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SUMMARY

A comprehensive study on the embryology of the thoracic venous system has been carried out, and its connection with the most frequent venous anomaly at this level, as well as the most significant elements for its diagnose. With the increasingly important use of diagnostic catheterization and corrective surgery in patients with congenital heart disease, the early detection and analysis of anomalies of the venous system during fetal life can significantly prevent morbimortality. This is why its study is imperative, especially for ultrasonographists, pediatricians, and cardio-genetic assessors interested in the evaluation of congenital heart diseases.

Key words:Persistent left superior vena cava. Fetal Echocardiography. Congenital Heart Diseases.

Rev Fed Arg Cardiol. 2012; 41(2): 84-88

INTRODUCTION
The development and variations of the normal venous system in human beings have been the object of study for many years [1]. Its anomalies may occur in isolation, often with little hemodynamic significance, or be associated to congenital heart diseases [2].

With the increase of the usefulness of therapeutic diagnostic catheterization and corrective surgery in patients with congenital heart diseases, the early identification and analysis of the anomalies of the venous system during the fetal life may significantly prevent morbidity. Echocardiography has been an essential tool in the diagnosis and management of these patients.[3]

Embryonic development
During the fifth week of embryonic life, the embryonic venous system is constituted by three pairs of veins that drain on each right and left horn of the venous sinus respectively, namely: the vitelline or omphalomesenteric vein, umbilical vein, and common cardinal vein [4]. The latter is made up, in turn, by the anterior and posterior cardinal veins, which drain the cranial and caudal areas, respectively, of the embryonic body [5]. (Figure 1A and B)

Figure 1: Outline of the development of systemic veins. Pay attention to the greater development that the right anterior cardinal will acquire (future superior vena cava) at the expense of its left counterpart by blood derivation that occurs through the innominate vein (A,B). Thus, the anterior cardinal vein or left superior cava will decrease its caliber until virtually disappearing (B,C). The absence of the innominate vein will originate a persistent left cava that in most cases will drain into the coronary sinus (D). Modified from Somoza et al [5].

During the eighth week of embryonic life, the left anterior cardinal vein undergoes a process of atresia, disappearing, after the innominate vein develops an anastomosis with the right anterior cardinal vein. The remaining part is the ligament of Marshall [6], a small vein located in the posterior wall of the left atrium. Therefore, only the right cardinal vein persists. The cephalic portion of this right cardinal vein finally originates the cephalic vessels, such as the internal jugular vein; the caudal portion of it originates the right superior vena cava (RSVC) that will receive all the blood of the upper half of the body, and which also receives the azygos vein and several small veins of the pericardium and other mediastinum structures.

The left horn of the venous sinus loses value, with the obliteration of the vitelline and left common cardinal veins. The coronary sinus remains as a remnant of this venous sinus [5].

There are four anomalies in the superior vena cava [3]:

1. Persistent left superior vena cava (PLSVC).
2. Levoatrial cardinal vein.
3. Right superior vena cava draining into the left atrium or in both atria.
4. Obstruction of the superior vena cava.

Persistent LSVC
The most common anomaly [7] of the venous system occurs in 0.3-0.5% of the general population [8], while its frequency in patients with congenital heart diseases is as high as 2-10% [9]. In the group of heterotaxy syndromes, with an incidence of 10/100,000 infants born alive and a frequency of 0.8% of congenital heart diseases, the anomalies of the venous system occur in more than 70% of the cases [10]. The PLSVCdrains into the right atrium through the coronary sinus in the great majority of cases (92%) and in the remaining 8% in the left atrium, whether directly by absence or unroofed coronary sinus (Raghib’ssyndrome) or through a left pulmonary vein (partial PLSVC) [3,11,12].

A failure in the atresia of the left anterior cardinal vein originates the PLSVC, consequently, there are two superior venae cavae [13]. The anastomosis that usually forms the left brachiocephalic vein may persist, be small or disappear. It has been reported that in a 65% of the cases, this innominate vein is absent [14]. (Figure 1C and D).

The dilatation of the coronary sinus as an essential element to suspect of PLSVC, is the most important electrocardiographic sign of this entity, in most of occasions, due to the presence of PLSVC draining inside of it. This increase in the size of the coronary sinus is observed as an echo-translucent image within the left atrium, in the echocardiographic long axis view (Figure 2B) and four chambers. In the latter, performing a more posterior plane, it can also be verified [15]. (Figure 2A) However, we should consider that the coronary sinus may be dilated because it receives other vessels, as one or more pulmonary veins [16].

Figure 2: Echocardiographic views of four chambers in a more posterior plane (A) and long axis of the left ventricle (B). In both cases, the arrow points to the dilated coronary sinus.

Several authors point out the significance of the view of the three vessels in the diagnosis of PLSVC, due to its easy identification [17], with the presence of a fourth vessel being observed at the left of the pulmonary artery. (Figure 3A)The transversal echocardiographic view of the aortic arch also is positive for the diagnosis of this malformation. (Figure 3B)

Another view that shows the presence of the anomalous left vein, is a longitudinal view, somehow in angle, where the entrance of the PLSVC is shown in the coronary sinus, in the shape of an intraluminal circular structure that reaches the lateral wall of the left atrium. (Figure 3C and D).

