MMM is an autoimmune disease with focal inflammation restricted to the muscles of mastication (temporalis, masseter, pterygoid and rostral digastricus) that are innervated by the mandibular branch of the trigeminal nerve.
Clinical signs:
- Jaw pain or inability to open the jaw
- Atrophy of the muscles of mastication
- Usually the lesions are bilateral; however, they can appear unilateral if one side is more severely affected.
- Breed predisposition: any breed can be affected; however, young large-breed dogs may be predisposed
Diagnosis:
- Rule out other causes for jaw pain and masticatory muscle atrophy (such as corticosteroid usage, trauma to the trigeminal nerve, etc)
- The serum 2M antibody test is the preferred diagnostic test for MMM (this test is an overseas referral test)
– Immunosuppressive corticosteroid doses can produce false negatives, especially if administered 7-10 days prior to testing.
– End-stage MMM with loss of myofibers and fibrosis can produce a false negative result. - Muscle biopsy is necessary to confirm polymyositis in patients negative for antibodies against type 2M fibres but where the disease is still suspected. In addition, muscle biopsy is helpful for determining long-term prognosis.
– The Temporalis muscle should be biopsied; take care to not biopsy the overlying frontalis muscle, as this muscle is not affected in MMM. - CBC, Chemistry & urinalysis
– Hyperglobulinaemia, mild anaemia and proteinuria have been documented in dogs with MMM but are not common.
– CK levels are often elevated in the acute phase, but become normal with chronic disease.
Differential diagnoses:
- Rule out generalized inflammatory myopathies/polymyositis
- Polymyositis (pain and reluctance to open jaw, but not MMM)
- Temporomandibular joint luxation, subluxation or fusion from chronic DJD
- Tetanus
- Craniomandibular osteopathy
- Retrobulbar abscess or other mass
- Extraocular myositis (referred jaw pain)
- Muscular dystrophy
- Foreign body
Reference: Melmed C, Shelton GD, Bergman R, Barton C. Masticatory muscle myositis: pathogenesis, diagnosis, and treatment. COMPENDIUM ON CONTINUING EDUCATION FOR THE PRACTISING VETERINARIAN-NORTH AMERICAN EDITION. 2004 Aug 1;26(8):590-605. https://www.semanticscholar.org/paper/Masticatory-Muscle-Myositis%3A-Pathogenesis%2C-and-Melmed-Shelton/7dad42b82c7b9e82340fa5cec1288158764e19d8
Reviewed: Jan 2024