A Human Model of Chronic Myalgia Induced by Acidic Saline

Human models are needed to study the aetiology of chronic pain conditions like the temporomandibular disorders. Lactic acid produced by exercise is an endogenous stimulus for muscle nociceptors, and acidic saline injections have been successfully used to induce acute myalgia in humans. In rats, repeated injections of low pH (4.0) saline into the masseter muscles induce allodynia that lasts for several weeks. Objective: To develop a model of chronic jaw muscle allodynia using acidic saline infusion in humans. Methods: Preliminary experiments were performed to establish a usable pH range. Saline solutions (pH 4.6, 4.8, 5.1, and 7.2) were infused for 20 minutes into one masseter of 14 healthy subjects. Pain intensity during infusion was assessed using a 200-mm Visual Analogue Scale (VAS) with a mid-point of 100 defined as the pain threshold. Pain to palpation (PP) and pressure-pain threshold (PPT) were assessed over 48 hours after the infusion. Results: pH 4.6-5.1 saline produced pain during infusion, and pH 5.1 was retained. It produced a mean pain of 133 (range 60-170). Mean PP was increased by 134% and mean PPT decreased 21% for at least 24 hours. Based on this preliminary data, a new protocol was developed using acidic (pH 5.1) or normal (pH 7.2) saline infusions into the masseter of 20 healthy subjects. PP, PPT, maximum mouth opening, chewing performance and chewing discomfort are tested prior to infusion and 1, 24, 48, and 72 hours post-infusion. Pain intensity and unpleasantness are measured during the infusion and after each jaw task. Conclusions: Infusion of acidic saline into the masseter muscle produces mild pain and allodynia of a few days duration. The present model may be a useful tool in the study of chronic muscle pain in humans. Supported by the Canadian Institutes of Health Research.