Abstract
Cor triatriatum sinister (CTS) is a relatively rare congenital condition characterized by an abnormal septum dividing the left atrium, morphologically presenting as three atria. Although most individuals with heart failure related to CTS undergo surgical treatment in childhood, those with larger fenestrations may remain asymptomatic until they reach an age where surgery is less viable, presenting treatment challenges. In our case study, we report on an elderly woman who declined all invasive treatments and developed heart failure due to severe functional mitral valve regurgitation triggered by atrial fibrillation. She opted for conservative treatment after a comprehensive evaluation of her condition using transesophageal echocardiography, cardiac magnetic resonance imaging, and right-heart catheterization. This multimodal evaluation highlights the importance of accurate diagnostic approaches and tailored treatments for elderly patients with CTS.