Clinical presentation, MRI, histopathology and outcome in a cat with immune-mediated masticatory myositis

CASE SUMMARY: A 4-year-old female spayed domestic shorthair cat was presented with facial swelling, ocular discharge and intermittent bilateral exophthalmos. Haematology revealed mild eosinophilia. Serum biochemistry showed a markedly elevated creatine kinase activity. MRI of the head revealed diffuse and severe changes of the masticatory muscles, including irregular areas compatible with fluid or necrosis within the abnormal muscle tissue. Cytological analysis of the left temporal muscle revealed eosinophilic and macrophagic inflammation. Bacterial and fungal cultures were negative. Serological titres against Toxoplasma gondii were compatible with previous exposure. A canine ELISA against masticatory muscle type IIM fibre proteins was positive at 1:4000 (reference interval <1:100). Histopathological examination of the left temporalis muscle revealed moderately severe and multifocal myositis. A diagnosis of immune-mediated masticatory myositis was made and immunosuppressive therapy was started. The cat initially responded to tapering doses of prednisolone, but subsequent relapses required therapy modulation. At the time of writing, 27 months after the initial diagnosis, the cat was in remission, but was diagnosed with diabetes mellitus, probably secondary to chronic glucocorticoid use. RELEVANCE AND NOVEL INFORMATION: To our knowledge, this is the first case report to describe the MRI appearance of masticatory myositis in a cat and the second to describe the clinical presentation, histopathology, response to treatment and outcome in a cat with this condition.