Effects of Conditioned Pain Modulation on Spread of Hyperalgesia after Intramuscular Capsaicin in Humans

Muscle pain can be associated with altered central pain processing, leading to hyperalgesia both locally and at distant body sites. The role of endogenous inhibitory mechanisms in hyperalgesia associated with muscle pain remains unclear. This study investigated the influence of conditioned pain modulation (CPM) on hyperalgesia at multiple body sites induced by intramuscular injection of capsaicin in the supraspinatus muscle. Thirty healthy males underwent CPM testing using the cold pressor test as conditioning stimulus and pressure pain threshold (PPT) as test stimulus. Muscle pain and hyperalgesia were induced by injection of capsaicin into the non-dominant supraspinatus muscle. Before and 5, 10, 15, 20, 30, 40, 50 and 60 min later, PPTs were recorded bilaterally at the supraspinatus, infraspinatus and deltoid muscles, ring finger and toe. Subjects were classified as inhibitory vs. facilitating CPM based on published reference values. Compared to baseline, PPTs decreased significantly at the supraspinatus, infraspinatus and deltoid muscle (p