An extremely rare congenital heart anomaly classified as the presence of a third atrium. Either atrium can be separated by a membrane to create a small chamber that disturbs the heart's function. Typically diagnosed within the first year of life, but can be discovered in childhood or adulthood. Signs and symptoms include dyspnea, murmur, cough, pallor, failure to thrive, palpitations, tachycardia, pulmonary congestion, and wheezing. Cor triatriatum can be successfully corrected with surgery. If left untreated, complications such as reoccurring pneumonia and bronchitis may result in congestive heart failure.