Developmental anomalies of the superior vena cava and its tributaries: What the radiologist needs to know?

Abstract

Deviations from the normal process of embryogenesis can result in various developmental anomalies of the superior vena cava (SVC). While these anomalies are often asymptomatic, they assume clinical importance during interventions such as central venous catheterisations and pacemaker implantations and during cardiothoracic surgeries while instituting cardiopulmonary bypass and for creation of cavo-pulmonary connections. Role of imaging in identifying these anomalies is indispensable. Cross-sectional imaging techniques like CT venography and magnetic resonance (MR) venography allow direct visualisation and consequently increased detection of anomalies. CT venography plays an important role in detection of SVC anomalies as it is readily available, has excellent spatial resolution, short acquisition times and potential for reconstruction of images in multiple planes. This pictorial review focuses on the developmental anomalies of the SVC and its tributaries highlighting their embryological basis, imaging appearances on CT venography and potential clinical implications, where relevant.

Introduction

The superior vena cava (SVC) is a valveless venous trunk, formed by the confluence of right and left brachiocephalic veins (BCV) on the right side of chest, which drains the upper part of the body into the right atrium (RA). ¹ A variety of developmental anomalies can occur with respect to the SVC secondary to deviations from the normal process of embryogenesis. It is imperative to be aware of these anomalies because of their clinical implications and to prevent potential complications during invasive procedures such as central venous catheterisations and pacemaker implantations, especially if performed via the left arm approach. ² Delineating the course of SVC is also crucial prior to instituting cardiopulmonary bypass and creation of cavo-pulmonary connections. ² Anomalies of SVC are usually detected incidentally upon imaging; however, may be seen associated with other cardiac anomalies. ³

Role of imaging

Role of imaging in identifying these anomalies is indispensable. On a chest radiograph, the SVC forms the upper part of the right heart border and abnormal leftward location of a central venous catheter on a post-procedure radiograph may indicate an SVC anomaly. Echocardiography has a limited role for characterising SVC anomalies due to poor acoustic window because of the overlying bones and the air-filled lungs. ³

Cross-sectional imaging techniques like CT venography and magnetic resonance (MR) venography provide direct visualisation of SVC and have resulted in increased detection of anomalies. CT venography acquired at 60–75 s after i.v. injection of iodinated contrast medium through a peripheral venous access provides optimum enhancement of SVC. CT provides excellent spatial resolution with short acquisition time and can generate reconstructed images in multiple planes as well as volume rendered images for better depiction. ³ Evaluation of SVC can also be done using MR angiography. ³ Although, it is devoid of radiation-related risks, its widespread use is precluded due to longer scan times, limited availability and frequent need for deep sedation, especially in paediatric patients with cardiac anomalies.

Relevant embryology

Normal development

Development of sino-atrial chamber

During embryogenesis, the sino-atrial chamber differentiates into the sinus venosus and the primitive atrium. With development of the interatrial septum, the sinus venosus communicates only with the RA. The lateral ends of sinus venosus are termed as horns. ⁴ The sinus venosus subsequently gets absorbed into the RA.

Formation of the primitive cardinal system

During the fourth week, there is formation of three primitive venous systems: umbilical system draining the placenta, vitelline system draining the gut and the cardinal veins draining the rest of embryo (Figure 1). ⁵ The cardinal veins are classified into anterior and posterior and are symmetrical (right and left). Bilateral anterior and posterior cardinal veins drain the cranial (head and upper limb buds) and caudal aspect of the developing embryo, respectively. They join to form the right and left common cardinal veins (ducts of Cuvier) respectively, which further drain into the ipsilateral horns of sinus venosus and then into the developing heart. (Figures 1 and 2). ⁵

Figure 1.

Schematic illustration showing the primitive venous drainage of the embryo at fourth week. (SV, Sinus venosus).

Figure 2.

