A proposed pathogenesis of cor triatriatum: Impingement of the left superior vena cava on the developing left atrium

The development of the abnormal membrane which constitutes cor triatriatum has been controversial, variably attributed to the maldevelopment of interatrial septae or to malincorporation of pulmonary veins. To examine the hypothesis that the membrane arises in relationship to the left superior vena cava (LSVC) we reviewed our 12 hearts with cor triatriatum and compared serial histological sections of nine normal human embryos of stages 14 to 21 and three early fetuses. Of the 12 hearts, five had classical cor triatriatum with a membrane dividing the left atrium into two compartments: one chamber with pulmonary veins and one with mitral valve and atrial appendage. In each heart a persistent LSVC ran along the line of insertion of the membrane. All five had normal pulmonary veins and normally located interatrial septae. Four hearts with a previously undescribed variant of cor triatriatum had a membrane separating the left atrial appendage from the main left atrial cavity and in each instance the LSVC ran along the line of insertion of the membrane. Three hearts had cor triatriatum dexter with a membrane which partially subdivided the right atrium into two chambers. The abnormal membrane dividing the right atrial cavity was interpreted as a retention of the septum spurium or right venous valve, an embryological structure which becomes the crista terminalis of the fully developed normal heart. Embryological studies showed that the LSVC courses along and may indent the left atrium where the abnormal membrane is virtually always located, but that normally the LSVC obliterates during early atrial development. The results suggest that cor triatriatum arises through induction of an abnormal left atrial membrane by impingement of a prominent or persistent LSVC.