Masticatory muscle myositis (MMM) in Cavaliers

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Masticatory muscle myositis (MMM) is an inflammatory condition that affects the muscles of mastication (chewing) in dogs.

MMM is caused by an immune-mediated process where the dog’s immune system mistakenly targets and attacks the specific muscle fibres (type 2M) found in the masticatory muscles. This leads to inflammation, pain, and muscle atrophy.

The muscles responsible for chewing are called masticatory muscles. When a dog develops MMM, these muscles become inflamed and painful, making it extremely difficult for the dog to open its mouth without experiencing severe discomfort. Consequently, affected dogs struggle with eating and chewing.

Certain dogs might be genetically predisposed to immune-mediated diseases, including MMM. Potential triggers for MMM include bacterial or viral infections, vaccinations, stress, exposure to allergens, adverse medication reactions, and contact with environmental toxins. However, in most cases, the specific cause of MMM remains unidentified.


Symptoms of masticatory muscle myositis in dogs, including Cavalier King Charles Spaniels, may include:

  1. Swelling or enlargement of the jaw muscles
  2. Pain or discomfort when opening the mouth or chewing
  3. Difficulty eating or drinking
  4. Weight loss due to reduced food intake
  5. Excessive drooling
  6. An inability to fully open or close the mouth (trismus or lockjaw)


A blood test is typically performed to diagnose MMM to detect and measure autoantibodies that target muscle fibers. Conducting this blood test before initiating treatment is essential, as administering corticosteroid anti-inflammatory medications may result in a false-negative outcome.

If you suspect your Cavalier King Charles Spaniel may have masticatory muscle myositis, it’s crucial to consult with a veterinarian for a proper diagnosis and treatment plan.

Treatment and prognosis for MMM

Treatment typically involves immunosuppressive medications, such as corticosteroids, to reduce the immune system’s attack on the muscles. Early diagnosis and intervention are crucial for successful treatment and minimising long-term complications. Additionally, your veterinarian may recommend supportive care, such as a soft diet, to help manage your dog’s symptoms during treatment.

The prognosis for a dog with MMM depends on the extent of muscle inflammation and the dog’s response to treatment. Early diagnosis and prompt initiation of therapy usually result in the dog regaining normal jaw function and the ability to open and close their mouth without pain. However, some dogs may develop scar tissue within the masticatory muscles, leading to permanent issues.

A potential complication of long-term prednisone therapy is muscle atrophy (the wasting away of muscles), which is also a clinical sign associated with the progression of MMM. This can hinder the complete recovery of muscle function. Dogs that have previously suffered from MMM may experience relapses, and treating subsequent episodes of the disease might prove more challenging.

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