Patterns of experimentally induced pain in pericranial muscles

Nociceptive mechanisms in the craniofacial muscle tissue are poorly understood. The pain pattern in individual pericranial muscles has not been described before. Experimental muscle pain was induced by standardized infusions of 0.2 ml 1 m hypertonic saline into six craniofacial muscles (masseter, anterior temporalis, posterior temporalis, trapezius, splenius capitis and sternocleidomastoid) in 20 healthy subjects. The pressure pain thresholds (PPTs) were determined before and after infusions. The subjects continuously reported intensity of saline-induced pain on an electronic visual analogue scale (VAS) and the perceived area of pain was drawn on anatomical maps. The pain areas were measured and the localization determined by a new centre-of-gravity method. The PPTs were lowest on the sternocleidomastoid muscle (anova: P<0.001), but the saline-evoked VAS pain scored highest following injection into the masseter muscle (anova: P<0.05). The centre-of-gravity measures demonstrated significantly different localization of the pain areas (anova: P<0.001). The trigeminally vs. the cervically innervated muscles had significantly different patterns of spread and referral of pain according to trigeminally vs. cervically innervated dermatomes (P<0.005). In conclusion, there appear to be characteristic pain patterns and pain sensitivity in different craniofacial muscles in healthy volunteers, which may be of importance for further research on different craniofacial pain conditions.