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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.
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.