What are masticatory muscle myositis and extraocular myositis?

Myositis is the medical term for inflammation of the muscles. Masticatory muscle myositis (MMM) is an inflammatory disease that affects the muscles of the jaw. "Masticatory" refers to chewing. Extraocular myositis (EOM) is an inflammatory disease that selectively affects the muscles that surround the eye. Extraocular myositis does not affect the muscles of the jaw. Neither type of muscle inflammation affects the muscles of the limbs. Masticatory muscle myositis and extraocular myositis are seen in dogs. Golden retrievers may be more likely to develop extraocular myositis than other breeds.

What causes masticatory muscle myositis and extraocular myositis?

Masticatory muscle myositis and extraocular myositis are considered immune-mediated diseases. An immune-mediated disease is a malfunction of the body's immune defense mechanism in which the immune system is turned against the body rather than its normal function of protecting the body. The immune system actually attacks part of the body, in this case, the muscles.

What are the signs of masticatory muscle myositis or extraocular myositis?

Typical signs associated with masticatory muscle myositis include sudden (acute) or long-term (chronic) pain when the jaw is opened and inability to pick up a ball or put food into the mouth. The muscles may be suddenly (acutely) swollen or the muscles may waste away over time (known as "progressive muscle atrophy"). Typical signs associated with extraocular myositis are sudden (acute) swelling of the muscles of the eye, leading to bulging of the eyes out their sockets. In long-standing cases, the eyes may be sunken within the sockets.

How is masticatory muscle myositis or extraocular myositis diagnosed?

MMM~Masticatory muscle myositis or extraocular myositis is diagnosed by medical history, physical examination, and laboratory testing. The veterinarian may suspect masticatory muscle myositis or extraocular myositis after examining the pet, but several tests are needed to confirm the diagnosis. Routine blood tests and urinalysis typically are normal. A blood test that measures a muscle enzyme (serum creatine kinase [CK]) may be abnormal. The serum creatine kinase may be normal or slightly elevated. A muscle biopsy (surgical removal and microscopic evaluation of a sample of the muscle) is required to confirm the diagnosis. Radiographs (X-rays) are performed to identify abnormalities of the jaw that may cause difficulties in jaw movement. An ultrasound examination (visualization of deep tissues by recording ultrasonic waves) of the region surrounding the eye may be performed to help identify any potential masses that could be causing the eyes to bulge out of their sockets.

How is masticatory muscle myositis or extraocular myositis treated?

Reducing the immune system's attack on the muscles treats masticatory muscle myositis or extraocular myositis. High doses of corticosteroids are required to suppress the immune system and arrest the disease process. Improved movement of the jaw, decreased pain, and a return to normal of previously elevated blood tests indicates a positive response to the medication in dogs with masticatory muscle myositis. Decreased swelling of the muscles of the eyes indicates a positive response to medication in dogs with extraocular myositis. Medication, at a low dose, should be continued for a minimum of 6 months.

What is the prognosis for animals with masticatory muscle myositis or extraocular myositis?

The prognosis (outcome) for dogs with masticatory muscle myositis is good if treated early with adequate doses of corticosteroids. Jaw mobility should return to normal unless the disease process has been long standing and significant scar tissue is interfering with jaw function. The prognosis for dogs with extraocular myositis typically is good as the disease is responsive to corticosteroid therapy.