Schematic illustrations showing (a) the right and left anterior and posterior cardinal veins joining to form the right and left common cardinal veins, respectively, which is seen draining into the ipsilateral horn of the sinus venosus. (b) The right anterior cardinal vein and the right cardinal vein form the superior vena cava (SVC) while the left anterior cardinal vein regresses to form the ligament of Marshall. (c) Persistence of the left anterior cardinal vein results in a persistent left SVC (PLSVC). (CS, Coronary sinus; IVC, Inferior vena cava; LBCV, left brachiocephalic vein; RBCV, right brachiocephalic vein).

Development of anterior cardinal anastomosis (superior and inferior transverse capillary plexus)

The development of SVC is characterised by formation of anastomosis between right and left side, so that blood from left side can be shunted to right. At eighth week of gestation, anastomosis derived from thyroid and thymic veins connects both the anterior cardinal veins cranial and caudal to the aortic arch (superior and inferior transverse capillary plexus, respectively) thus directing the venous drainage from left head and neck to the right anterior cardinal vein. ⁶ Subsequently, there is shortening of aortic arch that occupies the position of inferior plexus causing its involution. The blood is shunted to the superior plexus and the bridging communication between the anterior cardinal veins result in the development of normal supra aortic course of left BCV. ⁶

Formation of superior vena cava

There is obliteration of caudal left anterior cardinal vein below the level of anastomosis between the two anterior cardinal veins, forming the ligament of Marshall. ² This allows blood from left side of head and neck to drain into SVC via BCV. A part of left anterior cardinal vein forms left superior intercostal vein and adjoining left BCV. ³ The left sinus venosus horn and the left common cardinal vein forms the coronary sinus (CS) while the proximal right anterior and right common cardinal vein along with the right sinus venosus horn form normally positioned SVC on right side (Figure 2). ²

Anomalous development

Anomalies of superior vena cava

The left anterior cardinal vein and the adjacent part of common cardinal vein normally regresses; failure of which results in formation of persistent left SVC (PLSVC). PLSVC commonly drains into the RA via CS (Figures 2 and 3). An isolated PLSVC is formed due to regression of the right-sided anterior cardinal vein and persistence of left-sided anterior cardinal vein. ⁷

Figure 3.

Schematic illustrations showing (a) Normal development of the superior vena cava (SVC), (b) Development of persistent left SVC (PLSVC), and (c) Development of retroesophageal course of the left brachiocephalic vein. (AV, Azygous vein; CS, Coronary sinus; IVC, Inferior vena cava; LSIV, Left superior intercostal vein; LBCV, Left brachiocephalic vein; RA, right atrium; RBCV, Right brachiocephalic vein).

Anomalies of tributaries of superior vena cava

Failure of arch shortening as seen in right-sided aortic arch can cause compression and regression of the superior transverse capillary plexus. Also, pulmonary developmental anomalies including pulmonary stenosis and pulmonary atresia can lead to persistence of inferior capillary plexus. Persistence of the inferior plexus with failure of development of the superior plexus results in an anomalous retroaortic course of the left BCV. ⁶

The posterior cardinal veins form the azygous venous system on the right and hemiazygous-accessory hemiazygous venous system on the left. ⁸ The accessory hemiazygous vein and the hemiazygous vein drain into the azygous vein, coursing posterior to the aorta. The regression of both superior and inferior transverse capillary plexus can result in opening of the channel between left superior intercostal vein and accessory hemiazygous vein leading to a retroesophageal course of the left BCV (which drains via azygous vein into SVC) (Figure 3). ⁸

Developmental anomalies of superior vena cava

Anomalies of superior vena cava proper

Persistent left superior vena cava

PLSVC is seen in approximately 0.5–2% of the general population; however, in individuals with congenital heart disease, the prevalence of PLSVC is up to 10%. ⁹ There is presence of co-existent right-sided SVC in majority of the cases (Figure 4). ¹⁰ Its presence can be suspected on echocardiography upon visualising a dilated CS after ruling out other causes. Cross-sectional imaging can depict its course lateral to the aortic arch where it can mimic the imaging appearance of a vertical vein in cases of partial anomalous pulmonary venous connection (PAPVC) from the left upper lobe. PLSVC drains into RA via the CS in 90% cases (Figure 5). ¹⁰ Other alternate drainage sites include LA, and rarely the inferior vena cava (IVC) and hepatic veins. There are increased chances of associated congenital heart disease in cases of left atrial drainage or with associated absence of right-sided SVC. ¹⁰

Figure 4.

