Eustachian and Aortic Valve Endocarditis: A Rare Case of Concomitant Right- and Left-Sided Endocarditis.

Concomitant right- and left-sided infective endocarditis (IE) is an uncommon clinical entity, particularly when involving the Eustachian valve (EV), a vestigial remnant of the inferior vena cava. While right-sided IE is typically associated with intravenous drug use and indwelling cardiac devices, it can occur in non-traditional populations if anatomical niches are colonized. A 78-year-old female presented with heart failure and a "mixed" embolic signature, including pulmonary, renal, and cerebral infarcts, following a dental extraction. Imaging confirmed aortic valve endocarditis and a 1.5 cm vegetation on the EV. Blood cultures were positive for Streptococcus mutans. Despite lacking traditional risk factors for right-sided infection, the patient required emergent surgical intervention for refractory heart failure and high embolic burden. This case highlights the unique virulence of S. mutans and the necessity of comprehensive echocardiographic evaluation of fetal remnants when multisystem emboli are present.