If the membrane is extensively fenestrated and weblike in appearance, it is called a Chiari network. It can be shaped like a diaphragm, funnel, or band. 10, 11 The CTD dividing membrane can range from small to large, may contain fenestrations or be intact, and may obstruct blood flow to the RV. 9 However, in CTD, the partitioning membrane is attached to the interatrial septum, whereas a prominent EV ( Figures 4 and and5) 5) has no such attachment. Often a prominent EV can mimic CTD on TTE and TEE if the RA appears septated. 8 Incomplete or partial persistence of the right venous valve can result in a prominent EV whose margins variably extend superiorly along the crista terminalis and anteriorly merge with the Thebesian valve.
In extensive, potentially obstructive CTD, the entire right venous valve persists at the sinoatrial orifice, running from the SVC to the IVC, resulting in division of the RA into an inlet portion receiving venous flow from the venae cavae and an outlet portion communicating with the RA appendage and TV. During normal development, the right venous valve regresses between weeks 9 and 15, leaving the crista terminalis in front of the SVC superiorly and the EV of the IVC and the Thebesian valve of the coronary sinus inferiorly. The right horn of the sinus venosus joins with the expanding RA, becoming a smooth-walled venous sinus, and the primitive RA becomes the RA appendage. The right venous valve partitions the RA into two chambers, allowing oxygenated blood from the umbilical vein to pass from the IVC across the PFO into the left atrium and to the systemic circulation. The left venous valve eventually merges with the septum secundum, separating the left and right atria. The sinoatrial orifice, surrounded by the left and right venous valves, is the communication between the right horn of the sinus venosus and the primitive RA. The right horn transports venous blood from the right anterior cardinal vein (which becomes the SVC), right vitelline vein (which becomes the IVC), and right umbilical vein (which eventually obliterates) to the primitive RA. The left horn of the sinus venosus becomes the coronary sinus and oblique vein of the left atrium. The sinus venosus is a quadrangular cavity that precedes the RA. The RA develops early in fetal life when the right horn of the sinus venosus fuses with the primitive atrium. 5 However, incidentally detected isolated nonobstructive ff-CTD has been reported more frequently as a clinically insignificant anatomic variant that can be confused with a cardiac mass.
2, 4 Only a few cases of truly obstructive CTD in adult patients have been reported. Often, CTD is associated with other right-sided heart malformations, including Ebstein anomaly, and stenosis or atresia of the tricuspid or pulmonary valves with a hypoplastic RV. With the advent of echocardiography, CTD diagnoses have become more common. 1 Only 14 cases were reported between 1897 (when it was first characterized by Chiari) and 1972, all but one of which were identified postmortem. 2 Whereas the prevalence of cor triatriatum sinister in patients with congenital heart disease is between 0.1% and 0.4%, 3, 4 the true prevalence of CTD remains unknown, although estimates place it at < 0.01%. Obstructive CTD is an extremely rare congenital heart abnormality in which the RA is divided into two chambers by the complete persistence of the right valve of the sinus venosus.
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