|ECR 2018 / C-2260||
|All roads lead to R..ight atrium: general review of congenital anomalies of the systemic venous return|
Typically the venous return from the upper half of the body it’s done by the right-sided superior vena cava (SVC) formed by the confluence of the brachiocephalic veins.
The blood from the lower half of the body drains typically through the right-sided inferior vena cava (IVC), resulting from the confuence of the common iliac veins.
The azygos-hemiazygos venous system, running up the side of the vertebral column, may connect both systems and provide an alternative path to the right atrium when either of the venae cavae is interrupted.
Anomalies of the venous return are the result of complex variations in the persistence and regression of segments of the primitive venous network which in the first 2 months of fetal development consists of a dorsal systemic (formed by the anterior and posterior cardinal veins which reach the sinus venosus through the common cardinal veins, whose carries all the intraembryonic blood) and a double nutritional network (the vitelline, which carries the extraembryonic blood from the yolk sac and the umbilicoallantoic system carrying oxygenated blood from the placenta). These are initially paired and symmetric, but by a series of cross or transverse anastomoses, are converted into single major trunks in the right half of the embryo, whereas the left-sided vessels diminish in size and are largely obliterated.
They are usually asymptomatic, and such the major clinical significance of their recognition is to prevent misdiagnosis as adenopathy and in a preoperative setting.
Anomalous drainage to the left atrium is associated with many types of congenital heart disease but is rare if the heart is normal.
SUPERIOR VENA CAVA (SVC)
The SVC is a large valveless venous channel formed by the confluence of the brachiocephalic veins at the level of the 1st costal cartilage.
It receives blood from the upper half of the body (except the heart) and returns it to the right atrium at the level of the right 3rd costal cartilage.
The SVC has a relatively simple development compared to the inferior vena cava, and develops at a later time.
The venous plexuses of the upper limb fuse to form the subclavian vein, which end up to open into the anterior cardinal veins.
By week 8, a large anterior cardinal veins anastomosis (derived from the thymic and thyroid veins) channels the blood from the left anterior cardinal vein toward the right, giving rise to the future left brachiocephalic venous trunk.
The left anterior cardinal vein and left common cardinal vein regresses, persisting only a short segment which forms the left superior intercostal vein, and the coronary sinus, respectively.
INFERIOR VENA CAVA (IVC)
The IVC is formed by the confluence of the two common iliac veins at the L5 vertebral level.
It drains venous blood from the lower trunk, abdomen, pelvis and lower limbs to the right atrium of the heart, crossing the diaphragm at the caval hiatus at the T8 level, having a short intra-thoracic course.
It has a retroperitoneal course within the abdominal cavity, running along the right side of the vertebral column with the aorta lying laterally on the left.
Due to not being a midline structure, there is a degree of asymmetry of drainage, e.g. the gonadal and suprarenal veins drain into the IVC on the right side, but into the left renal vein on the left.
Normal IVC has a complex embryological development with many embryological veins contributing to different parts, in order of appearance, the posterior cardinal, the subcardinal, and the supracardinal veins, each predominating temporarily, then regressing, and remaining only partly in the final definitive system. It develops between the 6th and 8thweeks of embryonic life, and the definitive inferior vena cava is uch composed of diverse segments each resulting from different embryological veins (from cranial to caudal):
AZIGOS-HEMIAZIGOS VENOUS SYSTEM
The azygos-hemiazygos system is a paired paravertebral venous pathway in the posterior thorax with a H-shaped configuration, comprised of the azygos, hemiazygos, accessory hemiazygos veins and superior intercostal veins. It is responsible for draining the thoracic wall and upper lumbar region via the lumbar veins and posterior intercostal veins.
The azygos vein is a unilateral vessel that arises at the union of the right ascending lumbar vein and the right subcostal vein around T12-L2, enters the thorax via the aortic hiatus in the diaphragm and ascends along the anterolateral surface of the thoracic vertebrae, arching over the right main bronchus at T5-T6 and enters the SVC at T4. This arch of the azygos vein is an important anatomical landmark, and contains a valve halfway, which may led to accumulation of the intravenously injected contrast agent.
Although there is much variability, the azygos vein receives the accessory hemiazygos vein and the hemiazygos vein at the level of T8 and T9 respectively.
The hemiazygos vein is a similar structure on the opposite side of the vertebral column and similar to the azygos vein is formed by the confluence of the left ascending lumbar and left subcostal veins. It enters the thorax either through the aortic hiatus or directly through the diaphragmatic crura, ascends along the left anterolateral aspect of the thoracic vertebrae and at T8–T9 crosses dorsal to the descending thoracic aorta to join the azygos vein.
The accessory hemiazygos vein which drains the superior left hemithorax and left bronchial vein, arises from the 4-8thleft posterior intercostal veins, lies longitudinally on the left side of the vertebral bodies and joins the azygos vein behind the oesophagus at different levels, usually at the level of T8.
The right and left superior intercostal veins drains the left 2nd-4thintercostal spaces and join respectively the azygos vein and the left brachiocephalic vein.
The azygos venous system arises from the supracardinal veins embryologically: the azygos vein is considered to derive from the upper right supracardinal vein, the hemiazygos vein from the upper left supracardinal vein and the azygos arch from an upper segment of the right posterior cardinal vein.
In the abdominal cavity, the supracardinal veins become the infrarenal portion of the definitive inferior vena, as thus a comunication between both systems is possible.