title: Acidic saline-induced pain as a model for experimental masseter myalgia in healthy subjects.
abstract: BACKGROUND
  Repeated injection of acidic saline into skeletal muscles of the leg in rodents induces a prolonged bilateral mechanical hyperalgesia that persists for up to 30 days and may be useful to model widespread muscle pain conditions. In this study, repeated injection of acidic (pH 3.3) saline solution into the masseter muscle of healthy human subjects was undertaken to determine if these injections are painful and whether they would induce a prolonged period of muscle sensitization to artificial and/or natural mechanical stimulation of the masseter and temporalis muscles.
  
  
  METHODS
  Eighteen subjects (10 male, 8 female) participated in the study. Subjects received two injections of 0.5 mL acidic or regular isotonic saline 2 days apart, in a randomized, double blind, crossover design.
  
  
  RESULTS
  There was no significant difference in pain intensity ratings when acidic saline injections were compared with regular saline injections. Pain area drawings were, however, significantly larger in response to the first injection of acidic saline than to the second injection of acidic saline or to either the first or second injection of regular saline. Repeated injection of acidic saline did not significantly alter pressure pain thresholds from the masseter or temporalis muscles on either the injected side or the opposite side over the 10-day post injection monitoring period. There was also no effect of injections on chewing.
  
  
  CONCLUSIONS
  These findings indicate that, unlike in some rodent models, repeated injection of low pH solutions into jaw muscles of humans fails to induce a period of prolonged muscle hyperalgesia.
authors: Section of Clinical Oral Physiology, Department of Dentistry, Aarhus University, Denmark; Section of Orofacial Pain and Jaw Function, Department of Dental Medicine, Karolinska Institutet, Huddinge, Sweden.
authors: E E
authors: EE
authors: Castrillon
authors: B
authors: B
authors: Cairns
authors: T
authors: T
authors: List
authors: P
authors: P
authors: Svensson
authors: M
authors: M
authors: Ernberg
mesh_terms: D000143:Acids / Q000008:administration & dosage*; D000328:Adult; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006930:Hyperalgesia / Q000139:chemically induced*; D007273:Injections, Intramuscular / Q000379:methods; D008297:Male; D063806:Myalgia / Q000139:chemically induced*; D010146:Pain / Q000139:chemically induced*; D010147:Pain Measurement / Q000379:methods; D012965:Sodium Chloride / Q000008:administration & dosage; D055815:Young Adult
chemical_list: D000143:Acids; D012965:Sodium Chloride