Discussion

The development of pulmonary veins begins with the development of intrapulmonary venous plexuses in the bifurcating tracheo-bronchial tube which develops from the ventral aspect of primitive foregut. ^{Reference Anderson, Brown and Moorman3} A midline venous channel (in sagittal body axis) develops within the mediastinal tissues at 5 weeks of gestation and drains the developing intrapulmonary venous plexuses from both lungs. This pulmonary venous confluence (also labelled as common pulmonary vein in literature) located within dorsal mesocardium canalises to open into the atrial cavity, committing the pulmonary venous drainage between the pulmonary ridges into the primitive atrium (later into left atrium after development of primary atrial septum).

Total anomalous pulmonary venous connection results from complete failure of connection between the primitive intrapulmonary venous plexus and the common pulmonary vein (pulmonary venous confluence) or common pulmonary vein and left atrium. As a result, there is persistence of connection between the primitive pulmonary venous system, common pulmonary vein (pulmonary venous confluence), and cardinal venous system (which later develops into systemic veins). Mixed type of total anomalous pulmonary venous connection (defined as drainage of pulmonary veins into systemic venous side at multiple levels) is rare, accounting for less than 5% of patients with total anomalous pulmonary venous connection. ^{Reference Siddharth, Yadav, Bhoje and Hote4} The embryological basis of mixed drainage can be possibly explained by an insult at earlier period in gestation, resulting in persistence of aberrant connections at more than one level. ^{Reference Krishna, Gnanappa, Mervis, Forsey, Ayer and Orr5}

An extremely rare variant of total anomalous pulmonary venous connection, which is not separately classified in widely used “Darling” system, is “double drainage” type (being currently classified as a variant of mixed total anomalous pulmonary venous connection). In this type, which is extremely rare, all four pulmonary veins are traditionally described to drain into a common confluence, which further drains into two or more sites in systemic venous side. Dual drainage could be explained embryologically on the basis of initial collateral communications with the developing superior cardinal veins on both sides of the fetus. Careful observation of prior reported cases in literature shows that the vascular channel described in these patients as a common confluence is not a true single tubular channel in the sagittal body axis, which could have developed from an embryological confluence. ^{Reference Siddharth, Yadav, Bhoje and Hote4–Reference Lee, Wang, Wu, Chu and Lue7} Our patient had double drainage of a single vascular channel in sagittal body axis, draining all four pulmonary veins (a pattern most fitting to the embryologically described common pulmonary vein) into the left superior caval vein and coronary sinus.

The most common sites of double drainage reported in literature are coronary sinus and superior caval vein through left innominate vein via left vertical vein. ^{Reference Tanabe, Nakasato, Suzuki, Ishikawa and Hayasaka8} Drainage has also been reported to other sites like left brachiocephalic vein and right superior caval vein; portal vein and right superior caval vein and via two orifices into left superior caval vein. ^{Reference Siddharth, Yadav, Bhoje and Hote4,Reference Lee, Wang, Wu, Chu and Lue7,Reference Nagao, Tominaga, Kamei and Tanaka9} Our patient also had a co-existent significant right superior caval vein stenosis close to its entry into the right atrium, thereby leading to obstruction to drainage at supra-cardiac level.

Despite its rare occurrence, it is vital to preoperatively diagnose dual drainage to prevent post-operative persistent left to right shunt and cyanosis after successful surgical repair of one of the sites of total anomalous pulmonary venous connection. Echocardiography which is the initial imaging investigation may fail to identify double drainage of pulmonary veins. Cardiac CT angiography helps in comprehensive evaluation of pulmonary venous drainage including demonstration of length of the confluent channel and presence of morphologic obstruction. ^{Reference Siddharth, Yadav, Bhoje and Hote4} Surgical repair aims to establish wide and non-restrictive connection between all pulmonary veins and the left atrium. Variability in drainage sites in mixed type of total anomalous pulmonary venous connection requires an individualised approach for management based on sites of drainage. This approach is however different from surgical repair in “double drainage” types and in patients having pulmonary venous confluence. In most of the cases in literature with a similar pattern of dual drainage, the surgical approach included anastomosis of confluence of pulmonary veins to the left atrium with ligation of communication with coronary sinus and left vertical vein. ^{Reference Siddharth, Yadav, Bhoje and Hote4,Reference Tanabe, Nakasato, Suzuki, Ishikawa and Hayasaka8} Coil embolisation of the vertical vein can also be performed after primary surgical repair.