Masticatory Muscle Myositis: Pathogenesis, Diagnosis, and Treatment

CE Email comments/questions to compendium@medimedia.com, fax 800-556-3288, or log on to www.VetLearn.com Masticatory muscle myositis is an inflammatory myopathy in which patients most commonly present with jaw pain or an inability to open the jaw. This disease is an autoimmune process in which circulating antibodies specifically target the masticatory muscles. Patients can present either in the acute or, more commonly, chronic phase of the disease. Dogs generally demonstrate no other neurologic or physical abnormalities, which may help differentiate this disease from other causes of trismus. Masticatory muscle myositis requires early detection and aggressive immunosuppressive therapy to improve the prognosis. resent the acute and chronic phases of masticatory muscle myositis. The acute phase is characterized clinically by jaw pain, trismus (i.e., inability to open the jaw), and swelling, and the chronic phase is characterized by marked muscle atrophy. Without early recognition and aggressive treatment, myofiber loss and muscle fibrosis may result in irreversible jaw dysfunction and severe muscle atrophy. Although masticatory muscle myositis was once believed to be a form of polymyositis, further investigation has demonstrated that the disease represents a very unique myopathy. Initial studies comparing limb and masticatory muscle fibers demonstrated a significant difference in their fiber types. Although limb and masticatory muscles are both composed of type 1 and 2 fibers, limb muscle contains type 1A COMPENDIUM 590 August 2004 Masticatory Muscle Myositis: Pathogenesis, Diagnosis, and Treatment Caeley Melmed, DVM, DACVIM* Texas A&M University G. Diane Shelton, DVM, PhD, DACVIM University of California, San Diego Robert Bergman, DVM, DACVIM Claudia Barton, DVM, DACVIM Texas A&M University Now Online: CE Testing at VetLearn.com