In this chapter, we deal with abnormalities of position and connection of the major systemic venous channels that drain to the heart, and also with abnormal persistence of the valves of the embryonic systemic venous sinus. Many of these anomalies are incidental findings, having little haemodynamic significance. They may, however, complicate interventional procedures, and they may be associated with other more important congenital cardiac anomalies. A consideration of the morphogenesis of the normal systemic venous system is helpful in understanding the wide variety of systemic venous anomalies that exist. We will review briefly the presumed embryology of the systemic venous system, therefore, before considering the individual anomalies and their clinical significance.
MORPHOGENESIS OF THE SYSTEMIC VENOUS SYSTEM
Normal venous development is a process of progressive appearance of a series of paired venous structures, development of anastomotic channels between them, and the eventual selective regression of certain segments. The systemic venous tributaries are initially bilaterally symmetrical. They join together on each side, opening to the developing heart tube through paired channels known as the sinus horns. When first seen joining the developing atrium ( Fig. 23-1 ), there are no anatomical landmarks that permit distinction of a separate segment as a sinus venosus, such as exists in the hearts of lower vertebrates. 1 Initially, three major venous channels drain into each horn, namely the omphalomesenteric or vitelline veins medially, the umbilical veins in the middle, and the common cardinal veins laterally ( Fig. 23-2 ). Each common cardinal vein is formed by the union of the superior and inferior cardinal veins, which drain the cephalic and caudal regions of the embryo, respectively. Concomitant with development of anastomotic channels between the right- and left-sided venous channels formed throughout the embryo, there is preferential flow to the right-sided structures. The left horn then rotates beneath the atrioventricular junction, moving around the fulcrum provided by the developing pulmonary vein, so as to open in the right side of the developing atrial component of the heart tube ( Fig. 23-3 ). It is only at this stage, which is also the period of initial formation of the primary atrial septum, that it becomes possible to recognise the valvar structures marking the junction between the systemic venous tributaries and the developing morphologically right atrial chamber. These structures serve as the valves of the systemic venous sinus, and are often described simply as the venous valves (see Fig. 23-3 ). There then follows gradual regression of the left-sided vitelline and cardinal veins, with increasing prominence of the right sinus horn. Eventually, the entirety of the systemic venous sinus within the confines of the valves becomes incorporated as the smooth-walled venous component of the morphologically right atrium, with the opening of the right common cardinal vein becoming the orifice of the superior caval vein, and that of the right vitelline vein becoming the entrance of the inferior caval vein. The mouth of the left sinus horn then persists as the orifice of the coronary sinus ( Fig. 23-4 ).
The initial anastomotic connection between the right and left cranial cardinal veins enlarges, and becomes the left brachiocephalic vein, often illogically termed the innominate vein. What is the purpose of having an unnamed vein? In the process of normal development, the left superior cardinal vein below this point gradually regresses, with its remnant forming the ligament and oblique vein of Marshall ( Fig. 23-5 ). With ongoing development, there are further changes in the valves of the systemic venous sinus. The right-sided valve is prominent during embryonic life, while the left valve is less prominent, eventually fusing with the atrial septum. The right valve subsequently also regresses for its most part, except for portions which persist as folds related to the orifices of the inferior caval vein, the Eustachian valve, and the coronary sinus, the Thebesian valve.
The development of the pulmonary venous system is also of significance, considering the occasional co-existence of anomalies of pulmonary and systemic venous drainage. The developing lungs are derived from the primitive foregut. They share their early vascular supply with the gut via a splanchic plexus. 2 This plexus initially drains the intraparenchymal pulmonary tissues to the cardinal venous system. With normal development, these connections are lost as the pulmonary vein itself canalises in the dorsal mesocardium and gains entrance to the developing morphologically left atrium. 3
Initially, therefore, there are three systemic venous systems developing within the embryo, namely the vitelline, umbilical, and cardinal systems (see Fig. 23-2 ). The vitelline system largely becomes a vascular network draining the liver. These channels are predominant on the right side of the embryo, maintaining their connection to the right side of the systemic venous sinus through structures known as the hepatocardiac channels. As we have seen, it is the right-sided vitelline vein that ultimately becomes the terminal post-hepatic portion of the inferior caval vein (see Fig. 23-4 ).The umbilical veins are also paired structures when first seen. They lose their connection to the venous sinus, and anastomose to the hepatic sinusoids. The right-sided umbilical vein then regresses, while the left umbilical vein enlarges to become the only channel bringing blood from the placenta to the embryo, forming an anastomosis with the hepatocardiac channel which becomes the venous duct.
Further important changes occur within the developing abdominal region of the embryo that underscore normal and anomalous formation of the inferior caval vein. By the fifth week of gestation, paired subcardinal veins arise medial to the inferior cardinal veins, serving initially to drain the forming urogenital sinuses. When traced cranially, these subcardinal veins empty into the caudal cardinal veins on both sides ( Fig. 23-6 ). Anastomotic channels then develop between the two subcardinal veins, as well as between the right subcardinal and hepatocardiac channels. It is the right subcardinal vein that becomes predominant, developing into the infrahepatic segment of the inferior caval vein, and joining this segment to the right vitelline vein, which forms the entrance to the morphologically right atrium. With continued growth of the trunk of the embryo, yet another system of veins, the sacrocardinal veins, arises dorsal and caudal to the inferior cardinal system. Anastomotic channels between the subcardinal and sacrocardinal systems on the right side will form the renal segment of the inferior caval vein, while the distal portions of the right sacrocardinal vein enlarge and become its most inferior portion ( Fig. 23-7 ).
