Masticatory myositis is an inflammatory disorder that involves (primarily or exclusively) the muscles of mastication (chewing). 

The acute form of masticatory myositis is characterized by painful swelling of the muscles of mastication, particularly the temporal and masseter muscles. 

The chronic form of masticatory myositis is the more common form. Affected dogs have severe, progressive atrophy of the temporal and masseter muscles, accompanied by replacement of muscle tissue with fibrous connective (scar) tissue.  This results in a sunken appearance to their cheeks and the top of their head. 

Immunosuppressive doses of corticosteroids is used to treat this condition.

What is masticatory myositis?

• Masticatory myositis is an inflammatory disorder that involves (primarily or exclusively) the muscles of mastication (chewing).  The actual cause of masticatory myositis is unknown, but is suspected to be an immune-mediated mechanism.  This disorder can affect any dogs of any breed, but it seems to be seen most commonly in German Shepherds, retrieving breeds and other large breed dogs.  It is seen most commonly in young or middle-aged animals and there is no gender predisposition.  This has not been recognized in cats.

Clinical Signs

• Acute form:  Masticatory myositis is generally bilateral (both sides are affected).  The acute form of masticatory myositis is characterized by painful swelling of the muscles of mastication, particularly the temporal and masseter muscles.  Exophthalmos (protrusion of the eye from the socket) may be present, caused by pressure on the back of the eye from the swollen muscles.  This can cause inflammation of the optic nerve and can produce vision disorders.  Fever, enlarged lymph nodes, and inflammation of the tonsils may also be found.  Dogs with the acute form are reluctant to eat and may drool excessively.  They are painful on palpation of the muscles of mastication and on attempts to open the mouth.
• Chronic form:  The chronic form of masticatory myositis is the more common form.  It may occur after repeated bouts of the acute form or without any history of acute episodes.  Affected dogs have severe, progressive atrophy of the temporal and masseter muscles, accompanied by replacement of muscle tissue with fibrous connective (scar) tissue.  This results in a sunken appearance to their cheeks and the top of their head.  They have decreased ability to open their mouths, but are otherwise bright and alert. 

Diagnosis

• A diagnosis of masticatory myositis is suspected based on clinical signs.  The acute form must be differentiated from other disorders affecting the teeth, eyes, mouth, and temporomandibular joint.  The chronic form must be differentiated from trigeminal neuropathy (a disorder of the nerve which innervates the muscles of mastication).
• A complete blood count may show a mild decrease in red blood cells and increased white blood cells (primarily neutrophils, but occasionally an increase in eosinophils is seen).  A biochemistry panel may show increased creatine kinase (CK), aspartate aminotransferase (AST) and globulin (antibody) concentrations, especially in the acute phase of the disorder.  CK and AST are commonly increased with any muscle disorder.  In a few dogs, protein may be seen in the urine. 
• Electromyography (EMG) measures the electrical activity in muscles and can help confirm the presence of muscle disease.  However, this can be normal in some dogs, especially those with chronic disease.  EMG can help with selection of sites for muscle biopsy.  Muscle biopsy is used to make the definitive diagnosis of masticatory myositis. 

Treatment

• Immunosuppressive doses of corticosteroids should result in clinical remission of masticatory myositis.  Therapy is recommended for at least 4-6 months.  The dose of corticosteroids is gradually reduced over that time period, starting two weeks after therapy is initiated. 

Complications

• Some dogs may relapse after treatment is discontinued or each time the dose of corticosteroids is decreased.  These cases may warrant the use of additional immuno-suppressive drugs. 

Prognosis

• Recovery is usually rapid and complete if treatment is begun early.  Dogs with the chronic form of masticatory myositis, who have extensive fibrosis, may not respond well to therapy. 

References

• Nelson, Richard W. and C. Guillermo Couto.  Small Animal Internal Medicine.  Mosby, Inc.  St. Louis, 1998.
• Ettinger, Stephen J. and Edward C. Feldman.  Textbook of Veterinary Internal Medicine.  W. B. Saunders Company.  St. Louis, 2000.