Subdivided left atrium: an expanded concept of cor triatriatum sinistrum.

Twenty-four hearts with cor triatriatum were studied. On the basis of this material and a review of the literature a new classification of "subdivided left atrium" is proposed. Tye A, the most common form of subdivided left atrium, is the classic cor triatriatum with its multiple variations of partial anomalous pulmonary venous drainage; the fossa ovalis can be related to the proximal left atrial chamber (type A, a) or the distal left atrial chamber (type A, b). Type B hearts are related to (but not identical with) total anomalous pulmonary venous drainage into the coronary sinus; the coronary sinus opening is atretic in these hearts; abnormal defects connect the proximal left atrial chamber usually with the right atrium only, rarely also with the distal left atrial chamber. The Type C heart, first reported in this paper, has a superiorly and medially situated proximal chamber, located between the right and distal left atrium; it does not receive any pulmonary veins; the coronary sinus is normally formed. Current morphogenetic hypotheses that satisfactorily explain the Type A b and B heart fail to account for the Type A a and C heart.