Masticatory Muscle Myositis (MMM) in Dogs

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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: 

  • Swelling associated with facial and forehead muscles
  • Fever
  • Pain upon opening the mouth
  • Reluctance to eat or chew
  • Excessive salivation
  • Bulging of the eyes
  • Prolapse (protrusion) of the third eyelids
  • Shrinkage of head muscles
  • Inability to open the mouth normally

    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

  • History and physical exam
  • Complete blood count (CBC)
  • Biochemical profile
  • Serum creatine kinase
  • Serum type 2M myosin antibody titer
  • Muscle biopsy and immunocytochemistry
  • Electromyography (EMG)
  • Skull and dental radiographs (X-rays) or other imaging modality such as CT scan
  • Treatment of Masticatory Muscle Myositis in Dogs

  • Immunosuppressive doses of corticosteroids
  • Additional immunosuppressive drugs if clinically indicated

    Dogs that are severely affected and not immediately responsive to medical management may require a feeding tube.

  • Home Care and Prevention

  • Recheck examination every 3 to 4 weeks during the first several months of therapy is important to monitor response to treatment and allow the veterinarian to taper the medications appropriately. In many cases, long-term or even lifelong therapy may be necessary.
  • Feeding soft foods at home until the dog’s signs have improved is imperative. Home management of a feeding tube may be required as well.
  • Avoidance of rawhides or other chew toys is advisable.
  • There are no preventative measures to reduce risk of MMM.
  • 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:

  • Traumatic, inflammatory and cancerous conditions affecting the mouth, teeth, skull or eyes. This might include fractures, tooth root abscesses, oral ulcers or stomatitis (inflammation in the mouth), oral, nasal or retrobulbar (behind the eye) tumors, and luxation (dislocation) of the temporomandibular joint (where the jaw attaches to the skull). These conditions would likely mimic the signs seen with the acute form of MMM.
  • Polymyositis (PM). PM is a more widespread muscle disease that affects multiple muscles throughout the body. It is also believed to be the result of an immune mediated process. Muscle atrophy of the muscles of mastication is usually an important feature of this disease; however, the dogs have clinical signs of weakness, and possible lameness, as well as reluctance to exercise.
  • Trigeminal neuropathy. This disease is caused by loss of normal nerve function of the trigeminal nerve, which supplies nerve branches to the muscles of mastication. Loss of normal input from the nerve causes atrophy of the muscles; however, this results in a dropped jaw or inability to close the mouth, rather than inability to open it.
  • Infectious myositis. Myositis can also be caused by infectious organisms including: protozoal infections (Toxoplasmosis, Neosporosis, Hepatozoonosis), infections with spirochetes (Leptospirosis), and heartworm infection. These also cause diffuse muscle disease rather than disease localized to the muscles of mastication.
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