Axial (a), coronal (b) and volume rendered (c) images show presence of bilateral superior vena cava without any bridging vein between the two SVC. (1, Brachiocephalic artery; 2, Left common carotid artery; 3, Left subclavian artery; LSVC, Left-sided superior vena cava; RSVC, Right-sided superior vena cava).

Figure 5.

Volume rendered (a) and axial (b and c) images show presence of bilateral superior vena cava with a thin bridging vein (arrowheads) between the two SVC. (LSVC, Left-sided superior vena cava; RSVC, Right-sided superior vena cava).

Although PLSVC is frequently hemodynamically insignificant, it may pose several clinical problems making its detection imperative. Presence of PLSVC may cause difficulties during transvenous procedures performed via the left internal jugular vein or left subclavian vein such as central venous catheterisations, implantable cardioverter-defibrillator (ICD) or pacemaker implantations and biventricular pacing. ¹¹ Inadvertent introduction of the catheter and guidewire into the PLSVC and the CS, and subsequent manipulation can cause irritation of the CS which can result in arrhythmias, angina, shock or even cardiac arrest. ¹¹ Presence of PLSVC can cause anatomical abnormalities in the conduction system (sinoatrial and atrioventricular) leading to abnormalities in cardiac rhythm causing atrial fibrillation and even sudden cardiac death. ¹² PLSVC can also result in inadequate myocardial perfusion during retrograde cardioplegia. ¹³

Presence or absence of intercommunicating vein

In presence of bilateral SVC, an intercommunicating bridging vein is seen in about 30% cases, although it may be small (Figure 5). ¹⁰ When an intercommunicating vein is present, PLSVC is usually smaller in calibre as compared to the right-sided SVC. In the setting of a dilated intercommunicating vein and a small PLSVC, diagnosis may often be missed on echocardiography as the CS is not dilated (Figure 6). The presence and size of the intercommunicating vein during cavo-pulmonary anastomosis is imperative, as in the absence of the intercommunicating vein, a bilateral Glenn shunt surgery needs to be performed (Figure 7). ²

Figure 6.

Volume rendered (a) and coronal (b) images show presence of a thin persistent left superior vena cava (LSVC) with a good-sized bridging vein (asterisks) between the two SVC. (RSVC, right-sided superior vena cava).

Figure 7.

Oblique coronal image shows presence of bilateral cavo-pulmonary anastomosis. (LPA, Left pulmonary artery; LSVC, Left-sided SVC; RPA, Right pulmonary artery; RSVC, Right-sided superior vena cava).

Drainage of PLSVC to left atrium

PLSVC with defective separation of CS from LA is termed as Raghib syndrome. ⁵ It occurs due to deficiency of the anterior-superior wall of CS leading to its incorporation into LA. ⁵ This condition is usually asymptomatic with incidental detection at imaging; however, it may lead to cyanotic and thromboembolic complications requiring surgical correction. ⁵ In the setting of a co-existent right-sided SVC and adequate-sized bridging vein, ligation of PLSVC can be performed.

Isolated PLSVC

Persistent LSVC with absent right-sided SVC in the setting of viscero-atrial situs solitus is termed as isolated PLSVC (Figure 8). It is exceedingly rare, seen in 0.09–0.13% of patients with congenital heart diseases. ¹⁴ Due to abnormal angulation of CS into RA, there can be difficulties during pacemaker implantation or pulmonary artery catheterisations. PLSVC, especially the isolated form, is associated with other congenital abnormalities of the heart and great vessels such as atrial septal defects, left isomerism, CS ostial atresia and cor triatriatum. ⁹

Figure 8.

