MASTICATORY MUSCLE MYOSITIS (MMM)

WHAT IS IT?

Masticatory Muscle Myositis is an inflammatory disorder that specifically affects the muscles of mastication (chewing), which are situated in the jaw and above the eye sockets towards the front of the skull. The actual cause of MMM is unknown, but is suspected to be an immune-mediated mechanism. It is caused by the presence of a particular type of muscle fibre (known as 2M fibre) in the chewing muscles of the jaw and skull, which are not found anywhere else in the body. Because the structure of these 2M fibres closely resembles that of proteins found on the surface of bacteria, the immune system mistakenly recognises them as a foreign body and attacks them, resulting in inflammation. This disorder can affect any dogs of any breed, including the English Springer Spaniel, and it is seen most commonly in young or middle-aged animals. There is no difference in predisposition between males and females.

Acute form: Masticatory Muscle Myositis is generally bilateral (i.e. affecting both sides of the jaw and head). The acute form of MMM is characterised by painful swelling of the muscles of mastication, particularly the temporal and masseter muscles (those which open and close the jaw). Exophthalmos (protrusion of the eye from the socket) may be present, caused by pressure on the back of the eye from the swollen muscles. This can cause inflammation of the optic nerve and can produce vision disorders. Fever, enlarged lymph nodes, and inflammation of the tonsils may also be found. Dogs with the acute form will have difficulty eating and may drool excessively. It is painful to touch the muscles of mastication or to attempt to open the mouth.
The acute form must be differentiated from other disorders affecting the teeth, eyes, mouth, and temporomandibular joint (where the jaw meets the side of the skull). MMM is sometimes mistaken for other conditions, such as an abscess behind the eye socket, and therefore, if treated only with antibiotics, will not respond.

Chronic form: The chronic form of Masticatory Muscle Myositis is the more common form. It may occur after repeated bouts of the acute form or without any history of acute episodes. Affected dogs have severe, progressive atrophy (wasting or weakening) of the temporal and masseter muscles, accompanied by replacement of muscle tissue with fibrous connective (scar) tissue. This results in a sunken appearance to their cheeks and the top of their head. They have decreased ability to open their mouths, but are otherwise bright and alert. In the chronic form, the jaw muscles atrophy (waste or weaken) and scars will form in the masticatory muscles. The scars may make it impossible for the dog to open its mouth (a condition known as trismus).
Chronic MMM must be differentiated from trigeminal neuropathy (a nerve disorder which stimulates the muscles of mastication).

HOW DO I FIND OUT IF MY DOG IS AFFECTED?
A diagnosis of Masticatory Muscle Myositis must be based on clinical signs, so you will need to get to your Veterinary Surgeon as soon as you can to obtain a definitive clinical diagnosis.
A complete blood count may show a mild decrease in red blood cells and increased white blood cells (primarily neutrophils, but occasionally an increase in eosinophils is seen). A biochemistry panel may show increased creatine kinase (CK), aspartate aminotransferase (AST) and globulin (antibody) concentrations, especially in the acute phase of the disorder. CK and AST are commonly increased with any muscle disorder. In a few dogs, protein may be seen in the urine.

Electromyography (EMG) measures the electrical activity in muscles and can help confirm the presence of muscle disease. However, this can be normal in some dogs, especially those with chronic disease. EMG can help with selection of sites for muscle biopsy. Muscle biopsy is used to make the definitive diagnosis of MMM.

WHAT IS THE TREATMENT?

It is important in the treatment of MMM that the correct dosage of corticosteroids is used for an adequate length of time. An immunosuppressive dosage of prednisone should be used until the serum CK returns to the normal range (if elevated) and jaw function returns to normal. The dosage should then be gradually decreased, until the lowest dosage is obtained that will keep the dog free of clinical signs. This level of therapy should then be maintained for a period of 4 to 6 months.

If a dog is diagnosed with trismus (cannot open its mouth), it may be necessary to manipulate the jaw to open it while the dog is under anaesthesia.

Any dog affected by MMM may also require soft or even liquid food.

WHAT IS THE OUTLOOK FOR RECOVERY?

Whether or not the dog will regain 100% of its previous jaw function and muscle mass after treatment for MMM depends greatly upon how much damage has occurred to the muscle tissue.
Some dogs may relapse after treatment is discontinued or each time the dose of corticosteroids is decreased. These cases may warrant the use of additional immuno-suppressive drugs.
Azothiaprine therapy may be added in cases that do not respond optimally to corticosteroids alone, or where the side effects of the corticosteroids cannot be tolerated.

Where there is no evidence of significant fibrosis (scar tissue) and muscle fibre destruction, the outlook should be good for return of muscle mass and function. Recovery is usually rapid and complete if treatment is begun early. However, dogs with the chronic form of MMM, who have extensive fibrosis (scar tissue), may not respond well to therapy.