Velma came to see me as an emergency one summer afternoon. Her owner was confident she knew what the problem was. “I think she has a sore neck from roughhousing with other dogs about three days ago,” she said. Velma was a healthy young adult, weighing about 50 pounds. Apparently the dogs had been playing hard. “I heard her cry out while they were running around,” her owner continued. “She seems particularly uncomfortable moving her neck to the left.” I always appreciate input from owners. They know their pets best. Many are astute at noticing subtle signs and changes in their animals. But I also know not to be swayed by clients’ observations and assumptions without doing my own assessment.
I approached Velma to examine her. She was “bright, alert, and responsive.” In other words, she had normal mentation and did not seem disoriented or depressed. She was, however, visibly anxious. I tried to calm her, talking quietly and slowly, not making any sudden moves. I offered a delicious liver treat. Velma declined. That’s not unusual for nervous patients, but her owner mentioned that, although always a picky eater, Velma’s appetite had definitely been off since the playdate.
I held Velma’s head gently, moving through a series of maneuvers called “cervical flexion.” That means I slowly turned her head left, right, up, and down. Velma was tense, resisting my manipulations, but didn’t seem especially painful. She just wasn’t moving or holding herself in the way I expect in dogs with neck pain. And her owner reported Velma could jump in and out of the car without hesitation. That would be unusual for a pup with neck pain. Continuing my exam, I went to open Velma’s mouth. Velma screamed. I had not moved her neck at all. I had simply tried to look in her mouth, like I do with every animal I examine, every day, dog after dog, year after year. That particular scream almost always suggests to me a differential diagnosis including just two problems — and neither is a stiff neck.
One common cause of pain on opening the mouth in dogs is a “retrobulbar abscess,” an infection inside the tissues way in the back of the mouth. It’s hard to explain without diagrams. Put your finger on the side of your face where your top and bottom jaw hinge together. Now imagine that right behind that spot, deep inside, an infection is causing a pocket of pus to build up. 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. That hurts. Other signs associated with retrobulbar abscess may include swelling around the eye, bulging 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 anesthesia and surgically exploring the area to try to locate the abscess, lance, and drain it.
I looked at Velma’s face carefully. No bulging eye, no swelling, no third eyelid protrusion. She didn’t have a fever. Her face looked completely symmetrical and normal, other than the side eye she was giving me for having done that horrible thing to her. But there was another possible diagnosis for Velma’s pain.
Masticatory muscle myositis, also known as MMM. This is an odd autoimmune syndrome in which the dog’s own immune system attacks special strands called 2M fibers found in the muscles in the head used for chewing. No one knows why MMM occurs. At first, the muscles may swell and sometimes are painful to touch, but the most common presenting sign is simply a sudden onset of pain on opening the mouth. MMM is most common in young to middle-age large-breed dogs, with higher incidence in Labrador and golden retrievers, German shepherds, Doberman pinschers, Rottweilers, Weimaraners, and Cavalier King Charles spaniels. Definitive diagnosis requires either a muscle biopsy or a special blood test called a Type 2M muscle autoantibody test, which is positive in about 80 percent of MM cases.
When I was a wet-behind-the-ears senior student in clinical rotations in veterinary school, I was sent into an exam room to work up a middle-aged large dog who had been playing Frisbee with his owner the day before. The dog now had marked pain on opening his mouth. His owner proposed that the dog had hurt himself from repeatedly leaping up and catching the Frisbee in midair. Made sense to me. I went and happily presented the case to my professor and classmates at Medicine Rounds, where I was quickly and soundly berated by a disdainful clinician for missing my first case of masticatory muscle myositis.
Velma’s owner still wanted to rule out a stiff neck, so I sent her home with pain medication and an anti-inflammatory. I told her about my experience 42 years ago with that Frisbee-catching dog. There was just something about Velma’s demeanor that made me suspicious of MMM. “She’s the right age and the right size,” I said, “If she doesn’t improve, have your regular veterinarian evaluate her further to rule out both retrobulbar abscess and MMM.”
It’s important to diagnose MMM early and begin treatment promptly with immunosuppressive drugs, such as corticosteroids like prednisone. Left untreated, acute MMM becomes chronic. Muscles undergo fibrosis and atrophy, and inability to open the mouth may become permanent and largely irreversible. With prompt treatment, most dogs improve within the first week, but may require medications for months, or even lifelong. Stopping therapy too soon increases risk of recurrence. Patients who do not respond adequately to prednisone may need other, more aggressive immunosuppressants.
Three weeks later, Velma’s owner emailed me. Her MMM test had come back borderline positive. They were starting her on prednisone. I really am grateful to that Frisbee-playing dog 42 years ago. Thanks to him, I always remember how easy it is to mistake MMM for a play-related injury.
