Masticatory muscle myositis

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Trismus in a dog due to masticatory myositis, showing the maximum distance the patient could open his mouth whilst conscious[2]

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[3].

A breed predisposition has been reported in the German Shepherd and Cavalier King Charles Spaniel[4]. In the Cavaliers, concurrent eosinophilic stomatitis and bronchopneumopathy are also common[5][6].

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[7].

Although the masticatory muscles are primarily affected bilaterally, causing facial atrophy, trismus and ptyalism, the tongue may also be atypically involved[8].

Clinical symptoms also include dysphagia, conjunctivitis and exophthalmos[9].

In this disease, muscle-specific autoantibodies[10] and MHC class I and class II expression[11] 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[12], leading to fibrosis and cicatrization of masticatory muscles.

Diagnosis is based on clinical presentation supported with radiographic and ultrasonographic findings[13], 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[4].

A differential diagnosis would include tetanus, rhabdomyosarcoma, polymyositis, myasthenia gravis[14], thymoma[15], 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.

Treatment of masticatory muscle myositis involves gradual opening of the mouth, with medical treatment based on immunosuppressive therapy such as prednisolone[16] or cyclosporin[17].

Mandibular symphysiotomy may be required with tongues that have significant apprehension difficulties or dyspneoa associated with obstruction of the larynx due to tongue swelling[18].


  2. Reed F & Iff I (2012) Use of a laryngeal mask airway in a brachycephalic dog with masticatory myositis and trismus. Can Vet J 53(3):287-290
  3. Reiter AM & Schwarz T (2007) Computed tomographic appearance of masticatory myositis in dogs: 7 cases (1999-2006). J Am Vet Med Assoc 231(6):924-930
  4. 4.0 4.1 Taylor SM (2000) Selected disorders of the muscle and the neuromuscular junction. Vet Clin North Am Small Anim Pract 30:59–75
  5. German AJ et al (2002) Eosinophilic diseases in two Cavalier King Charles spaniels. J Small Anim Pract 43(12):533-538
  6. Joffe DJ & Allen AL (1995) Ulcerative eosinophilic stomatitis in three Cavalier King Charles spaniels. J Am Anim Hosp Assoc 31(1):34-37
  7. Evans J et al (2004) Canine inflammatory myopathies: A clinicopathologic review of 200 cases. J Vet Intern Med 18:679–691
  8. Ito D et al (2009) Symptomatic tongue atrophy due to atypical polymyositis in a Pembroke Welsh Corgi. J Vet Med Sci 71(8):1063-1067
  9. Ryckman LR et al (2005) Dysphagia as the primary clinical abnormality in two dogs with inflammatory myopathy. J Am Vet Med Assoc 226(9):1519-1523
  10. Wu X et al (2007) Autoantibodies in canine masticatory muscle myositis recognize a novel myosin binding protein-C family member. J Immunol 179(7):4939-4944
  11. Paciello O et al (2007) Expression of major histocompatibility complex class I and class II antigens in canine masticatory muscle myositis. Neuromuscul Disord 17(4):313-320
  12. Pavone LM et al (2012) Role of serotonergic system in the pathogenesis of fibrosis in canine idiopathic inflammatory myopathies. Neuromuscul Disord 22(6):549-557
  13. Bishop TM et al (2008) Imaging diagnosis - masticatory muscle myositis in a young dog. Vet Radiol Ultrasound 49(3):270-272
  14. Clooten JK et al (2003) Myasthenia gravis and masticatory muscle myositis in a dog. Can Vet J 44(6):480-483
  15. Hackett TB et al (1995) Third degree atrioventricular block and acquired myasthenia gravis in four dogs. J Am Vet Med Assoc 206:1173–1176
  16. Pitcher GD & Hahn CN (2007) Atypical masticatory muscle myositis in three cavalier King Charles spaniel littermates. J Small Anim Pract 48(4):226-228
  17. Gatineau M et al (2008) Locked jaw syndrome in dogs and cats: 37 cases (1998-2005). J Vet Dent 25(1):16-22
  18. Nanai B et al (2009) Life threatening complication associated with anesthesia in a dog with masticatory muscle myositis. Vet Surg 38(5):645-649