Cardiac leiomyoma associated with advanced atrioventricular block in a young dog.

A 1 year old intact female Labrador retriever dog presented for evaluation of syncope caused by severe bradycardia. Advanced atrioventricular block was diagnosed. A mass infiltrating the interventricular septum and the tricuspid annulus was identified with echocardiography. Histopathological evaluation of the mass was consistent with cardiac leiomyoma, and immunohistochemical staining for smooth muscle actin confirmed the diagnosis. To the authors' knowledge, this is the first report of cardiac leiomyoma in the canine species.