The most common indication for biopsy of the temporal and masseter muscles is as part of acomplete work-up for patients with suspected masticatory muscle myositis (MMM). MMM is an idiopathic, autoimmune, focal inflammatory myopathy with clinical signs restricted to the muscles of mastication. The temporal, masseter, and medial and lateral pterygoid muscles are all composed of type 2M fibers that contain a unique myosin and are innervated by the trigeminal nerve. The digastricus muscle, however, contains type 2A fibers and thus is not affected by the condition. Dogs with MMM have circulatingautoantibodies that specifically target type 2M fibers. The autoimmune response (infiltration of inflammatory cells after immune complex formation in muscle tissue) results in necrosis, phagocytosis, and fibrosis of affected muscles. Serum 2M antibody testing is commercially available, non-invasive, and highly sensitive (85-90 %) and specific (100 %) for MMM and should be coupled with muscle biopsy for a definitive diagnosis.1 Corticosteroid therapy up to 10-days prior to blood collection, however, mayproduce a false negative titer. Patients often present with inability (or unwillingness) to open the mouth. In the acute phase, muscle swelling, pain, lethargy, fever, decreased activity, regional lymphadenomegaly and ocular signs can be present. As thedisease becomes more chronic, marked masticatory muscle atrophy and progressive inability to open the mouth can occur. Computed tomography (CT) imaging with contrast agent injection has been shown to be a useful aid in ruling out many differential diagnoses for MMM and selecting an appropriate site for muscle biopsy. Muscle biopsy allows for microscopic documentation of the severity of tissue damage and the amount of fibrosis present. Immunohistochemical staining of sampled muscle tissue may also revealimmune complexes surrounded by inflammatory cells, which is sufficient to make a diagnosis of MMM even in the absence of a positive 2M serum titer.
展开▼
