chapter  8
Problems with Muscles, Bones, and Joints
Pages 26

DEFINITION Masticatory muscle myositis (MM) is an autoimmune, focal inflammatory myopathy of the muscles of mastication that causes necrosis, phagocytosis, and fibrosis of the myofibers1-3.

ETIOLOGY AND PATHOGENESIS Masticatory muscle fibers have a distinct embryologic origin and contain type 2M fibers. These fibers differ histochemically and biochemically from muscle fiber types of the limbs3-5. This unique type 2M myofiber isoform is likely to be related to the different motor nerve branches that develop during embryologic development6. Autoantibodies against the specific type of myosin in these type 2M fibers are responsible for the immune-mediated reaction. The reason for the autoimmune response is still unknown. Some theories suggest that cross-reaction with self-antigens may be a causative factor1. Antibodies directed against bacterial antigens could potentially cross-react with these myofibers7. Inflammatory mononucleate cells infiltrate and phagocytose the muscle fibers causing necrosis and subsequent fibrosis1,3. Cellular infiltrates in MM selectively affect the muscles innervated by the mandibular branch of the trigeminal nerve, including the masseter, temporalis, pterygoid, tensor tympani, tensor veli palatine, and digastricus 1,5,8.