Dilatation of the right atrium in a dog with polymyositis and myocarditis.

A nine-year-old, female, neutered golden retriever was referred for investigation of weakness. Immune-mediated polymyositis with mixed mononuclear cell inflammation and fibrosis was diagnosed in muscle biopsy specimens from several sites. A soft tissue opacity was noted radiographically cranial to the cardiac silhouette. The dog failed to respond to treatment with prednisolone and was euthanased. At post-mortem examination, the structure observed radiographically was showed to be an enlarged right atrium. Histopathological examination of samples collected at post-mortem confirmed polymyositis. Additionally, mononuclear cell infiltrations and fibrosis were observed in areas of endocardium and/or myocardium in the walls of all four cardiac chambers. Myocarditis has not been reported previously in canine polymyositis but is well recognised in people. Enlargement of the right atrium has also been reported in association with polymyositis and myocarditis in people.