Third-degree atrioventricular block secondary to intracardiac paraganglioma in a dog

A 14-year-old, 6-kg dog was referred for lethargy, exercise intolerance, and syncope. Electrocardiography revealed complete atrioventricular (AV) block with a narrow QRS escape rhythm at 40 bpm. Echocardiography identified a solitary mass involving the interatrial septum with extension into the interventricular septum, associated with valvular regurgitation and chamber enlargement. Despite medical management of congestive heart failure, the dog died suddenly 10 days later. Necropsy revealed a large mass infiltrating the interatrial and AV septa, including Koch's triangle and the septal vestibule. Histopathology and immunohistochemistry supported the diagnosis of an intracardiac paraganglioma exhibiting locally invasive histological features. This case describes an intracardiac paraganglioma associated with an AV block and illustrates that intracardiac neoplasia is a rare differential diagnosis for AV conduction disturbances in older dogs.