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What is Masticatory Muscle Myositis?

Soo Owens
Soo Owens

Masticatory muscle myositis (MMM) is a muscle disease that causes inflammation in the masticatory muscles, the muscles used for chewing, in dogs. Symptoms usually include jaw pain, excessive drooling and swelling of the jaw muscles. Chronic MMM can lead to an inability to open and close the mouth, called trismus. It is the most common type of myopathy, or muscular disease, to afflict dogs.

MMM is an autoimmune disorder in which the immune system targets the lower jaw muscle. This is because of the presence of certain fibers in the masticatory muscles that are absent from the rest of the body. These masticatory muscle (2M) fibers resemble the protein structures of certain bacteria, causing the animal's immune system to target those fibers as foreign bodies and attack them. This process causes inflammation of the muscles and, if it is chronic, atrophy of the involved muscles.

Dobermans are prone to masticatory muscle myositis.
Dobermans are prone to masticatory muscle myositis.

There are two types of masticatory muscle myositis: acute and chronic. The signs for each can vary, with the chronic form being the most common of the two. Acute MMM usually involves pain and masticatory muscle swelling as well as a fever and enlarged mandibular lymph nodes. Exophthalmus, known as protruding eyeballs, and blindness from optic nerve compression also can occur. Chronic MMM might contain any of the aforementioned symptoms but can also exhibit recurring inflammation and even atrophy of the mastication muscles.

Larger dog breeds comprise most cases of masticatory muscle myositis. German shepherds, Cavalier King Charles spaniels, Doberman pinschers and retrievers can be more susceptible to the disease, though it can occur in any breed. Most reported incidents involve young or middle-aged dogs.

Diagnosis of masticatory muscle myositis is done via a 2M antibody assay or a masticatory muscle biopsy. A false negative is possible from a 2M antibody test if the disease has already caused the destruction of the 2M fibers and led to severe atrophy. Thus, an animal that has received a negative result from a 2M antibody test but continues to exhibit signs of MMM still retains a 15 percent chance of having masticatory muscle myositis.

Corticosteroids over a four- to six-month period are the usual method of treatment for MMM. The medicine is administered in decreasing doses over the treatment period. This typically results in restored function, though the extent of this function is dictated by the amount of atrophy and muscle damage already present. It is possible for MMM to recur even after treatment has ended and the animal has appeared to have been healed.

Frequently Asked Questions

What is Masticatory Muscle Myositis and how does it affect dogs?

Masticatory Muscle Myositis (MMM) is an autoimmune condition in dogs where the body's immune system mistakenly attacks the muscles used for chewing, leading to inflammation and muscle atrophy. This can result in difficulty opening the mouth, chewing, and pain. Early diagnosis and treatment are crucial to prevent permanent damage to the jaw muscles.

What are the symptoms of Masticatory Muscle Myositis in dogs?

Symptoms of MMM include swelling of the jaw muscles, pain when opening the mouth, difficulty eating, and atrophy of the muscles involved in chewing. Some dogs may also exhibit a 'dropped jaw' where they cannot close their mouth, and in chronic cases, the muscles may become stiff and hard, limiting jaw movement even further.

How is Masticatory Muscle Myositis diagnosed in dogs?

Diagnosis of MMM typically involves a combination of clinical signs, blood tests to detect elevated levels of muscle enzymes, and a specific test for 2M antibodies, which are often present in MMM. A muscle biopsy may also be performed to confirm the diagnosis and rule out other diseases that can cause similar symptoms.

What breeds are most susceptible to Masticatory Muscle Myositis?

While MMM can occur in any breed, it is more commonly seen in larger breeds such as German Shepherds, Labrador Retrievers, Doberman Pinschers, and Golden Retrievers. However, it's important to note that MMM is not exclusive to these breeds and can potentially affect any dog.

Can Masticatory Muscle Myositis in dogs be treated or cured?

MMM can often be treated successfully with immunosuppressive drugs like corticosteroids, which reduce inflammation and immune system activity. Early intervention is key to improving the chances of recovery. In some cases, long-term medication may be necessary to manage the condition, and physical therapy can help maintain jaw function.

Is there a way to prevent Masticatory Muscle Myositis in dogs?

Currently, there is no known method to prevent MMM as it is an autoimmune disorder with unclear causes. However, maintaining regular veterinary check-ups can help in early detection and treatment, which is crucial for a better prognosis. Genetic research may eventually provide insights into prevention strategies for breeds at higher risk.

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    • Dobermans are prone to masticatory muscle myositis.
      By: doberman
      Dobermans are prone to masticatory muscle myositis.