When “Tight-Lipped” Is Not a Good Thing!
By Dr. Gail Mason,
DVM, MA, DACVIM
Chief Medical Officer, Portland Veterinary Emergency & Specialty Care
Masticatory myositis is a disorder of the immune system in which the dog’s muscle tissue is attacked by cells in its own body. “Masticatory” muscles are those involved in chewing activity. They include both the jaw and temporal muscles. This syndrome does not affect limb muscles and occurs either gradually or sudden in onset. The masticatory muscles can be swollen and painful but may not necessarily be symmetrical. In severe or chronic cases, the dog’s ability to open its mouth may be severely compromised. This is called “trismus.” The dog often drools, refuses to eat, and vocalizes when its mouth is touched. In contrast, injury to the nerve supplying these muscles (trigeminal nerve) would cause the opposite syndrome of “drop-jaw.”
What causes this?
The cause of this immune disorder in dogs is yet unknown. The disease does not involve other musculature. It is known that muscle fibers in masticatory muscles have a different molecular structure than any other muscles in the dog’s body.
It is important to note that masticatory muscle myositis (MMM) must be distinguished from other, more common causes of facial muscle/jaw pain. These include a foreign body (stick, bone, toy, etc. stuck in the mouth), an abscess behind the eye (retrobulbar abscess), dislocation/fracture of the jaw, dental pain, tetanus infection, or other types of muscle disorders. It most typically occurs in large breed dogs, with an increased frequency observed in Golden retrievers, Doberman pinschers, German shepherds, Labrador retrievers, Rottweilers, and Weimaraners. The average age at presentation is 3 years old.
Diagnosis:
The most effective first step should be a full oral examination by a veterinarian, while the patient is under anesthesia. This allows the doctor to assess if and how far the patient’s jaw can be opened. Evaluation of the teeth, gums, tongue, and temporomandibular joints are key. Ocular pressures may be assessed digitally to see if they are uneven, which is often noted in the case of a retrobulbar abscess.
If the oral examination is otherwise normal, your veterinarian will likely request a blood test that measures abnormal antibodies directed against the masticatory muscle fibers (an MMA or 2M antibody test). There can be a 15% false-negative test result, so a muscle biopsy may be obtained at the time of the oral exam to confirm the diagnosis. A more common test, measuring an enzyme called creatinine kinase can often be done in house and be of value in disease monitoring. This enzyme is not specific for MMM but can be useful in the long term.
Treatment:
The treatment of choice is to use immunosuppressive (about 1 mg/lb. of weight/day) dose of corticosteroids (usually prednisone or dexamethasone) until the jaw regains full range of motion. The creatinine kinase should return to normal as well. The dosage should be slowly tapered until the lowest, effective, every-other-day dosage is reached. This should be continued for at least six months. Some patients will need lifelong treatment. Despite many clients’ worry about the length of prednisone use, it is important to use an appropriate dose for an appropriate time. Using lower doses for shorter periods of time may result in relapsed disease. MMM is more difficult to control after one or more relapses. In certain patients that do not respond to prednisone alone or are intolerant of it, other immunosuppressants such as azathioprine, cyclosporine, mycophenylate, or oclatinib can be considered.
Prognosis:
If MMM is diagnosed and treated early in its course, the prognosis is usually favorable. Semi-liquid, high-calorie diets may be used during the acute phase of the illness to maintain the dog’s caloric intake. The owner can then consider feeding moistened kibble and/or canned food as the dog improves. Encouraging the use of chew toys can be useful physical therapy. If diagnosis and treatment are delayed, then permanent scar tissue may form which may result in resistance to medical management.