A closer look: Masticatory Myositis in Dogs
The term “masticatory muscle myositis” is an exact description of this condition. The masticatory muscles are the muscles involved in opening the mouth and chewing. Myositis refers to inflammation of muscles.
Left untreated, MMM can lead to weight loss, nutritional deficiencies, and long-term muscle damage due to atrophy.
Acutely, the chewing muscles may be visibly swollen. With chronic cases, muscle fibers waste away and are replaced by scar tissue, resulting in loss of muscle mass on the head.
Prognosis depends on the severity of the disease and the amount of muscle loss present at the time of diagnosis. Prognosis is good with early diagnosis and treatment, but relapses are common.
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Any age or breed of dog may be affected by MMM and it is common. Both male and female dogs are susceptible to MMM and it is most commonly diagnosed in young adult, large-breed dogs.
MMM is an immune-mediated disease. The exact cause is unknown but the symptoms are due to the immune system attacking muscle fibers of the affected muscles.
Symptoms of MMM are related to severe muscle pain and include:
- Pain with jaw manipulation
- Difficulty picking up toys/food
- Difficulty chewing
- Dropping toys/food
- Reluctance to open mouth
- Vocalizing when trying to eat
- Refusal to eat due to pain rather than reduced appetite
The pain associated with MMM is so severe that dogs often vocalize quite loudly if an attempt is made to open their mouths.
Testing and diagnosis
There are multiple conditions associated with reluctance or inability to open the mouth. Testing to rule out these possible conditions includes:
- Physical examination
- Blood work
- Sedated oral/dental exam
- CT scan
Definitive diagnosis is made with a specialized test that identifies antibodies to the muscle fibers. Testing includes a blood test and/or muscle biopsy.
Steps to Recovery
Treatment requires long-term, tapering doses of corticosteroids to suppress the immune system’s attack on the muscle fibers. In some cases, dogs may require additional treatment such as:
- Pain medication
- Nutritional support with softened food or a feeding tube
- Additional immunosuppressive medications
Rarely, cases may spontaneously resolve.
Steroids generally improve MMM symptoms within a few weeks, but the dose of steroids is tapered very slowly. At least six months of steroid use is usually required, and relapses are common. Some dogs require lifelong treatment.
Even dogs that go into complete remission may never regain normal muscling of their heads. Prolonged corticosteroid use can also lead to muscle atrophy.
This condition cannot be prevented and is not contagious.
Is Masticatory Myositis in Dogs common?
MMM is common in dogs. It is more common in large-breed dogs, but it can occur any time regardless of breed, age, or sex.
- Pain medication
- Nutritional support
- Immunosuppressive medications