Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - JANUARY 2002

Masticatory muscle myositis in a 13 week old male Saluki mix dog
Contributed by Drs. Robert Bergman and Caeley Malmed
Texas A&M University
College Station, TX

Clinical History
One week prior to presentation, the dog's muscles of mastication became acutely swollen and painful (Fig. 1 and Fig. 2). The following morning the dog had difficulty eating which then progressed over a few days to the inability to open the mouth. At the onset of the muscle swelling, both submandibular lymph nodes were enlarged. He was unable to eat solid food but could drink. He was current with vaccinations and worming. There was no history of trauma or other medical problems. The RDVM had started Amoxicillin 250 mg PO TID 3 days prior to presentation at the VMTH.

Physical and Neurological Examination
No remarkable physical examination findings were identified other than abnormalities of the masticatory muscles and jaw. These muscles seemed swollen and were painful. The jaw could only be opened 3-4 cm. Both submandibular lymph nodes were enlarged. No other neurological abnormalities were identified. The lesion was localized to the muscles of mastication.

Figure 1.

Figure 2.

Diagnostic Testing

CBC: No significant abnormalities

Serum Chemistry: Creatine kinase 1503 U/L (reference range 68-400 U/L)

Toxoplasma gondii IFA titer: Weak positive at 1:16

Neospora caninum IFA titer: Negative

Submandibular lymph node aspirate: No abnormalities

Type 2M antibody titer: Serum antibodies were detected against masticatory muscle type 2M fiber proteins by ELISA up to a dilution of 1:1000.

Radiographs of skull- No significant abnormalities

Electromyography: Awake: occasional positive sharp waves and fibrillation potentials. The next day under anesthesia and after a dose of prednisone: No abnormal activity with reduced insertional activity.

Muscle Biopsy: A biopsy was taken from the left temporalis muscle (Fig. 3 and Fig. 4) No muscle fibers were present within the biopsy sections (Fig. 3 H&E). There was a mixed population of cellular infiltrates including multifocal clusters of mononuclear cells composed predominantly of presumed lymphocytes and macrophages (Fig. 4, esterase stain for macrophages), scattered neutrophils, and fibroblasts. Irregular collagen bundles within the perimysium were separated by edema. Regenerating muscle fibers were not observed.

Figure 3.

Figure 4.

Diagnosis
Severe inflammatory myopathy/masticatory muscle myositis with fibrosis, edema, and antibodies against masticatory muscle type 2M fibers.

Treatment
Prednisone was initiated at immunosuppressive dosages (20 mg prednisone PO BID, weight 23 lbs). Clindamycin was also given (150 mg PO BID) pending results of the infectious disease titers. Supportive care included syringe feeding of liquid diet.

Outcome
The dog was re-evaluated 20 days later. The owners reported that the dog could eat moist food but had difficulty chewing dry food. It was drinking without problems and was able to use its tongue. Water consumption was increased due to the prednisone, but there were no other reported side effects. On physical examination, there was severe atrophy of the muscles of mastication (Fig. 5 and Fig. 6) and the dog was unable to completely close its mouth with poor jaw tone. The serum CK concentration was 156 U/L.

Comments
This case was presented to illustrate three points regarding masticatory muscle myositis. 1) This immune-mediated disease can occur in dogs as young as 3 months of age. 2) This case illustrates how an acute onset of marked inflammation can result quickly in myofiber destruction. 3) Marked myofiber atrophy can result once corticosteroids have been initiated and the inflammation subsides.

Figure 5.

Figure 6.