Cor triatriatum dexter results from persistence of the right sinus venosus valve, leading to an obstructive flap or false septum across the right atrium dividing it into 2 separate chambers. The upper chamber receives deoxygenated blood from the superior and inferior caval flow, while the lower chamber incorporates only the right atrial appendage. Most of the systemic venous return is directed towards a co-existent atrial septal defect into the left atrium, resulting in a right-to-left shunt. The membrane usually has multiple perforations, allowing flow into the lower chamber and to the right ventricle.
The eustachian valve is an embryologic remnant of the valve of the inferior vena cava (see discussion under embryology). During fetal life, the eustachian valve directs oxygen-rich blood coming from the inferior vena cava (IVC) toward the foramen ovale and away from the tricuspid valve. After birth it does not have a specific function. In echocardiographic examinations, the eustachian valve may appear as a thin flap originating from the orifice of the IVC, or it may be totally absent. It may be prominent in some individuals, but the echocardiographic appearance of a divided right atrium, as the consequence of a large eustachian valve (See image below), may be confused with that of a cor triatriatum dexter which is extremely rare. A large eustachian valves may increase right-to-left shunting in patients with atrial septal defects or persistent foramen ovale defect (PFO) by directing inferior caval flow through the defect into the left atrium. This may cause cyanosis as described as orthodeoxia in some patients with PFO defects. Sometimes it may appear as a rigid, elongated structure in the inferior portion of the right atrium or as a membranous, undulating, echogenic mass within the right atrial cavity.
There is a large variability in size, shape, thickness, and texture of the persistent eustachian valve, and in the extent to which it encroaches on neighboring structures such as the atrial septum. At one end of the spectrum, the embryonic eustachian valve disappears completely or is represented only by a thin ridge. Commonly, it appears as crescentic fold of endocardium arising from the anterior rim of the IVC orifice. The lateral horn of the crescent tends to meet the lower end of the crista terminalis, while the medial horn joins the thebesian valve, a semicircular valvular fold at the orifice of the coronary sinus. At the other extreme, it persists as a mobile, elongated structure projecting several centimeters into the right atrial cavity appearing as an undulating membrane on echocardiography; and when it is quite large, it may be confused with right atrial tumors, thrombi, or vegetations. Occasionally, the eustachian valve crosses the floor of the right atrium from the orifice of the IVC and inserts into the lower portion of the interatrial septum adjacent to the atrioventricular valves.
Chiari's network is a congenital remnant of the right valve of the sinus venosus. It is found in 1.3% to 4% of autopsy studies and is believed to be of little clinical consequence. The cranial end is attached to crista terminalis and the caudal end attaches to Thebesian valve.
In early cardiac development, 2 venous valves guard the right horn of the sinus venosus. The smaller left valve is incorporated into the septum secundum, and the right valve partitions the right atrium. In normal development, the right valve regresses between gestational weeks 9 and 15, its cephalic portion remaining as the crista terminalis and its caudal portion dividing to form the eustachian and thebesian valves. Failure of regression of the right valve gives rise to the fenestrated Chiari's network, which attaches to the superior vena cava or crista terminalis. This must be differentiated from a prominent eustachian valve, and from complete atrial partitioning as found in cor triatriatum dexter.
Legends: VV = Vitelline vein, UV = Umbilical vein, CC = Common cardinal vein, SA = Sinoatrial orifice, RSH, LSH right, left sinus horn, RA, LA = right, left atrium Copyright © 2011 European Heart Rhythm Association of the European Society of Cardiology (ESC)
Legends: LVV, RVV = left, right venous valve, SS = Septum spurium SA = Sinoatrial orifice, OCS = oriface of the coronary sinus, RAu = Right auricle, SV = Sinus venarum, CT = Crista terminalis, OCS = Oriface of the coronary sinus, LVV, RVV = left, right venous valve,
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