Lipomatous hypertrophy of the interatrial septum in a dog: echocardiographic, gross pathologic, and histologic findings.

A 12-year-old, 20-kg, Cocker spaniel was evaluated for the recent onset of marked weakness. Cardiologic assessment revealed myxomatous degeneration of the atrioventricular valves, moderate left atrial dilation, and a sustained narrow QRS complex tachycardia primarily consistent with focal atrial tachycardia. Moreover, two-dimensional imaging of the interatrial septum showed a hyperechoic, multifocal thickening (maximum thickness: 10 mm). Differential diagnoses included neoplastic infiltration, intracardiac thrombosis, and mural endocarditis. Despite prompt hospitalization in the intensive care unit and combination antiarrhythmic therapy, adequate control of the tachyarrhythmia was not achieved, and euthanasia was ultimately elected. Necropsy confirmed myxomatous mitral valve degeneration and excluded cardiac neoplasia, thrombosis, and mural endocarditis. Histologic examination of the interatrial septum revealed abnormal infiltration by mature adipose tissue, causing full-thickness separation and dissociation of septal myofibers. Based on antemortem and postmortem findings, the diagnosis was lipomatous hypertrophy of the interatrial septum, a condition not previously reported in dogs but well recognized in human cardiology.