Issue Description Masticatory muscle myositis (MMM) is an
inflammatory disease in dogs affecting the muscles of mastification
(chewing). MMM is the most common inflammatory myopathy in dogs. The
disease mainly affects large breed dogs. Other Names Masticatory Muscle Myositis, Atrophic Myositis, Eosinophilic
Causes MMM is caused by the presence of 2M fibers in
the muscles of the jaw. 2M fibers are not found elsewhere in the body,
but they are close in structure to proteins found on the surface of
bacteria. The immune system recognizes these proteins as foreign to
the body and attacks them, resulting in inflammation.
Symptoms MMM appears as an acute (sudden) and chronic
form (more common). Canines with the acute form may be lethargic,
feverish, and reluctant to eat or chew. The patient may exhibit
swelling of facial and forehead muscles, excessive salivation, bulging
eyes, prolapsed third eyelids, shrinkage of head muscles, sunken
appearance and enlarged local lymph nodes. Dogs can still be active
and alert. Dogs with the chronic form cannot open the mouth normally
or at all.
Diagnosis Diagnosis of MMM involves patient history and
physical exam. A complete blood count (CBC) can indicate the acute
form with perhaps mild anemia and an elevated neurophil count, or the
chronic form with normal results. A biochemical profile can assess the
kidney and liver and other systems. A serum creatine kinase may show
elevated CK measurements. The Serum Type 2M myosin antibody titer (2M
Test) from the University of California, San Diego, can measure the
level of antibody attacking the masticatory muscle.
Treatment Treatment of Masticatory Muscle Myositis should
suppress the immune system to stop the antibodies from destroying the
muscles. Immunosuppressive doses of corticosteroids, including
prednisone or dexamethasone may help. Prednisone may result in
clinical improvement. If steroid therapy does not yield a successful
response, additional immunosuppressive drugs may be prescribed Side
effects of corticosteroids may make the canine drink and urinate more
and have an increased appetite. Dogs can experience gastrointestinal
upset, bladder infection, bleeder or ulcer, vomiting, diarrhea, and
dark and tarry stool.
Suppressing the immune system may lead to problems including risk of
secondary infection. Affected areas include urinary and respiratory
tracts. Signs include straining to urinate and blood in urine.
Coughing and/or nasal discharge may be present. A skin infection may
develop showing red, irritated or itchy skin.
A feeding tube that involves a small bore tube through the nose to the
esophagus or a surgically placed tube can help the patient that cannot
open its mouth.
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.