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:
The latter two signs are recognized the most often, as they are associated with the chronic form of the disease.
Diagnosis of Masticatory Muscle Myositis in Dogs
Treatment of Masticatory Muscle Myositis in Dogs
Dogs that are severely affected and not immediately responsive to medical management may require a feeding tube.
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: