We are seeing more cases of masticatory muscle myositis lately so I thought we'd look into this interesting diagnosis together. Let's start by defining it.
Masticatory Muscle Myositis (MMM) is an inflammatory, autoimmune myopathy affecting the muscles of mastication (temporalis, masseter, pterygoid and rostral digastricus). All of these muscles are innervated by CN 5, specifically the mandibular branch.
Why only these muscles?
The muscles listed above have a specific myofiber type (type 2M), which is only present in these muscles and some fetal muscles. All other muscles in the body are 1A and 2A fiber types! Therefore, these muscles present a unique antigen for the immune system to target.
What does the clinical picture look like?
There is an acute phase, and a chronic phase. The acute phase occurs first and often includes signs such as jaw pain, swollen muscles, and difficulty chewing. (Note NOT drop jaw or difficulty closing the jaw....that is a different disease.)
If left untreated, patients will then progress into the chronic phase, which includes signs such as muscle atrophy and an inability to open the jaw. Muscle fibers are replaced with scar tissue during the chronic phase making the dog unable to open their mouth more than a few centimeters over time.
Must we take a chunk of muscle to make this diagnosis?
No, you don't! Although you will obtain a diagnosis in both the acute and chronic phase, there is an easier, and less invasive, way. The serum 2M antibody test is both highly sensitive (85-90%) and highly specific (100%) and is the preferred diagnostic test for MMM. Send it to the Comparative Neuromuscular Lab in La Jolla CA (https://vetneuromuscular.ucsd.edu/) if you live in the states. I think there are other choices worldwide but I'm not as aware of those.
Steroids are the best, and most effective treatment.
Immunosuppressive steroids are the most effective treatment for this disease and are strongly recommended in the acute phase. Your goal is to stop that inflammation in its tracks so that the patient doesn't progress to the chronic phase. Once they replace the muscle fibers with connective tissue, the game is over. The dog cannot open its mouth, chew, or be safely intubated or vomit. The chronic stage results in poor quality of life. Steroid protocols vary by neurologist. My protocol starts with 1 mg/kg PO q12h prednisone for 30 days, and then tapers slowly from there. Repeat 2M antibody titers every 1-2 months is recommended. Once the numbers return to normal, it is generally safe to stop the steroid administration.
If any dog has painful muscles of mastication, struggles to open its jaw without pain, and has an elevated CK on lab work a 2M antibody titer is recommended. Don't forget: neospora caninum and toxoplasma gondii can cause myositis and do attack the muscles of mastication. Concurrent testing for these two diseases in all dogs is strongly encouraged before starting immunosuppressive prednisone. I have been taken by surprise how often these tests are positive when I suspect this is "just" MMM.
Hopefully this helps you address muscle pain in your patients. If you have a question about a patient, please contact me via email or schedule a consultation online! Have a great week!