle and plasma. In the trapezius muscle NPY increased during isometric contraction, while development of pain was associated with a high precontraction plasma level of NPY. High precontraction level of NPY in the masseter muscle was associated with development of pain after isometric contraction of this muscle