Muscle Myositis in Dogs

Canine muscle myositis is an inflammatory disease affecting muscles of the jaw or eye muscles. It's most common in large breed dogs and three specific breeds:

  • Cavalier King Charles Spaniels

  • German Shepherds

  • Golden Retrievers

There are a couple forms of myositis. Each form requires specific treatment. The most common involves the muscles that perform chewing.

Canine Masticatory Muscle Myositis

Masticatory muscle myositis (MMM) occurs when the masseter, rostral digastricus, pterygoid and temporalis muscles become inflamed. These muscles contain 2M muscle fibers that only appear in these muscles. In some dogs, the immune system mistakenly targets these 2M fibers as bacteria. As antibodies attack the 2M fibers, inflammation occurs.

Because these four muscles work together to help dogs rip apart meat and chew their food, the dog may refuse food due to the pain. As the inflammation worsens, the dog develops problems opening his mouth. If your dog has MMM, it's important to feed him canned foods until treatment helps improve muscle function.

Symptoms of MMM include:

  • Excessive drooling

  • Fever

  • Pain when opening mouth or chewing

  • Protruding eyeball

  • Protruding third eyelid

  • Red eyes

  • Swollen jaw

The symptoms are often missed by pet owners until the disease has started damaging the muscle. At this point, corticosteroids are used to stop the immune systems attack, relieve inflammation and loosen the jaw muscles again. The longer MMM goes untreated, the higher the risk of scar tissue causing permanent muscle damage. If the jaw doesn't open by itself, the veterinarian may need to do it while the pet is under anesthesia.

Corticosteroid dosages are gradually decreased after six months. In some situations, the medication must be given for the rest of the dog's life. Follow your veterinarian's treatment plan. If you stop your dog's medications too early, it's likely that MMM will relapse.

Extraocular Myositis in Dogs

Extraocular myositis (EOM) is extremely rare and generally only affects Golden Retrievers. The disease, first diagnosed in 1991, occurs when the extraocular muscles become inflamed. Unlike MMM that occurs when the antibodies attack 2M fibers, it's unknown why the eye muscles are attacked.

Dogs with this form of myositis have a startled expression with wide open eyes that may appear to pop out. Pet owners might notice their dogs third eyelid is protruding. The dogs do not appear to be in pain and are usually one to three years old when EOM becomes noticeable.

Because this is a relatively new disease, research is still ongoing. In studies performed on animals showing the signs of EOM, almost all dogs were female and of breeding age. There are a handful of male dogs who've developed EOM, but research finds it is most common in females by a ratio of about four to one.

Treatment for this inflammatory disease involves corticosteroids. Prednisolone is the prescription medication used and many dogs are back to normal within a month. Dogs given the corticosteroids for only a few days usually had a relapse of the disease after approximately two to three months. For this reason, it is important to follow your veterinarian's treatment plan.