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Masticatory Muscle Myositis - MMM

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Overview

Masticatory Muscle Myositis (MMM) is an inflammatory condition that causes muscles to be so rigid, the animal cannot open its mouth. MMM involves the muscles of mastication, whether just one muscle or a group. The meaning of Myositis comes from “myo” muscle and “sitis” inflammation. The muscles affected include the master (cheek muscle), temporalis (muscle on top of the head), and pterygoid muscle (thick, quadrilateral muscle of mastication). The cause is a response to an infectious agent, parasite or virus. An immune mediated process targets muscle fibers of masticatory muscles. Antibody production leads to the destruction of the fibers. Inflammation and cell death result. The cause may refer to cancer or a precancerous chance of cancer elsewhere that could trigger the immune reaction.

This attack on the pet’s own masticatory muscles can occur in all breeds of dogs, but not in cats. Breeds include German Shepherd Dog, Doberman Pinscher and Retrievers. MMM afflicts young and middle-aged dogs. The average canine patient is three years old, either female or male.


Symptoms and Diagnosis

MMM appears as an acute (sudden) and chronic form (more common). Canines with the acute form may be lethargic, feverish, and reluctant to eat or chew. The patient may exhibit swelling of facial and forehead muscles, excessive salivation, bulging eyes, prolapsed third eyelids, shrinkage of head muscles, sunken appearance and enlarged local lymph nodes. Dogs can still be active and alert. Dogs with the chronic form cannot open the mouth normally or at all.

Diagnosis of MMM involves patient history and physical exam. A complete blood count (CBC) can indicate the acute form with perhaps mild anemia and an elevated neurophil count, or the chronic form with normal results. A biochemical profile can assess the kidney and liver and other systems. A serum creatine kinase may show elevated CK measurements. The Serum Type 2M myosin antibody titer (2M Test) from the University of California, San Diego, can measure the level of antibody attacking the masticatory muscle. A muscle biopsy and immunocytochemistry can reveal inflammatory cells and resultant muscle cell death. Other tests include an electromyography (EMG), skull and dental radiographs, CT scan or other imaging tests.


Treatment

Treatment of Masticatory Muscle Myositis should suppress the immune system to stop the antibodies from destroying the muscles. Immunosuppressive doses of corticosteroids, including prednisone or dexamethasone may help. Prednisone may result in clinical improvement. If steroid therapy does not yield a successful response, additional immunosuppressive drugs may be prescribed Side effects of corticosteroids may make the canine drink and urinate more and have an increased appetite. Dogs can experience gastrointestinal upset, bladder infection, bleeder or ulcer, vomiting, diarrhea, and dark and tarry stool.

Suppressing the immune system may lead to problems including risk of secondary infection. Affected areas include urinary and respiratory tracts. Signs include straining to urinate and blood in urine. Coughing and/or nasal discharge may be present. A skin infection may develop showing red, irritated or itchy skin.

A feeding tube that involves a small bore tube through the nose to the esophagus or a surgically placed tube can help the patient that cannot open its mouth.

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