Masticatory muscle myositis is the most commonly recognized myositis in the dog. This is an inflammatory muscle disease in which animals most commonly present with jaw pain or inability to open the jaw. The disease is an autoimmune process in which circulating antibodies specifically target the masticatory (lower jaw) muscles. There are two forms of the disease: acute and chronic. Dogs generally demonstrate no other neurologic or physical abnormalities, which may help differentiate this disease from other causes of prolonged spasm of the muscles of the jaw. A common clinical sign to both forms is trismus (prolonged spasm of the muscles of the jaw), however, it may or may not be present.
In the acute form, dogs can present with masticatory muscle swelling and pain. Ocular signs, which include exophthalmus (marked protrusion of the eyeball), conjunctivitis, and even blindness from optic nerve compression, may also occur. The dog may have a fever, swollen tonsils, and enlarged mandibular (lower jaw) lymph nodes. The chronic form usually has a more insidious presentation, with recurrent bouts of inflammation. There is a variable degree of atrophy of the muscles of involved in chewing.
Definitive diagnosis can be made with masticatory muscle biopsy and/or positive 2M-antibody titers in addition to clinical signs. However, animals that exhibit signs and have a negative 2M-antibody test may have up to a 15% chance of being affected with masticatory myositis. Hence, a negative result does not completely rule out this disease. Additionally, previous corticosteroid treatment may produce a false negative antibody titer. Masticatory muscle myositis requires early detection and aggressive immunosuppressive therapy to improve the prognosis. The goal of treatment is to restore function, followed by long-term control to prevent recurrence, which is common, especially if animals are tapered off steroid therapy over a short period of time. Inappropriate therapy is the most common cause of treatment failure in dogs with masticatory myositis. High doses of corticosteroids are maintained until CK values return to normal and/or resolution of clinical signs. The dose can then be gradually tapered, until the lowest alternate day dose is achieved that maintains the dog symptom free. This dose is generally maintained for 4-6 months. Additionally, physical therapy can be implemented, especially in chronic cases, to gradually facilitate jaw motility. Under no circumstances should the jaw be forcibly opened, even under sedation or anesthesia, as this may result in mandibular fracture.