Case Presentation: A 48-year-old female presented with dyspnea and pleuritic pain 3 weeks after a month-long hospitalization due to septic shock with methicillin susceptible staphylococcus aureus (MSSA) bacteremia. Initial workup had revealed no definitive source. Transthoracic echocardiogram had shown preserved ejection fraction without vegetation or significant valvular abnormality. Transesophageal echocardiogram (TEE) was attempted but had been mechanically unsuccessful. With infectious disease consult, she was undergoing a full 6 weeks of IV antibiotics for presumed MSSA endocarditis. She now, on week 6 of cefazolin, was admitted to the intensive care unit with hypoxic respiratory failure. Bilateral pulmonary emboli were seen on CT imaging, though embolic burden did not correlate with her severe degree of hypoxia. Pulmonary edema was also present and persisted despite diuresis. Ultimately, TEE was successfully performed and revealed severe aortic regurgitation and left coronary sinus echolucency suggestive of abscess. Heart failure, rather than pulmonary emboli, was the more significant cause of hypoxia. Cardiothoracic surgery was performed and revealed an aortic annular abscess pocket with leaflet desiccation, and a left aorta-atrial fistula. Treatment included repair of the fistula and mechanical aortic valve replacement.

Discussion: Bacterial endocarditis can be a life-threatening infection with an 18% overall in-hospital mortality. It may be present in up to 19% of patients with native heart valves and Staphylococcus aureus bacteremia. Clinical associations include unknown source of S. aureus bacteremia and embolic events. Complications include heart failure, perivalvular abscess, fistula formation, and valvular dissection. Heart failure, primarily due to valvular regurgitation, is the most common indication for surgical treatment. The incidence of perivalvular abscess among cases of infective endocarditis (IE) is 30-40%, and is associated with nearly a two-fold increased risk of systemic embolization and death. Additionally, there may be infectious extension into adjacent myocardium. Fistula formation is rare with an incidence of only 1.6% of IE cases, and can be fatal if it leads to aortic dissection, aneurysm, or myocardial infarction.

Conclusions: Staphylococcus aureus is the most common pathogenic source of bacterial endocarditis in patients with native valves and without history of IV drug use. Clinicians must have a high index of suspicion for endocarditis in patients with unidentified sources of S. aureus bacteremia and aim for early definitive diagnosis. Treatment invariably includes pathogen-specific antibiotics. Additionally, recognition of complications that may necessitate surgical management such as abscess, valvular dysfunction, and rarely, fistula formation, is imperative for survival.