Masticatory Myositis in Dogs

Key Takeaways

Masticatory muscle myositis (MMM) is a condition in dogs where the immune system attacks the chewing muscles. Myositis is the term for inflammation of muscle tissue. 

• The chewing muscles are on the top and sides of the head and are responsible for opening the mouth and chewing

• Dogs with MMM experience severe pain when opening their mouths and are unwilling or unable to pick up their food and toys, or chew effectively

• MMM may cause muscle swelling and severe pain and leads to muscle atrophy (wasting) over time

• MMM diagnosis requires a specialized blood test and/or muscle biopsy

• Treatment for MMM involves corticosteroid therapy for several months or longer

• Some dogs require life-long treatment

• Prognosis depends on severity of disease and response to treatment

• There is a high likelihood of relapse

A Closer Look: What is MMM 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.

Risk Factors

In chronic cases, difficulty opening the jaw can lead to: 

• Weight loss

• Weakness

• Lethargy

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.

Possible Causes

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.

Main Symptoms

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 mouth.

Testing and Diagnosis

There are multiple conditions associated with reluctance or inability to open the mouth. Testing to rule out these possible conditions include: 

• Physical examination 

• Blood work

• Sedated oral/dental exam

• X-rays

• 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 life-long treatment.

Even dogs that go into complete remission may never regain normal muscling of their head. Prolonged corticosteroid use can also lead to muscle atrophy.

Prevention

This condition cannot be prevented and is not contagious.

Is MMM Common in Dogs?

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.

Typical Treatment

• Corticosteroids 

• Pain medication 

• Nutritional support

• Immunosuppressive medications