masticatory muscle myositis in dogs

Masticatory Muscle Myositis (MMM) in Dogs

Overview of Masticatory Muscle Myositis in Dogs

Masticatory muscle myositis (MMM) is an inflammatory condition involving the muscles of mastication or chewing in the dog. MMM is caused by an immune-mediated process targeted against specific muscle fibers; the immune system attacks the body’s own muscles.

MMM occurs in all breeds of dog, but appears to be more common in German shepherds, Doberman pinschers, and retrievers. Young and middle-aged dogs are most commonly affected. MMM has not been reported in the cat.

The disease occurs in both acute (sudden) and chronic forms, and the signs may vary with each form. The chronic form is seen more commonly.

What to Watch For

Symptoms of masticatory muscle myositis in dogs may include:

Diagnosis of Masticatory Muscle Myositis in Dogs

Treatment of Masticatory Muscle Myositis in Dogs

Home Care and Prevention

In-depth Information on Masticatory Muscle Myositis in Dogs

MMM is an inflammatory condition which specifically affects the muscles of mastication or chewing. These include the masseter (cheek muscle), temporalis (muscle on top of the head), and pterygoid muscles. These muscles are composed of a unique type of fiber that is not found in other skeletal muscles. The disease is believed to be an immune-mediated or autoimmune process (these terms are synonymous) because the immune system inappropriately makes antibodies directed against these special muscle fibers. The end result of the antibody production is destruction of the muscle fibers, resulting in inflammation and cell death.

Historically, two distinct diseases were thought to exist, but the current belief is that one disease process exists, and that it may be seen clinically as either acute or chronic disease. The chronic disease is much more commonly recognized, and dogs are usually presented with the complaint of inability to open the mouth normally or sometimes even at all. Despite limited jaw mobility, these dogs are not usually in pain.

Owners may also notice atrophy of the head muscles, which may cause a pointy or skull-like appearance to the head and a sunken appearance to the eyes. Otherwise, these dogs are bright, alert and active. Dogs with the acute form of the disease may be lethargic, have a fever, react painfully to touching the head, experience excessive salivation and be reluctant to chew or eat. They may also have enlargement of local lymph nodes. Although the chronic disease may be a sequela to episodes of acute disease, dogs with signs of chronic disease rarely have a history of previous episodes consistent with acute disease.

Other diseases that may cause signs similar to MMM include:

In-depth Information on Diagnosis

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:

In-depth Information on Treatment

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.

Follow-up Care for Dogs with Masticatory Muscle Myositis

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

Administer all prescribed medications as directed. Alert your veterinarian if you are having problems treating your dog. 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.