Sunny, a 1 ½ year-old female spayed Golden Retriever, presented to her referring veterinarian in January 2009 for pain and swelling around the right eye. The veterinarian prescribed an antibiotic and a glucocorticoid which resolved the dog’s pain and swelling. In March, she presented for difficulty with prehending food. On sedated oral exam, it was found that she had drastically reduced range of motion of her jaw. Upon recovery from sedation, her tongue became entrapped between her carnassial teeth on the right side. Sunny presented to the CUHA Emergency and Critical Care Service on March 12, 2009 for evaluation of her reduced mandibular jaw range of motion and therapy for the entrapped tongue. On presentation, Sunny was bright and alert. She was panting, but otherwise, her vital parameters were within normal limits. Her temporal and masseter muscles were atrophied and her saggital crest prominent. Her mandibular jaw range of motion was 3 cm from measured from the maxillary central incisor tip to mandibular central incisor tip. Her tongue was protruding from the left side of her mouth to the level of the frenulum. It was significantly swollen with a soft fluctuant area around the frenulum. The tip of the tongue was dry but not discolored. Other physical exam findings included a cranial drawer sign in the left stifle and a BCS of 7/9.
Sunny was admitted to the hospital that evening and a tracheostomy was performed due to airway restriction caused by the lingual swelling. Feline Finochietto rib spreaders were placed between her incisors and slowly opened over the night to free the tongue and increase mandibular jaw movement. Steroid therapy was initiated. The following morning, Sunny’s tongue was placed back in her mouth. A CT-scan of her head was performed, and muscle biopsies were taken from her temporal and masseter muscles on the left side. The CT-scan showed atrophy of the temporal and masseter muscles on the right side and swelling of those muscles on the left. Blood was also submitted for a Type 2M autoantibody test. Her 2M antibody test was positive (1:400) and her muscle biopsies were also positive for masticatory muscle myositis.
Sunny was started on immunosuppressive doses of prednisone (50mg BID). She regained the ability to prehend food within a few days. She was discharged to the care of her owners with instructions to feed soft food, and a diet was initiated to decrease the strain on her injured stifle. Sunny’s masticatory muscles responded well to the prednisone and she slowly regained acceptable range of motion of her jaw. She is now able to eat normal food and play with her toys. She currently has both cranial cruciate ligaments ruptured. Her steroid dose has been tapered down to 5mg BID and she has been lost to follow-up. Masticatory muscle myositis is an autoimmune disorder that affects dogs. Autoantibodies are formed that are specific to type 2M myofibers. These myofibers are found only within the dorsal group of muscles innervated by the mandibular nerve. These muscles include the temporalis, masseter, pterygoideus, tensor veli palatini and tensor tympani muscles. Clinically, the disease manifests as an inflammatory stage where there is an active immune reaction to the 2M myofibers, followed by fibrosing of the myofibers where the necrotic muscle is replaced by collagen.