What is masticatory myositis?
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 genetically predisposed to immune-mediated diseases, including MMM. Suspected triggers of MMM include bacterial and/or viral infection, vaccination, stress, exposure to allergens, reactions to medication, and exposure to environmental toxins. Unfortunately, most of the time, the trigger of MMM will never be known.
Are some breeds 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 three years, but there have been cases of dogs as young as four months with the disease. Several breeds are identified as more likely to develop MMM, including:
- Golden retrievers
- Doberman pinschers
- German shepherds
- Labrador retrievers
- Cavalier King Charles spaniels
- Rottweilers
- Weimaraners
- Large-breed dogs
How is MMM diagnosed?
Symptoms of MMM include an inability to open the mouth, inability to eat, swollen chewing muscles, and excruciating face pain are symptoms.
A blood test will diagnose MMM. The test measures circulating antibodies that attack the muscle fibers (autoantibodies). The blood test must be done before any treatment is provided, as treating with corticosteroid anti-inflammatory medication may cause the blood test to register a false negative.
In cases where circulating autoantibodies are not detected, a muscle biopsy may be used as a diagnostic test. The veterinarian removes a small section of affected muscle for evaluation. The muscle biopsy is used to determine the level of inflammation present as well as the severity of fibrosis that occurs as the muscle tissue deteriorates.
How is MMM treated?
The current, recommended traditional treatment for MMM is an immune-suppressive dose of corticosteroid medication (usually prednisone), given over a period of months. Early aggressive treatment offers the highest chance for remission. Once the symptoms are under control, the dose of prednisone is reduced to keep the symptoms at bay. Some dogs will require prednisone therapy for the rest of their lives, but others can be weaned off. Side effects of prednisone include excessive appetite, excessive thirst, and increased urination (from increased water intake).
Other immunosuppressive drugs, such as azathioprine, cyclosporin, mycophenolate mofetil, or oclacitinib may also be recommended. Pain medications may be required during the initial phases of the disease to help make the dog more comfortable.
No matter the treatment, dogs with MMM will require changes to their feeding practices. Softening kibble with warm water may be enough to encourage eating. Some dogs may do better with canned food. Your veterinarian can assist with feeding recommendations.
What is the prognosis for dogs with MMM?
The outlook for an individual dog depends on the degree of inflammation in the muscles and the dog’s response to therapy. If MMM is diagnosed early and treatment is started 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 of the problems with long-term prednisone therapy is muscle atrophy (when muscles waste away), a clinical sign that also happens with the progression of MMM. Muscle atrophy may interfere with the return of full muscle function. Dogs who have experienced MMM may relapse, and they may be more difficult to treat during subsequent bouts of the disease.
