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Masticatory Muscle Myositis
Masticatory muscle myositis is an inflammatory muscle disease in which animals most commonly present with jaw pain or inability to open the jaw. This 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. Masticatory muscle myositis requires early detection and aggressive immunosuppressive therapy to improve the prognosis.
Masticatory myositis is the most commonly recognized myositis in the dog. 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 marked protrusion of the eyeball (exophthalmus), conjunctivitis, and even blindness from optic nerve compression, may also occur. The dog may have a fever, swollen tonsils, and enlarged lower jaw (mandibular) 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 (mastication).
Diagnosis
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.5
Treatment
Treatment usually involves immunosuppressive doses of corticosteroids. 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., Rapid improvement
is generally seen in cases of acute masticatory myositis. Persistent atrophy is common
even after treatment. Other immunosuppressant drugs, such as azathioprine, can be
added to the treatment protocol as needed. 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.
Go Pets America recommends seeking the advice of your local veterinarian for the most appropriate vaccination program and for the diagnosis and treatment of your pet's health problems. For vaccination requirements please contact your state and local licensing authorities.
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