Many of the most commonly asked questions regarding the diagnosis and treatment of masticatory muscle myositis (MMM) are addressed in the January 2010 Special Feature in the Case of the Month section on this web site. A good review of masticatory muscle disorders can also be found in the August 2004 issue of Compendium (Melmed et al 26:590-605). Many more questions, however, have been asked regarding the 2M antibody test for the diagnosis of MMM. In the past couple of years we have a developed a new test for detection of 2M antibodies and now use a sensitive and specific ELISA method (See Figure below). It has been known for many years that MMM is an autoimmune disease with the characteristic presence of autoantibodies against the unique muscle fiber type present in the masticatory muscle group, type 2M fibers. These autoantibodies are not detected in other muscle diseases that can affect this muscle group including generalized inflammatory myopathies such as polymyositis, or inherited and congenital myopathies such as the muscular dystrophies where jaw pain or restricted jaw mobility may also be a problem. Thus, this test is specific for masticatory muscle myositis.
Q: Do I need to do a muscle biopsy or is the 2M antibody assay enough to make the diagnosis?
A: The 2M antibody test, if positive, will confirm a diagnosis of MMM. However, it cannot give prognostic information regarding the possibility of return of muscle mass or jaw function. In acute cases, where loss of muscle mass is not extensive, the antibody test may be enough. However, in chronic cases, where there is extensive loss of muscle mass, it is important to know how much muscle mass remains and the extent of fibrosis to guide therapeutic decisions. If there is extensive loss of muscle mass with fibrosis, and very little in the way of muscle mass remaining, the chances of regaining a normal muscle mass and jaw function is poor. So, the risk of using immunosuppressive dosages of prednisone in this case may outweigh any benefits. This information can only be determined by collection and evaluation of biopsy specimens from a masticatory muscle such as the temporalis. For collection of temporalis muscle biopsies, refer to our June 2012 Case of the Month.
Q: Does previous corticosteroid therapy affect the antibody test?
A: Yes, autoantibody production is affected by corticosteroid therapy and autoantibody titers therapeutically lowered. This may result in a negative or borderline test. It is always a good idea to collect the serum BEFORE beginning corticosteroid therapy. If the serum cannot be submitted at that time, refrigerate or freeze until shipped. The autoantibodies should be stable in the refrigerator for up to 2 weeks and in the freezer indefinitely.
Q: How do I interpret the antibody titer?
A: A negative result is an antibody titer <1:100. This titer does not support a diagnosis of MMM. If a diagnosis of MMM is still suspected, a masticatory muscle biopsy should be performed.
A borderline antibody titer is 1:100. This titer may be found in cases of recent onset of MMM where the antibody titer may be rising and is not yet high enough to be detected in the positive range, or in cases where the dog has been on corticosteroid therapy lowering the antibody titer. A muscle biopsy is suggested in these cases to confirm the diagnosis.
A positive result is an antibody titer of 1:500, 1:1000 or 1:4000, diagnostic of MMM
Q: Can I follow the antibody titer to determine response to therapy?
A: No, because the treatment of choice is immunosuppressive dosages of corticosteroids, and autoantibody titers will be therapeutically lowered. This lowering of the antibody titer is not a result of remission of the disease but a result of the drug. Thus, rely on return of jaw mobility to a normal range of motion, and absence of jaw pain, to determine response to therapy. Don’t forget that corticosteroid therapy itself can also result in muscle atrophy, so do not misinterpret additional muscle atrophy as worsening of the disease. It may just be the myopathic effect of corticosteroids.
Acknowledgement: Thanks to Darwin Pagaduan for expert photography of the ELISA plates