Figure 3: Echocardiographic view (A) of the three vessels. The arrow points to a fourth vessel that corresponds to the persistent left superior vena cava (B). Transversal view of the aortic arch where the left superior vena cava (LSVC) and the right superior vena cava (RSVC) at both sides of the aortic arch (indicated) are observed (C). Longitudinal echocardiographic view of the short axis of the aorta, with transducer angulations, where the PLSVC entry, in the shape of an arch, is shown in the coronary sinus (D). Color Doppler superimposed on the two-dimensional image, where the PLSVC is indicated, entering the coronary sinus (CS).

Less frequently, PLSVC presents with absence of the right superior vena cava. As we already mentioned, in the embryo, the system of the cardinal veins is responsible for the superior venae cavae. As the embryo grows, the left anterior cardinal vein will lead its blood to the right anterior cardinal vein, through the innominate vein, with this situation causing an increasingly significant development of the future right superior vena cava, at the expense of the left anterior cardinal vein. [5,17,18] The presence of a PLSVC could be attributed to the persistence of the proximal part of the left anterior cardinal vein. If the flow through the innominate vain is reverse, the right cephalic region drains into the left anterior cardinal vein, causing atretic or absent RSVC.

In these cases, fetal echocardiography by the view of the three vessels shows the unusual arrangement of the vessels where the first located at the left corresponds to the PLSVC, and next the pulmonary artery and the aorta (Figure 4A). When performing a higher echocardiographic view, we would observe the innominate vein going within the LSVC, which is responsible of gathering the blood of the right side of the neck and the head (Figure 4B and C). In the echocardiographic view of both cavae, it is not possible to visualize the right superior cava (Figure 4D).

Figure 4: (A) View of the three vessels. The unusual arrangement of the vessels is observed, where the first, located at the left, corresponds to the persistent left superior vena cava (PLSVC), next the pulmonary artery (PA), and the aorta (Ao). (B) Higher echocardiographic slice, with the innominate vein is shown entering the left superior vena cava (LSVC), where when superimposing the encoded Doppler to color, in the two-dimensional image, the blood in the right side of the neck and the head toward the PLSVC is shown in blue (C). In the echocardiographic view of both cavae (D), it is not possible to see the right superior vena cava. IVC: inferior vena cava.

The PLSVC may drain directly into the left atrium, crossing through the left pulmonary artery, between the left superior pulmonary artery and the left appendix. As this occurs as a rare defect, some have suggested that this type of connection of the PLSVC is the result of a failure in the arrangement between the left horn of the venous sinus and the left atrium (with the subsequent failure to develop or absence of the coronary sinus) and persistence of the left anterior cardinal vein (Raghib’s syndrome) [12]. Other authors, however, suggest the defect is the result of an extensive unroofed coronary sinus.

Levoatrial cardinal vein
The levoatrial cardinal vein is an embryological connection between the capillary plexus of the embryonary anterior intestine (very related to the pulmonary veins, which originate normally, in the primitive pulmonary buds usually located next to the periesophageal plexi, embryologically connected in turn, to the anterior intestine) and the cardinal venous system, which persists when atrial pressures are high, during the establishment of the obstruction of the left ventricular outflow in the early stages of the pregnancy, thus allowing the drainage from the left atrium into the innominate vein or the right superior vena cava [19-20].

The echocardiographic view that contributes the most to its diagnosis, is the transversal view of the three vessels, where the presence of a fourth vessel is evident at the left of the pulmonary one. When using color Doppler overlain on the two-dimensional image, a retrograde flow becomes evident, toward the superior vena cava or innominate vein. This characteristic is essential when making the differential diagnosis with the persistent left superior vena cava, since in both cases, a fourth vessel is seen at the left of the artery, but unlike the levoatrial cardinal vein, the flow is anterograde to the coronary sinus where it most oftenculminates. The levoatrial cardinal vein has its origin in the roof of the left atrium, heading behind the left pulmonary vein and to the front of the left bronchus, without causing compression or obstruction of the structures it goes through [21]. (Figure 5)

Figure 5: Anatomical piece, where the presence of the levoatrial cardinal vein (LV) is indicated. The vein originates in the roof of the left atrium, in a direction opposite to the pulmonary veins (PV)

Right superior vena cava draining directly into the left atrium or in both atria.
The direct connection from the RSVC with the left atrium is a rare anomaly attributed to the left and superior shift of the right horn of the venous sinus [22]. In the cases of atrial septal defects of the venous sinus (of the superior vena cava type), the same embryological event appears, and in the biatrial connection of the SVC, the distinctive trait is the venous sinus defect of the SVC type, so we can infer that these two lesions are variants of the same anomaly [3].

Other reports suggest that both variants are secondary to the unroofed right pulmonary veins above the RSVC joint with the right atrium, i.e. the usual division between the posterior portion of the RSVC and the anterior portion of the right pulmonary veins does not exist [23]. Hemodynamically, cyanosis is the result of the first variant, and right volumetric overload of the second. The relationship between the RSVC and both atria can be obtained in the longitudinal view of both venae cavae.

Obstruction of the superior vena cava
Extremely rare entity during the fetal life. Occasionally, it is the result of the compression by thrombi or tumors [24].

It may present in cases of aneurysms of the vein of Galen, which cause huge SVC by exaggerated venous return. In infants, they may occur in the post-operative period of corrective “switch” of transposition of the great vessels, in patients in whom correction of partial anomalous drainage of pulmonary veins have been carried out, and in the cases of Glenn operation.

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Publicación: Junio 2012