Concomitant with these developments of the paracardinal systems, and apart from their most proximal portions, the inferior cardinal veins themselves gradually involute. This involution occurs concomitant with the transformation of the supracardinal venous system into the azygos and hemiazygos veins on the right and left sides, respectively, which terminate cranially in the right cardinal venous system ( Fig. 23-8 ). The definitive inferior caval vein, therefore, has multiple embryological origins. Its terminal suprahepatic portion is derived from the right vitelline vein. The infrahepatic segment arises from the right subcardinal vein, the intermediate segment from the anastomotic channels initially developed between the subcardinal and sacrocardinal veins, and the most inferior segment from the right sacrocardinal vein. The azygos and hemiazygos veins are derived from the supracardinal system ( Fig. 23-9 ). Should there be abnormal development of these multiple abdominal venous channels, it is possible for the supracardinal venous system to become dominant. This then results in the venous return from the lower abdomen returning through the azygos system of veins to either the right- or left-sided superior caval vein. The inferior caval vein is then itself interrupted between the abdominal and hepatic segments, the hepatic segment draining only the blood from the liver and the portal venous system back to the heart.
The venous development discussed thus far has presumed the development of anastomotic channels and connection of the systemic venous sinus to the right-sided atrium. It is this process that produces the morphologically right atrium, which contains the entirety of the embryonic systemic venous sinus. The left side of the initial atrial component of the heart tube, together with the developing pulmonary veins, becomes the morphologically left atrium. Concomitant with these changes, the appendages balloon from each side of the atrial component of the heart tube. The appendage growing to the right assumes its typically triangular shape, incorporating the cephalic part of the valve of the systemic venous sinus to form the terminal crest and the spurious septum. The appendage growing from the pulmonary side is much narrower. It has no relation with the valves of the systemic venous sinus, and therefore lacks a terminal crest. All of this occurs concomitant with development of anastomoses from the left to the right side. Should the anastomoses develop in right-sided to left-sided fashion, then the left-sided atrium will incorporate the systemic venous sinus and become the morphologically right atrium, while the right-sided atrium becomes the morphologically left atrium. This will produce a mirror-imaged atrial arrangement.
In certain circumstances, however, the process of lateralisation does not occur, and the systemic venous tributaries retain their bilateral symmetry. This lack of lateralisation occurs in two forms. In the one, the valves of the systemic venous sinus are incorporated bilaterally, along with bilateral right atrial appendages. This process squeezes out the anlagen of the primary pulmonary vein so that there is either no atrial site, or else a restricted anomalous site, for pulmonary venous drainage. Such right isomerism is characterised by presence bilaterally of appendages with right morphology, bilateral sinus horns together with bilateral systemic venous drainage, as well as anomalous pulmonary venous drainage. In the other pattern, there is bilateral growth of morphologically left structures. Consequently, the pulmonary venous component predominates in relation to the systemic venous tributaries. The major systemic venous channels then make anomalous connections to the atrial chambers, which have morphologically left appendages bilaterally. The common cardinal veins are more successful in this respect, tending to enter the atrial roof on each side in absence of a terminal crest. The hepatic veins also enter the atrium directly, often bilaterally. The venous return from the lower body, in contrast, usually continues through the supracardinal azygos system, draining to one or other of the superior cardinal veins to produce azygos continuation of the inferior caval vein. In those cases with isomeric left appendages in which the inferior caval vein does achieve a direct atrial connection, it tends to do so separately from the hepatic veins. Because of these developmental associations, anomalous systemic and pulmonary venous connections are part and parcel of the syndromes of isomerism and visceral heterotaxy (see Chapter 22 ).
CLASSIFICATION OF SYSTEMIC VENOUS ANOMALIES
Even though the account of development given above provides the background to understand their morphology, an organised classification of the multiple anomalous systemic venous connections can be most easily achieved by using anatomical subdivisions. Thus, we can consider aberrations of development in the superior and inferior caval veins and the coronary sinus, as well as totally anomalous systemic venous connection, and abnormal persistence of the embryonic valves of the systemic venous sinus. Prevalence, pathophysiology, clinical and laboratory findings, and management will be discussed where appropriate for each anomaly.
Anomalies of the Superior Caval Vein
A left-sided superior caval vein draining to the coronary sinus is the most common systemic venous anomaly. The venous channel, which follows the course of the embryonic left cranial cardinal vein (see Fig. 23-5 ), enters the pericardial cavity to the left of the left upper pulmonary veins, and runs posterior to the dome of the left atrium, having the mouth of the left atrial appendage to its left side. The lesion has been noted in almost one in every 200 postmortems in the general population, 4 but occurs in up to one-twentieth of patients with congenitally malformed hearts. 5 The left-sided venous channel usually co-exists with a right-sided superior caval vein. A brachiocephalic vein connects these two structures in three-fifths of patients, with its size varying inversely with that of the left vein ( Fig. 23-10 ). When present, the left vein usually is positioned anteriorly relative to both the aortic arch and the left pulmonary artery, but can pass posterior to these structures ( Fig. 23-11 ). (See also Fig. 23-12 .)
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