Volume-rendered (a and b) images show presence of an isolated left superior vena cava (LSVC) draining into the right atrium via coronary sinus (CS) in the setting of viscero-atrial situs solitus. (RBCV, Right brachiocephalic vein).

Superior vena cava aneurysm

Aneurysmal dilatation of SVC is a rare venous anomaly often detected incidentally and usually asymptomatic. SVC aneurysms can be congenital or caused by inflammation or degeneration of the wall but in most cases the cause is not known. ¹⁵ Post-traumatic and post-surgical causes have also been implicated. They can be saccular or fusiform. The fusiform variety usually require conservative treatment. However in cases of saccular aneurysm, anticoagulation or surgical excision is preferred due to associated complications including thrombosis and pulmonary embolism. ¹⁶

Drainage of SVC into left atrium

Biatrial or left atrial drainage of SVC is extremely rare. ¹⁷ Posterior wall of the SVC is adjoining the anterior wall of the right upper lobe pulmonary vein. ¹⁷ A defect in this common wall causes shunting of blood from the SVC into the unroofed pulmonary vein and ultimately into the LA. Along with the normal drainage of SVC into the RA, this alternate pathway results in a haemodynamic biatrial drainage of SVC. In the setting of preferential drainage of SVC into the LA, the connection with RA may become stenotic/atretic, and subsequently lead to exclusive left atrial drainage of the SVC. ¹⁷ Most complications such as cyanosis and thromboembolism arise due to right-to-left shunting and requires surgical correction. ¹⁷

Low right atrial insertion of superior vena cava

Anomalous low insertion of SVC into the RA is extremely rare. In reported cases, SVC is seen taking an extra mediastinal course, passing through the right lung, and draining into the lateral aspect of the RA. This variation occurs due to developmental failure of the sino-atrial fold with incomplete differentiation of sinus venosus and its tributaries. ¹⁸

Anomalies of tributaries of superior vena cava

Anomalous course of left BCV

Retroaortic course of left BCV

Normally, the BCV courses obliquely downwards, antero-superior to the aortic arch and its branches. Its anomalous course has been reported in 0.2–1.7% of all congenital heart diseases and in 0.02% of surgical patients without congenital heart disease. ¹⁹ Retroaortic BCV courses inferior to the aortic arch, traversing between the ascending aorta and trachea and joins the SVC caudal to the drainage of azygous vein (Figure 9). The commonest association is with tetralogy of Fallot, especially with a right aortic arch; other associations being right atrial isomerism and septal defects. Right ventricular outflow obstruction in the form of pulmonary atresia or stenosis is also significantly associated with retroaortic course of BCV. ²⁰ On echocardiography, the descending portion of retroaortic BCV can be misinterpreted as PLSVC and the retroaortic portion can be mistaken for the right pulmonary artery in cases of pulmonary atresia. ⁶ Although often clinically insignificant, identification of this anomaly is imperative prior to surgeries and transvenous cannulations. There can be difficulties during central venous catheterisation and pacemaker implantation, when done via a left arm approach. During cardiac bypass, SVC cannulations should be more caudal to avoid obstruction of this vein. ²¹ It can also complicate pulmonary artery exposure while performing Glenn shunt surgery and can obscure the surgical field during patent ductus arteriosus ligaton. ²¹

Figure 9.

Volume-rendered (a), axial (b) and sagittal (b) images show retroaortic course of the left brachiocephalic vein (asterisks). (RSVC, Right-sided superior vena cava).

