Illustrated Encyclopedia of Human Anatomic Variation: Opus II: Cardiovascular System: Veins: Head, Neck, and Thorax

Azygos Vein

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The azygos vein receives the hemiazygos vein, on its left margin, as a side or double branch at about the eighth and ninth vertebra. Slightly higher, at the sixth or seventh thoracic vertebra, it receives the accessory hemiazygos vein as a single trunk in 72% of individuals. The hemiazygos and accessory hemiazygos veins are incompletely formed in 15% of individuals. When this occurs the posterior intercostal veins on the left side of the thorax may drain into the azygos vein independently. In this case the azygos vein lies on the midline.

In one clinically significant variation, the azygos curves over and grooves the right lung before entering the superior vena cava, just before it enters the heart. This is called the azygos lobe of the lung.

In one study of 200 bodies, the azygos vein showed variation in 26%. The cross connections of the hemiazygos to the azygos occurred with almost equal frequency at the eight or ninth vertebra. At least two cross connections were present in 100 %; three cross connections occurred in 4%, and four or more in 6%.

Rarely, the azygos vein passes through the aortic hiatus of the diaphragm. It may open directly into the right atrium (ascribed to the embryonic disappearance of the right common cardinal vein).

The azygos occasionally opens into the right brachiocephalic or even the right subclavian (believed to be caused by an elongated embryonic right common cardinal vein).

If the hemiazygos is under developed, its branches open directly into the azygos vein.

The azygos may receive either or both supreme intercostal veins and any combination of intercostal and lumbar veins. The left superior intercostal communicates with the azygos system in 57% of individuals.

The azygos may drain into the hemiazygos, which in turn drains into the right atrium (Gruber). The pulmonary veins have been found opening into the azygos vein.

Unusual tributaries include the right internal thoracic, superior phrenic, bronchial, and gonadal veins.

The azygos vein may be absent or doubled.

The inferior vena cava has been seen continued into the azygos vein, which is then of course extremely large. The testicular vein and, on the left side, the renal and suprarenal veins have been seen terminating in the same manner.

Poynter (1922) discussed these veins as follows: The embryology of these veins has not been worked out for man, but as Evans (1912) pointed out , there is no doubt but that the posterior cardinal veins are concerned in the formation of these veins. All of the variations I (Poynter) have encountered can be explained by the familiar figures of Hochstetter. A study of 200 bodies showed 26% variation, Soloweilschik (1899) found 22%. Many of these are of a very minor nature. The cross connections of the hemiazygos to the azygos is almost as frequent at the 8th as at the 9th vertebra and in 10% both are present. In 4%, three connections are maintained. In 12 bodies, 4 or more connections are found. In 4, bodies the left spermatic and suprarenal veins joined the accessory hemiazygos, while in 12%, a branch from or all of the left renal opened into this vein, Zumstein (1895). In 4% the accessory hemiazygos does not join the hemiazygos, but enters the coronary sinus. In 14 cases, the right and left systems do not connect, but the left forms a separate system flowing into the left anonyma except in one case which enters a persistent left cava.

Gruber (1864) saw a case in which the azygos opened into the hemiazygos, and this in turn into the right atrium. "The azygos may curve from the fourth or fifth vertebra, grooving the lung to enter the superior vena cava just before it enters the heart . " (From Poynter, 1922)

Image 98, Image 205, Image 211, Image 369, Image 439

References

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