Visiting Vet: Masticatory myositis

When it just hurts too much to open the mouth.

Masticatory myositis, inflammation of the chewing muscles has a higher incidence in Labrador and golden retrievers -Chris McClave

When Jacob, the Labrador retriever, was just a pup, he underwent corrective surgery for a congenital joint problem. Nowadays, at 3 years old, he takes nutritional supplements to keep his joints healthy, but he can run around to his heart’s content without any pain or lameness. This summer he had a grand old time with all the company coming and going. Catching the ball. Going to the beach. Playing with his toys. Then one morning, right after Labor Day, his owners went to give him his daily joint supplement and Jacob cried out in pain. They thought it seemed to hurt him when he tried to open his mouth, and that he was looking forlornly at his dry dog food … as though he wanted to eat but was afraid to try. They soaked the kibble to soften it, and Jacob ate, chewing gingerly.

The next day, Jacob came to my office, where Dr. Buck was covering for me while I was off-Island with my daughter. Sure enough, when she tried to open his mouth, he let out a yelp. Everything else seemed normal, but there was no way to get a good look in his mouth without sedating him. Since he had just eaten a nice, big breakfast, that wasn’t a good idea. So what could be going on in there?

One of the most common causes of pain on opening the mouth in dogs is called retrobulbar abscess. This occurs when something pokes a pup way in the back of the mouth (behind the last molars), leading to infection. It’s hard to explain without diagrams, but put your finger on the side of your face at your temporomandibular joint (TMJ). That’s where your top and bottom jaw hinge together. Now imagine that right behind that spot, but deep inside the tissues, infection is brewing. Purulent material (the fancy word for pus) is building up inside, forming an abscess. But because it is trapped inside the tissues, with no way to drain, the pus pushes up into the space behind the eyeball — hence the term “retro” meaning behind and “bulbar” meaning bulb or ball. Every time you open your mouth, it squeezes that abscess … and that hurts. Other signs associated with retrobulbar abscess may include bulging eye, swelling around the eye, third eyelid protrusion, ocular discharge, conjunctivitis, fever, and lethargy. Treatment starts with antibiotics and pain medication. If things don’t improve, the next step is usually anesthesia and exploring the area. Sometimes by making a little incision behind that last molar and probing with a blunt instrument, we can find the abscess and drain it.

Jacob didn’t have any other signs besides the pain, but since retrobulbar abscess was still a possibility, Dr. Buck prescribed the appropriate antibiotics and pain medication. But there was another possibility on her differential-diagnosis list: masticatory myositis. Mastication mean chewing. Myositis means muscle inflammation. So masticatory myositis (MM) is inflammation of the chewing muscles — the masseter muscle on the side of the face and the temporalis muscle on the top of the head. This weird syndrome is thought to be the result of a dog’s immune system getting confused and attacking special muscle strands called 2M fibers, which are only found in those particular muscles. No one knows why this occurs. At first, the muscles may swell, and sometimes are painful to touch, but the most common presenting sign is simply sudden onset of pain on opening the mouth.

Although some sources indicate no breed or age predispositions, others suggest that MM is most common in young to middle-aged large-breed dogs with a mean age of 3 years, and a higher incidence in Labrador and golden retrievers, German shepherds, Doberman pinschers, Rottweilers, Weimaraners, and Cavalier King Charles spaniels. Jacob, a 3-year-old Labrador, fits the picture. After a few days on antibiotics without much improvement, we decided to run tests. Routine chemistry profiles and complete blood counts are rarely helpful. There may be increased blood cells called eosinophils, and/or an elevation in the enzyme creatine kinase, but often everything is normal. Definitive diagnosis can be made with a muscle biopsy, but that is invasive. We decided instead to draw blood for a “2M muscle autoantibody” test. This assay looks for those weird antibodies, and is positive in about 80 percent of MM cases.

Left untreated, acute MM progresses to the chronic form, in which the muscles fibrose and atrophy. This basically means they turn into nonfunctional scar tissue, and the inability to open the mouth becomes permanent and largely irreversible. So it’s important to diagnose and start treatment promptly. Treatment usually consists of corticosteroids, like prednisone, that suppress the immune system so it stops attacking those muscle fibers. Most dogs show improvement within the first week, but MM requires four to six months of treatment, and after that owners need to monitor carefully for recurrence. Stopping therapy too soon increases the risk of relapse. If patients do not respond adequately to prednisone, more aggressive immunosuppressants have to be tried.

In severe cases, dogs may need feeding tubes placed if they can’t open their mouths to eat. Luckily, Jacob still had enough jaw mobility to eat soft foods, but because of the importance of early intervention in MM, we considered starting a therapeutic trial of prednisone even before receiving the autoantibody test results, which could take 10 days. But the very day his owner was poised to give the first dose of corticosteroids, Jacob started to improve. His chewed on a rawhide. The next day, he found a tennis ball and was able to carry it in his mouth. Looks like we won’t need to proceed with anesthesia, surgery, radiographs, or biopsies. Perhaps he did have a small retrobulbar abscess, or a minor injury, or even a mild case of MM. We’ll see what the 2M test results say, and continue to observe him carefully. But for now, Jacob is back to his old self, and ready for fun. Play ball!