Most Commonly Asked Questions about Masticatory Muscle Myositis
Contributed by the Comparative Neuromuscular Laboratory Staff

Masticatory muscle myositis (MMM) is a common focal inflammatory myopathy (myositis) in dogs selectively affecting the muscles of mastication and sparing the limb muscles. Clinical signs may range from acute swelling of the temporalis and masseter muscles, restricted jaw movement, jaw pain, and exophthalmos to muscle atrophy (Fig. 1) with or without restricted jaw movement (Fig. 2). A classical clinical sign of MMM is inability to open the jaws under anesthesia. The following is a list of frequently asked questions about this disorder.

Figure 1.

Figure 2.

Q:  What are the recommended diagnostic procedures for MMM? Do I have to take a muscle biopsy?
A:  A serum creatine kinase (CK) concentration should be determined since, if elevated, is a good parameter to monitor during therapy. Both a biopsy of the temporalis or masseter muscle and the serum assay for circulating autoantibodies against masticatory muscle type 2M fibers (2M antibodies) are advised. When the temporalis muscle is biopsied, make sure the frontalis muscle is incised, the thick fascia overlying the temporalis muscle is incised and retracted, and the temporalis muscle itself is biopsied. A common problem encountered with temporalis muscle biopsies is that the muscle biopsied was not the temporalis but the frontalis, a muscle which is not affected in MMM.

Q: What is the 2M antibody assay? What is the sensitivity and specificity?
A:  The 2M antibody assay is an Enzyme-Linked Immunosorbant assay (ELISA) incubating serum from a dog with possible MMM directly into plastic wells coated with masticatory muscle antigens. Type 2M fibers and proteins are present only in the group of muscles supplied by the mandibular branch of the trigeminal nerve (i.e.: the masticatory muscles), providing the specificity necessary to separate this disorder from polymyositis which affects the masticatory and limb muscles. We have not yet detected antibodies against type 2M fibers in denervating disorders or other neuromuscular diseases. False negatives may occur if the dog has been on immunosuppressive dosages of corticosteroids for longer than 7-10 days or if the MMM is end-stage with destruction of type 2M fibers and marked fibrosis. For additional information, please refer to our December 2007 case of the month.

Q:  Can I just perform the 2M antibody assay or do I have to do a muscle biopsy too?
A:  Ideally it is best to do both the 2M antibody assay and the muscle biopsy. The 2M antibody assay will provide the diagnosis (in the absence of corticosteroid therapy or end-stage disease) but does not provide information about degree of myofiber destruction or fibrosis which are important in determining a prognosis. The owner will want to know about return of jaw function and muscle mass. Without determining the severity of pathological changes within the muscle biopsy this cannot be said with certainty.

Q: What sample do I need for the 2M antibody assay? How do I send it? Does the dog need to be fasted? Does hemolysis or lipemia affect the assay?
A: A serum sample (1-2 ml) should be sent by an overnight or second day express service on cold packs. Please refer to our sample submission form for the submission address. As for any other testing, a fasting sample is recommended. We have not found the assay to be affected by hemolysis or lipemia unless severe.

Q:  What other conditions can be mistaken for masticatory muscle myositis?
A:  Atrophy of the masticatory muscles may occur as a result of chronic corticosteroid therapy and should not be misdiagnosed as MMM. The masticatory muscles are very sensitive to the effects of corticosteroids and significant atrophy may occur. Prior to assuming a diagnosis of MMM, it should be determined if atrophy occurred after the initiation of corticosteroid therapy for another problem. Disorders of the temporomandibular joints should also be ruled out with radiographic evaluation. Atrophy of the masticatory muscles and muscle pain may also occur in endocrine disorders including hypothyroidism and Cushing's syndrome. Lastly, atrophy of this muscle group may occur with subclinical limb muscle involvement in polymyositis. The 2M antibody assay in all these conditions would be negative.

Q: Does corticosteroid therapy affect the assay?
A:  The 2M antibody assay is based on detection of circulating antibodies. Immunosuppressive dosages of corticosteroids for longer than 7-10 days will lower antibody levels. If you are not certain that you want to run this test, it is a good idea to collect a blood sample prior to beginning corticosteroid therapy and freeze the serum.  If you do plan at a later date to run the test, a pre-steroid sample is available eliminating the steroid problem.

Q: How do I treat masticatory muscle myositis?
A:  A common problem is the incorrect treatment of MMM. First, MMM cannot be diagnosed by physical examination alone so testing should be performed early and a diagnosis reached prior to initiation of therapy. Second, immunosuppressive dosages of corticosteroids should be used (2 mg/kg/day) until the normal range of jaw motion returns and serum CK, if elevated, returns to the normal range. The dosage should then be gradually decreased until the lowest alternate day dosage is reached that will keep the dog free of clinical signs. This dosage should then be continued for at least 6 months. While this disorder is usually very steroid responsive, occasionally other immunosuppressive agents such as azathioprine must be added. Inappropriate dosages of corticosteroids for inappropriate periods of time are commonly encountered problems and result in relapse of the disorder and worsening of the fibrosis. The best way to achieve a favorable outcome with MMM is with an early and accurate diagnosis and appropriate treatment.

Q:  What is the prognosis for MMM?
A:  With an early, correct diagnosis and appropriate therapy the prognosis for MMM is good. If the correct diagnosis is not reached and the appropriate therapy not initiated until there is marked atrophy and fibrosis, the prognosis is then poor for return of jaw function and muscle mass. In our experience, inappropriate therapy is the most common cause of a poor clinical outcome.

If you are a veterinarian with additional questions, please email the Comparative Neuromuscular Laboratory technicians at musclelab@ucsd.edu

Pet owners: Please be advised that for legal reasons, we are unable to provide clinical advise or treatment recommendations directly to you. We ask that your veterinarian contact us at the above email address.

Malmed C, Shelton GD, Bergman R, Barton C. Masticatory muscle myositis: Pathogenesis, diagnosis and  treatment. Comp Cont Ed Pract Vet 2004;26:590-605.

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