Masticatory Muscle Myositis (MMM) and Extraocular Myositis (EOM) in dogs

Masticatory Muscle Myositis (MMM) and Extraocular Myositis (EOM) in dogs

What are MMM and EOM?

Masticatory Muscle Myositis (MMM) and Extraocular Myositis (EOM) are two diseases that can affect the muscles of the face in a dog. (Myositis is the medical term for muscle inflammation.) None of these muscle inflammations will affect the muscles of the limbs; they are limited to the face. For reasons not yet fully understood, Golden Retrievers are more likely to develop Extraocular Myositis (EOM) than other dog breeds.

If a dog suffers from Masticatory Muscle Myositis (MMM), the muscles of its jaw are inflamed. (The term ‘masticatory’ is the medical term referring to chewing.)

When a dog suffers from Extraocular Myositis (EOM), its muscles around the eyes are inflamed.

How can my dog catch Masticatory Muscle Myositis (MMM) and Extraocular Myositis (EOM)?

As far as veterinary experts know, this is not disease that you ‘catch’. MMM and EOM are both immune-mediated disease where the dog’s own immune system attacks the body instead of attacking intruding organisms. When the muscles around the jaw are attacked by the immune system it is called MMM, and the when the muscles around the eyes are attacked it is called EOM.  

Symptoms of Masticatory Muscle Myositis (MMM) in dogs

One of the most common symptoms of MMM in dogs is sudden or long-term chronic pain that makes is hard to the dog to open its jaws. The dog will for instance no longer be able to pick up food or catch a ball during playtime without experiencing intense pain. In some dogs, the muscles in the jaw will suddenly swell up. In dogs with Masticatory Muscle Myositis, the muscles in the jaw can also begin to waste away, a condition known as progressive muscle atrophy.  

Symptoms of Extraocular Myositis (EOM) in dogs

A dog with Extraocular Myositis will normally develop a sudden swelling of its eye muscles. This is easy to spot for the dog owner since the eyes of the dog will start bulging out of their sockets. In long-standing cases of Extraocular Myositis in dogs, the eyes can start sinking back into the sockets – often abnormally far back.

Diagnosing Masticatory Muscle Myositis (MMM) or Extraocular Myositis (EOM) in dogs

Both these diseases are diagnosed using similar tests. To begin with, the veterinarian will naturally examine your dog and check the affected area, i.e. the jaw or the eyes. The vet will then perform routine blood tests and urine analysis. In many dogs suffering from MMM or EOM, a blood test will discover abnormal levels of muscle enzyme (serum creatine kinase [CK]). The vet will also need to do a muscle biopsy before a definite diagnosis can be made. During a muscle biopsy, the vet will remove a muscle sample from your dog and carry out a microscopic evaluation.

In the case of suspected MMM, radiographs (x-rays) can be used to locate abnormalities of the jaw, since such abnormalities can cause chewing problems.

When EOM is suspected, the vet can carry out an ultrasound examination of the region around the eyes in search of masses that could make the eyes bulge. During an ultrasound examination, deep tissues are visualized with the help of ultrasonic waves. 

Masticatory Muscle Myositis (MMM) treatment for dogs

In order to treat Masticatory Muscle Myositis, the vet must decrease the immune system’s attack on the dog’s jaw muscles. This can be achieved by giving the dog high doses of corticosteroids which will suppress the immune system. Unfortunately, high doses of corticosteroids can have significant side effects. The MMM treatment must be continued for at least 6 months. If the MMM disease is diagnoses early and the dog is given adequate doses of corticosteroids, the prognosis for dogs with Masticatory Muscle Myositis is good. When the disease is detected at an early stage, the dog can regain its normal jaw mobility with the help of corticosteroids. If the disease has been allowed to proceed for a longer period of time, the amount of scar tissue can cause permanent problems for the dog.

Extraocular Myositis (EOM) treatment for dogs

Extraocular Myositis in dogs is treated just like Masticatory Muscle Myositis – with corticosteroids capable of suppressing the immune system. Keep in mind that a suppressed immune system can make your dog susceptible to a wide range of other diseases. Once the vet has found a correct corticosteroids dose for your dog, you will be able to notice decrease swelling of the eye muscles, and the prognosis for dogs with Extraocular Myositis is good if they receive adequate treatment. A low dose of corticosteroids must be administered for at least 6 months.

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