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Masticatory Muscle Myositis

By: Dr. Erika de Papp

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Diagnosis In-depth

A thorough history and physical exam are imperative to making an appropriate diagnosis. Although there are many disease processes that may have signs similar to acute MMM, there are fewer causes of signs consistent with chronic MMM. Physical exam findings are important in order to determine the appropriate course of action with respect to the diagnostic work-up. Tests may include:

  • Complete blood count. The CBC evaluates both the red and white blood cells. In acute cases of MMM, it is not uncommon to see mild anemia (low red blood cell count), a neutrophilia (elevated neutrophil count) and sometimes an eosinophilia (elevated eosinophil count). The neutrophils and eosinophils are two different types of white blood cells. The CBC is usually within normal limits in chronic cases of MMM.

  • Biochemical profile. The biochemical profile is an appropriate way to assess other organ systems, including the liver and kidneys. It is useful in ruling out systemic disease, as may be seen with some of the infectious causes of myositis. It also evaluates the proteins in the blood, which are composed of albumin and globulin. Elevated globulin levels may sometimes be seen with acute MMM, and can be a marker of inflammation and/or antibody production.

  • Serum creatine kinase (CK). CK measurements may be elevated in dogs with the acute form of the disease. This is a muscle enzyme that increases in response to muscle damage. A lack of CK elevation does not rule out the disease, but a high CK level may support the diagnosis.

  • Serum type 2M myosin antibody titer. The 2M antibody titer is a test which detects circulating antibodies against the specific muscle fibers (2M) that are damaged in MMM. This is a very useful test, as 81 percent of dogs with MMM will have positive test results. However, the absence of a positive test does not rule out the disease. Additionally, the test may become negative if dogs have been treated with corticosteroids prior to obtaining the serum sample.

  • Muscle biopsy. Muscle biopsy is the gold standard for diagnosing MMM. The acute form is characterized by specific inflammatory cell infiltrates in the muscle and resultant muscle cell death. The chronic form of the disease can also demonstrate inflammatory cells, but there is also muscle fibrosis (scarring) seen, due to the prolonged nature of the condition. Muscle biopsy can help distinguish MMM from infectious causes of myositis as well as neurologic causes of muscle atrophy.

  • Immunocytochemistry. This special staining technique is performed on the biopsy tissue to evaluate for the presence of antibodies against the 2M muscle fibers. It is similar to the 2M antibody titer, but it looks for antibodies at the tissue level rather than in the blood. As many as 85 percent of dogs with MMM have positive test results with immunocytochemistry.

  • Electromyography (EMG). EMG studies evaluate the muscles by looking at their electrical activity. This test can be useful for determining if muscle disease is limited to one set of muscles, as is the case with MMM. It can also help distinguish between primary muscle disease and disease caused by neurologic dysfunction of the nerves supplying the muscles in question. It is not useful, however, for distinguishing between different causes of muscle disease.

  • Radiography or CT Scan. Radiographs or CT scan images of the head and/or teeth may be useful in cases where traumatic or cancerous causes of the dog's clinical signs are suspected. They may not be particularly useful in chronic cases of MMM.

    Treatment In-depth

    The therapy of MMM, whether acute or chronic, is centered around suppressing the immune system. Because the disease is the result of an immune mediated process, the goal of therapy is to inhibit the immune system, thereby halting the antibody induced destruction of the muscles. Immunosuppressive therapy is not a benign process, as it makes the body more susceptible to infection. Therefore, it is important that a confirmed diagnosis be established prior to starting this type of treatment.

  • Corticosteroids at high doses are the mainstay of treatment for MMM. Corticosteroids are widely used in veterinary medicine, and different doses have differing effects. High dose steroid therapy must be used in order to achieve suppression of the immune system. Most dogs show clinical improvement quite rapidly following institution of this therapy. Most common is prednisone.

  • Additional immunosuppressive drugs may be required if the patient does not show adequate response with steroid therapy alone or if there are adverse side effects seen with the steroid use. A drug in this class is azathioprine (Imuran®). This also suppresses the immune system, but does so via a different mechanism than corticosteroids.

  • Feeding tubes may occasionally be required in dogs that do not show rapid improvement and are not able to open their mouths far enough to eat. There are various styles of feeding tubes available. The least invasive form of tube feeding involves placement of a small bore tube through the nose into the esophagus. Tubes can also be surgically placed or placed with the aid of an endoscope into the esophagus or stomach for feeding purposes.

    Follow-up

    Optimal treatment for your pet involves a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve.

    Administer all prescribed medications as directed. Alert your veterinarian if you are having problems treating your pet. Follow-up after the initial 3 to 4 weeks of therapy is very important, as drug dosages may be gradually decreased depending on the pet's clinical response to therapy. Corticosteroids must be very slowly tapered over a several month period, and should never be abruptly discontinued, as this can make your dog quite ill. Some dogs need to be treated with lifelong therapy if disease relapses occur.

    Because of the increased risk of secondary infection when being treated with immunosuppressive drugs, owners must watch for problems. Infections are most commonly seen in the urinary and respiratory tracts, as well as the skin. If you notice straining to urinate or blood in the urine, red, irritated or itchy skin, or coughing and/or nasal discharge, these could all be signs of infection. Contact your veterinarian if any of these symptoms arise.

    Certain side effects of high dose corticosteroid therapy are to be expected: Dogs will drink and urinate more than normal, may pant, and may have an increased appetite. Adverse effects of this therapy may include gastrointestinal upset and even bleeding or ulcer formation. This may manifest as vomiting, diarrhea, dark and tarry stools, or general weakness. Any of these adverse effects should be brought to the attention of your veterinarian immediately.

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