Cor triatriatum can be either cor triatriatum dexter or sinister (also called sinistrum). Usually we mean cor triatriatum sinister when we say just cor triatriatum. Original description dates back to as early as 1868 . The term literally means a heart with three atrial chambers instead of two. When it involves the left side, there is a partition in the left atrium so that there are two atrial chambers on the left side and one atrial chamber on the right side. When it is right sided (dexter), there is a partition in the right atrium so that there are two atrial chambers on the right side and one on the left.
The left sided variety may present as mitral stenosis . In the sinister variety (also called sinistrum) the upper chamber usually receives the pulmonary venous inflow while the lower chamber has the left atrial appendage, fossa ovalis and connects to the mitral valve. It is when the separation between the two chambers is restrictive that they can present like mitral stenosis with left ventricular inflow obstruction.
The right sided variety is much rarer than the left sided variety. The upper chamber receives the venous inflow from the superior and inferior vena cavae and the lower portion has the right atrial appendage and the connection with the tricuspid valve.
Church WS. Congenital malformation of heart: abnormal septum in left auricle. Trans Path Soz. 1868;19:188-190