Retroesophageal course of left BCV

Left BCV coursing posterior to trachea and oesophagus is rare with an incidence of ~0.19% in the setting of congenital heart diseases (Figure 10). ⁸ Various associations described in literature include ventricular septal defect with or without pulmonary atresia, patent ductus arteriosus and right aortic arch. ⁸ Demonstration of this variation is important especially during preoperative evaluation, as its manipulation can lead to inadvertent bleeding during posterior mediastinal surgeries. It also has clinical implications while planning cavo-pulmonary shunts. ⁵

Figure 10.

Volume-rendered (a and b) and axial (c) images show retroesophageal course of the left brachiocephalic vein (asterisks). (RSVC, Right-sided superior vena cava; arrowhead in (b) indicates azygous vein).

Partial or total anomalous drainage of pulmonary veins to SVC

In supracardiac type of PAPVC or total anomalous pulmonary venous connection (TAPVC), there is anomalous drainage of all or at least one pulmonary veins, respectively, into the SVC or its tributaries (Figure 11). It occurs due to failure of the common pulmonary vein to connect to the splanchnic plexus and persistence of the communication between splanchnic plexus and the cardinal venous system. ²² In supracardiac TAPVC, all the pulmonary veins form a common channel (vertical vein) that drains into the left BCV in most of cases. However, drainage can also occur into the right-sided SVC, PLSVC (Figure 12) or azygous vein. Obstruction usually occurs at the origin or drainage site of the vertical vein. ²³ The most common form of PAPVC is the drainage of right upper lobe pulmonary vein into the RA or SVC (Figure 13). It is most frequently associated with sinus venosus type of atrial septal defect. Left-sided pulmonary veins may anomalously drain into left BCV or CS. ²⁴

Figure 11.

Volume-rendered (a and b) images show left-sided pulmonary veins (white arrowheads) joining to form a vertical vein (asterisks) which drains into the left brachiocephalic vein (LBCV). The right-sided pulmonary veins (black arrowheads) drain into a dilated coronary sinus (CS) – suggestive of mixed type of total anomalous pulmonary venous connection. (RSVC, Right-sided superior vena cava).

Figure 12.

Volume-rendered (a–c) images show all pulmonary veins joining to form a vertical vein (asterisks) which drains into a dilated left superior vena cava (LSVC) – suggestive of supracardiac total anomalous pulmonary venous connection. (RSVC, Right-sided superior vena cava).

Figure 13.

Axial (a), coronal (b) and volume rendered (c) images show right superior pulmonary vein (asterisk) draining into the right-sided superior vena cava (RSVC) – suggestive of partial anomalous pulmonary venous connection. Oblique axial (d) image shows presence of superior caval type of sinus venosus atrial septal defect (indicated by star).

Azygous/hemiazygous continuation of inferior vena cava

In cases of intrahepatic interruption of IVC, the blood is deviated via the retrocrural azygous vein or through the hemiazygous vein. The intrahepatic IVC is absent or hypoplastic with blood draining via dilated azygous vein into the SVC. ²⁵ The drainage of left-sided IVC via the hemiazygous vein has many variations as follows: (i) Hemiazygous vein into the azygous vein and right-sided SVC, (ii) via accessory hemiazygous vein into PLSVC and CS, and (iii) via accessory hemiazygous into LSIV and then into left BCV (Figure 14). ¹

Figure 14.

Volume-rendered (a and b) and axial (c and d) images show hemiazygous continuation (asterisks) of the inferior vena cava into the left-sided superior vena cava (SVC). (DTA, Descending thoracic aorta).

Conclusion

A wide gamut of anomalies of SVC and its tributaries can be detected incidentally on imaging. It is imperative for radiologists, cardiologists, cardiothoracic surgeons and anaesthesiologists alike to be aware of such anatomical variations for treatment planning and to avoid potential complications. CT venography has an important diagnostic role in the detection of such anomalies providing excellent spatial resolution with multiplanar reconstructions and volume-rendered images. With the advent of recent generation CT scanners and use of advanced dose-reduction techniques, the requisite information can be obtained at low radiation doses.