Left atrial thrombosis in a dog with advanced myxomatous mitral valve disease.

An 11-year-old Cavalier King Charles spaniel with a previous diagnosis of preclinical myxomatous mitral valve disease (MMVD) was presented with respiratory distress and abdominal distension. Lung edema and ascites were diagnosed. Echocardiography revealed a progression of the MMVD as it was associated with a moderate enlargement of the left-sided cardiac chambers and an atrial septal defect (ASD). The latter was hypothesized to be primarily due to a rupture of the interatrial septum caused by MMVD. Moreover, a hyperechoic irregular mass was documented inside the left atrium. At that time, the primary differential diagnosis included intracardiac thrombosis (ICT) and mural endocarditis. Comprehensive diagnostic tests subsequently ruled out extracardiac prothrombotic triggers; moreover, both blood and urine cultures tested negative. Despite the administration of cardiac and supportive therapies (including antithrombotic drugs), the dog died 138 days after presentation. Necropsy confirmed the presence of MMVD (type IV lesions according to Pomerance and Whitney's classification system) associated with remodeling of the left-sided cardiac chambers, multiple left atrial (LA) jet lesions, and ASD. Endocarditis was ruled out, and the LA mass was demonstrated to be an ICT entrapped in the ASD. In light of premortem and postmortem findings, the turbulent blood flow secondary to the mitral valve insufficiency and ASD, along with the extensive LA endothelial damage, were considered likely triggering factors for the development of ICT. This case suggests that, although ICT represents an extremely rare complication of cardiac diseases in dogs, it can potentially develop when canine MMVD is particularly advanced.