Masticatory Myositis in Dogs

My dog suddenly lost the ability to open his mouth. He would try to eat and instead scream in pain. The veterinarian called it “masticatory myositis” and tried to explain it, but I am not sure I understand what is going on with my poor dog.

Masticatory muscle myositis (MMM) is an immune system disorder in which the dog’s immune system identifies the dog’s own muscle tissue as foreign and attacks it. The masticatory muscles are the muscles involved with chewing. MMM causes these muscles to become swollen and painful. Dogs with MMM cannot open their mouths without excruciating pain. They cannot eat, chew, or pick up their toys.

Some dogs may be predisposed genetically to immune-mediated diseases, including MMM. Suspected triggers of MMM include bacterial and/or viral infection, vaccinations, stress, exposure to allergens, reactions to medication, and exposure to environmental toxins. Unfortunately, most of the time, the actual trigger of MMM will never be known.

Early in the course of MMM, the muscles used for chewing can actually look swollen and feel warm to the touch. Over time, these muscles begin to waste away. The contours of the dog’s skull become visible, and the eyes appear to sink back into the eye sockets.

Who develops MMM?  Are there some breeds that are more susceptible to MMM than others?

MMM can occur in any breed, and in both males and females. The average age of MMM patients is 3 years, but dogs as young as 4 months have been reported in the veterinary literature. There are several breeds that are identified as more likely to develop MMM, and these include:

• Golden Retrievers
• Doberman Pinschers
• German Shepherds
• Labrador Retrievers
• Cavalier King Charles Spaniels

How is MMM diagnosed?

A dog’s inability to open his mouth, inability to eat, swollen chewing muscles, and excruciating face pain are all highly suggestive of MMM. That said, the definitive diagnostic test for MMM is a muscle biopsy. The veterinarian removes a small section of affected muscle for evaluation under the microscope by a pathologist. The muscle biopsy demonstrates the level of inflammation present as well as the severity of the fibrosis that occurs in this disease as the muscle tissue deteriorates.

There is now a blood test for diagnosing MMM. The test looks for and measures circulating antibodies that attack the muscle fibers, and is offered by the Comparative Neuromuscular Laboratory at the University of California – San Diego. The blood test must be drawn before any treatment is provided. Treatment with corticosteroid anti-inflammatory medication can cause the blood test to register a false negative.

How is MMM treated? 

The current recommended traditional treatment for MMM involves an immune-suppressive dose of corticosteroid medication -- usually prednisone -- given over a period of months. Early aggressive treatment offers the highest chance for remission from MMM. Once the symptoms are under control, the dose of prednisone is reduced to the lowest every-other-day dose that keeps the symptoms at bay. Some dogs will require prednisone therapy for the rest of their lives, but many can be weaned off.

Side effects of prednisone include excessive appetite, excessive thirst, and increased urination (from increased water intake).

Another approach to MMM treatment is neuromodulatory acupuncture. Treating MMM with acupuncture involves inserting very thin needles into specific spots on the face and head in order to modulate the inflammation associated with MMM by affecting the nervous system. While this is not an approach that is discussed much in the veterinary literature, veterinary acupuncturists report excellent results in a majority of cases of MMM.

No matter the treatment chosen, dogs with MMM will require changes to their feeding. Softening kibble with warm water may be enough to encourage eating. Some dogs may do better with canned food. Your veterinarian can assist you with feeding recommendations during a bout of MMM.

What does the future look like for dogs with MMM?

The outlook for individual dogs depends upon the degree of inflammation in the muscles and the dog’s response to therapy. If MMM is diagnosed early and treatment is initiated right away, a dog usually regains normal jaw function and the ability to open and close his mouth without pain. In some dogs scar tissue can form within the masticatory muscles causing permanent problems.

One problem with long-term prednisone therapy is muscle atrophy -- a clinical sign that also happens with the progression of MMM. Atrophy itself may interfere with a return of full function. Dogs who have experienced MMM may relapse, and they may be more difficult to treat during subsequent bouts of the disease.