Masticatory muscle myositis
Masticatory muscle myositis (MMM) is a immune-mediated neuromuscular disease of the masticatory muscles. In these affected dogs, there is an absence of clinical abnormalities of any other skeletal muscles.
A breed predisposition has been reported in the German Shepherd and Cavalier King Charles Spaniel. In the Cavaliers, concurrent eosinophilic stomatitis and bronchopneumopathy are also common.
This syndrome is commonly referred to as locked jaw syndrome and is characterized by trismus (inability to open the mouth) due to inflammation of the masticatory muscles, including the temporalis, masseter, medial and lateral pterygoid muscles, and rostral portions of the digastricus muscles.
In this disease, muscle-specific autoantibodies and MHC class I and class II expression have been detected. The initiating immune-mediated process associated with MMM presumably involves the subsequent release of serotonin (5-HT), since there is a strong expression of 5-HT in muscle tissues of MMM-affected dogs, whereas the amine is normally absent in the muscles of healthy dogs, leading to fibrosis and cicatrization of masticatory muscles.
Diagnosis is based on clinical presentation supported with radiographic and ultrasonographic findings, histopathological examination of masseter muscle biopsies and 2M-immunohistochemistry. Masticatory muscles are composed primarily of unique type 2M myofibers that are not present in limb muscles, allowing a definitive diagnosis to be established with this latter technique.
Blood tests are usually unrewarding, but serum creatine kinase activity may be increased and a leukocytosis attributable to a neutrophilia or, less commonly, an eosinophilia may be present.
A differential diagnosis would include tetanus, rhabdomyosarcoma, polymyositis, myasthenia gravis, thymoma, dermatomyositis, extraocular myositis, temporomandibular joint ankylosis due to fracture, mandibular osteosarcoma, trigeminal nerve paralysis, temporomandibular joint luxation and dysplasia, temporomandibular osteoarthritis, retrobulbar abscess and severe otitis externa.
Medical intervention must be cautious with these dogs, especially during anesthetic induction due to difficulties associated with breathing. Mask induction is recommended.
Mandibular symphysiotomy may be required with tongues that have significant apprehension difficulties or dyspneoa associated with obstruction of the larynx due to tongue swelling.
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