Masticatory Myositis: A Painful Problem to Chew Over

Does it hurt your dog to open his mouth? It may be a condition that affects the chewing muscles, making it difficult for him to eat or yawn.

If your dog has painful masticatory myositis, he probably can’t yawn or even lick. By: ralphandjenny

How often do you yawn?

You probably aren’t even aware when you yawn, we do it so often. But in dogs there is a condition where yawning becomes excruciatingly painful or even a physical impossibility.

The pain is so bad that it prevents the dog from opening her mouth to yawn, chew, drink or even lick.

Amber’s Story

Take, for example, Amber, your typical waggy, food-obsessed Labrador — except that she wasn’t eating. Her human was especially worried because, as we all know, a Labrador refusing food goes against nature.

Amber’s cheek was swollen on one side. When I tried to look inside her mouth, this usually stoic dog whimpered and sank to the ground to avoid my touch.

Amber’s mom had her own explanation: “She was playing with a bee earlier, and I think she was stung again.”

This is a reasonable deduction — insect stings are associated with pain and swelling. But even so, something wasn’t quite right. For starters, bee stings are usually puffy and tender, whereas Amber’s cheek muscle was hard, hot and very painful.

Stranger still, this was Amber’s second identical episode, which made me suspicious the problem wasn’t an insect sting but a condition called masticatory myositis.

Masticatory Myositis

Amber was indeed suffering from masticatory myositis (MM). But don’t let the name intimidate you:

  • “masticatory” means chewing
  • “myo” means muscle
  • “-itis” means inflammation

String it all together and you get “swollen chewing muscles.”

Breeds prone to MM are German Shepherds, Golden Retrievers and spaniel breeds, which is ironic because the cases I’ve seen were in Labradors, Border Collies and Staffie crosses.

This poor vizsla has trouble drinking water because of his masticatory myositis:

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MM is a condition where the immune system attacks itself — in this case, in the cheek muscles.

In the early stages, it hurts when the muscle moves (think what it’s like when you sprain a muscle), and it is pain that stops the mouth from opening. Over time, ongoing damage causes scar tissue to form that stops the muscle from stretching, and when there’s no longer enough “give,” the mouth stays shut.

In the same way cardiac muscle fibers are found only in the heart, the particular muscle fibers of the cheek are only in the dog’s face. This makes cheek muscles uniquely open to attack by the dog’s own body.


The signs to be alert for include:

  • No yawning: Yawning becomes so painful that the dog may start and then yelp or open his mouth only partway.
  • Not playing with toys: Picking a ball up becomes impossible because the dog can’t open his mouth wide enough. Watch for a sudden loss of interest in a favorite ball or toy.
  • Messy eating: The dog becomes a messy eater. He either drops food out of his mouth or pushes the food around his bowl but isn’t able to eat.
  • Change of shape: The immune attack causes hardness of the muscle and swelling. Look at your dog head-on to see if both sides of the face look identical. If one side is more swollen than the other, this isn’t normal; a veterinary check is in order.
  • Muscle wastage: Another form of MM acts more slowly and without symptoms. However, the end result is the damaged muscles waste away. You may see this as a hollow cheek on one side. If you’re uncertain, compare the dog with photos taken months before to see if there is a change in appearance.
High doses of corticosteroids can help your dog recover after a bout of masticatory myositis. By: ishane

Diagnosis and Treatment

Confirming the diagnosis can be tricky, but the best option a vet can offer is a blood test that measures the amount of antibodies against cheek muscle protein (the dog’s immune response) circulating in the bloodstream.

It’s also a good idea to X-ray the jaw to rule out fractures or trauma as a source of pain.

MM is an auto-immune disease, and treating the patient means “switching off” this misplaced attack on the body’s own cells. Corticosteroids top the treatment options because of their strong anti-inflammatory action and inhibitory effect on the immune system. To settle a flare-up, high doses are needed, but these can be reduced once the dog improves.

A Happy Outcome for Amber

I started Amber on steroids, and she improved quickly. Prompt treatment reduces the risk of permanent damage.

Amber was soon munching treats again and playing fetch with her ball. But her mom remains vigilant for flare-ups when higher doses of meds might become necessary.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.