Discussion

Isolated drainage of the RSVC to the LA or biatrial drainage of the RSVC is a rare anomaly of the systemic venous connection. In review of the literature, we found 10 reported cases of biatrial drainage of RSVC published by Van Praagh and another case was reported by Alghamdi.^{Reference Van Praagh, Geva, Lock, Del Nido, Vance and Van Praagh2,Reference Alghamdi, Elfaki, Al-Habshan and Aljarallah3} Our case describes biatrial drainage of the RSVC with preferential flow into the LA in the presence of sinus venosus ASD and partial anomalous pulmonary venous return. Most of the reported cases in the literature were identified in association with persistent left superior caval vein, sinus venosus ASD, partial anomalous drainage of the right upper pulmonary veins, tetralogy of Fallot, pulmonary atresia, transposition of great vessels, and patent ductus arteriosus.^{Reference Van Praagh, Geva, Lock, Del Nido, Vance and Van Praagh2} The most common presentation was cyanosis with normal clinical findings in the childhood period. Symptoms of dyspnoea, fatigue, effort intolerance, cerebral emboli, or abscess were also described in adults.^{Reference Shapiro, Al-Sadir, Campbell, Thilenius, Anagnostopoulos and Hays4} The drainage of the RSVC to the LA leads to right-to-left shunting that can cause cyanosis and left ventricular overload. Our patient had preferential flow to the LA with smaller right atrial orifice causing significant right-to-left shunt.

Van Praagh proposed that sinus venosus defects appear to be secondary to a defect in the wall shared between the anterior wall of right upper pulmonary vein and the posterior wall of the superior caval vein.^{Reference Van Praagh, Geva, Lock, Del Nido, Vance and Van Praagh2} The absence of this wall unroofs the right pulmonary veins, resulting in overriding of the atrial septum by the superior caval vein and anomalous drainage of right upper pulmonary vein to the right side of the heart.^{Reference Van Praagh, Geva, Lock, Del Nido, Vance and Van Praagh2} Kirsch and associates postulated that drainage of the RSVC to the LA is likely caused by malposition of the right horn of the sinus venosus during fetal life.^{Reference Kirsch, Carlsson and Hartmann5} Another theory by Braudo et al suggested that the cephalic portion of the right valve of the sinus venosus fuses with the atrial septum superior to the coronary sinus inlet, forming a seal that prevents the superior caval vein from draining into the RA.^{Reference Braudo, Beanlands and Trusler6}

The prevalence of sinus venosus ASD varies from 5 to 10% of patients with ASDs.^{Reference Webb and Gatzoulis7} Approximately 90% of patients with a superior sinus venosus defect have anomalous right pulmonary venous drainage, with right upper pulmonary vein being the most common anomalous connection.

Sinus venosus defect and anomalous venous connection are often difficult to diagnose with transthoracic echocardiography alone. Trans-oesophageal echocardiography is considered a sensitive and cost-effective imaging modality.^{Reference Pascoe, Oh, Warnes, Danielson, Tajik and Seward8,Reference Oliver, Gallego, Gonzalez, Dominguez, Aroca and Mesa9} Cardiac MRI and CT are very useful in confirming the diagnosis and assessment of anomalous pulmonary venous drainage and their precise point of drainage. Cardiac MRI can also help with quantifying Qp: Qs prior to surgical correction.