What IS Masticatory Muscle Myositis?
Masticatory muscle myositis, or 3M, is an autoimmune disease that targets a muscle fiber type ONLY found in the masticatory muscles. The muscle fiber type is the 2M fiber, which is embryologically different from other muscle fiber types and is ONLY found in the muscles that help animals chew. This disease affects mostly young/middle aged dogs but has been reported (rarely!) in cats and mink. (Yep, mink.)
How do you diagnose 3M?
Clinical signs are consistent with inflamed muscles on the dog's head. As you might expect, this includes: difficulty and pain opening the jaw and swollen, painful muscles.
The 2M fiber type is unique to this muscle group therefore the antibody directed at the fiber is unique as well. As a result of all of this uniqueness, we can titer for the immune response to make a diagnosis! The 2M titer, developed many years ago, has a very high sensitivity (about 100%) and terrific specificity (about 85-90%) for 3M (the disease). I suggest sending samples to the Neuromuscular lab in LaJolla California (http://vetneuromuscular.ucsd.edu).
Clinical suspicion? = draw a titer --> if positive, the disease is confirmed.
What is the treatment, and response?
Treatment is immunosuppression, typically with corticosteroid administration. I prefer prednisone at 1mg/kg PO q12h x 30 days, then taper as possible. You may need to continue immunosuppression longer than 30 days. Repeat the titer (or monitor the CPK) every 1-3 months to assess for resolution of the disease.
What ISN'T 3M?
Recently, I consulted on a patient with unilateral muscle atrophy, and no history of evidence of muscle pain/pain opening the jaw. We performed titers to be confident it wasn't 3M, but the unilateral atrophy and lack of discomfort highly suggested disease of cranial nerve 5.
A second case last week involved a dog with periocular swelling, and a history of pain when brushing the teeth prior to the onset of jaw/muscle pain. This patient had clinical signs outside of the masticatory muscle group so 3M was very unlikely. Remember, 3M is an immune reaction to the very unique muscle fiber in this region and therefore the disease cannot "spread" elsewhere!
Finally, there was a nice report in JAVMA this month (March 15, vol 256m No 6, pg 647) from a good friend and colleague that reported a case in which unilateral muscle pain and swelling was secondary to infectious myositis, presumably from a penetrating foreign body (stick). Infectious myositis will often show clinical signs that reach outside of the masticatory muscle group which will tip you off that it might not be 3M. Infectious myositis may be secondary to hematogenous or direct penetration of bacteria, or from neospora or toxoplasma infections.
What do you do if you suspect 3M but titers are negative?
Negative titers may suggest the disease ISN'T 3M or that the 3M has progressed so far that the immune response has naturally run it's course and the 2M fibers are being replaced with fibrosis. Once the 2M fibers disappear, the immune response will decrease. If you strongly suspect 3M, but titers are negative, several well placed muscle biopsies should help confirm the diagnosis. Not comfortable performing biopsies? Give me a call!
Thanks for the terrific consults; keep them coming. I look forward to working with you more!