A Unique Variation of Azygos System of Veins

Abstract

Knowledge of variations of azygos and hemiazygos veins is of importance to cardiothoracic surgeons and radiologists during various surgical, radiological, and echography techniques. We report some unique variations of azygos system of veins observed during dissection classes for undergraduate medical students. The azygos vein was formed as usual by the union of right subcostal and ascending lumbar veins. The vein ascended upward and to the left to reach the midline at the level of the 9^th thoracic vertebra. After ascending till 5^th thoracic vertebra, it gradually inclined to the right of midline and terminated by opening into the superior vena cava at the level of the 3^rd thoracic vertebra. There was no major variation in the tributaries of the azygos vein on the right side, except that the right superior intercostal vein crossed behind the azygos vein from right to left and opened into the left side of the azygos vein. Further, two anastomotic veins connected the 10^th, 11^th and 12^th posterior intercostal veins with each other to form two anastomotic circles on the right side of 10^th to 12^th thoracic vertebrae. The hemiazygos vein bifurcated on the left side of the 10^th thoracic vertebra and the two ends opened into the azygos vein at the level of 9^th and 10^th thoracic vertebrae forming a venous circle in front of the 10^th thoracic vertebra. The course of accessory hemiazygos vein was noteworthy. Instead of its classic descending course, the vein ascended upward from the left side of the 8^th thoracic vertebra till the 6^th thoracic vertebra before opening into the azygos vein.

INTRODUCTION

Azygos vein is the vein that drains the venous blood from the upper part of the posterior abdominal wall and the thoracic wall. It is very important for the reason that it connects the superior vena cava with the inferior vena cava, and it has very few to no valves. It helps to shunt the blood in both ways due to this reason.[1] Although its formation is variable, generally it is formed at the level of the 1^st lumbar vertebra by the union of right ascending lumbar vein and the right subcostal vein. It ascends on the right side of the vertebral column and terminates by opening into the superior vena cava. It receives right superior intercostal vein, 5^th to 11^th right posterior intercostal veins, hemiazygos vein, accessory hemiazygos vein, pericardial, bronchial, and esophageal veins.[1] The hemiazygos vein looks like the mirror image of the lower part of the azygos vein. It is formed by the union of left ascending lumbar vein and subcostal vein. It ascends to the level of the 8^th thoracic vertebra and opens into the azygos vein. On its way, it receives 9^th, 10^th, and 11^th left posterior intercostal veins. The accessory hemiazygos vein collects blood from 4^th or 5^th intercostal spaces to 8^th intercostal spaces through posterior intercostal veins and drains into azygos vein at the level of 7^th thoracic vertebra. Sometimes, the hemiazygos and accessory hemiazygos veins drain into azygos vein through a common trunk. We report a unique pattern of azygos system of veins, the knowledge of which could be very useful to cardiothoracic surgeons, vascular surgeons, radiologists, and other clinical disciplines.

CASE REPORT

During dissection classes for medical students, we found some venous variations in the posterior mediastinum of and adult male cadaver aged approximately 70 years. Variations were noted in the course and tributaries of the azygos vein. The azygos vein was formed as usual by the union of right subcostal and ascending lumbar veins on the right side of the 12^th thoracic vertebra. The vein ascended upward and to the left to reach the midline at the level of 9^th thoracic vertebra. Keeping its median position, it ascended till 5^th thoracic vertebra and then gradually inclined to the right of midline [Figure 1]. It ascended till the right side of 3^rd thoracic vertebra, where it terminated by opening into the superior vena cava. There was no major variation in the tributaries of the azygos vein on the right side, except that the right superior intercostal vein crossed behind the azygos vein from right to left and opened into the left side of the azygos vein [Figure 1]. Further, two anastomotic veins connected the 10^th, 11^th and 12^th posterior intercostal veins with each other to form two anastomotic circles on the right side of 10^th to 12^th thoracic vertebrae [Figure 2]. The hemiazygos vein received 9^th to 12^th left posterior intercostal veins. It bifurcated on the left side of the 10^th thoracic vertebra, and the two ends opened into the azygos vein at the level of 9^th and 10^th thoracic vertebrae forming a venous circle in front of the 10^th thoracic vertebra [Figure 3]. The accessory hemiazygos vein received 5^th to 8^th left posterior intercostal veins and opened into the azygos vein at the level of the 6^th thoracic vertebra. The course of this vein was noteworthy. Instead of its classic descending course, the vein ascended upwards from the left side of 8^th thoracic vertebra till the 6^th thoracic vertebra before opening into the azygos vein [Figure 3]. A schematic representation of all these variations is collectively shown in Figure 4.

Figure 1.

Photograph of dissection of posterior thoracic wall showing the azygos system of veins. AV = Azygos vein, TD = Thoracic duct, SICV = Left superior intercostal vein, HAV = Hemiazygos vein, S = Superior, I = Inferior, L = Left, R = Right

Figure 2.

Photograph of dissection of lower part of the posterior thoracic wall as viewed from the right side. AV = Azygos vein, TD = Thoracic duct, T11 = Eleventh thoracic vertebra, AC = Anastomotic venous circle, 10 = Right tenth posterior intercostal vein, 11 = Right eleventh posterior intercostal vein, 12 = Right subcostal vein, S = Superior, I = Inferior, L = Left, R = Right

Figure 3.

Photograph of dissection of the posterior thoracic wall as viewed from the left side. AV = Azygos vein, T11 = Eleventh thoracic vertebra, T10 = Tenth thoracic vertebra, T6 = Sixth thoracic vertebra, HAV = Hemiazygos vein, AHAV = Accessory hemiazygos vein, S = Superior, I = Inferior, L = Left, R = Right

Figure 4.

A schematic drawing of the azygos vein variation. AV = Azygos vein, HAV = Hemiazygos vein, AHAV = Accessory hemiazygos vein, SICV = Superior intercostal vein, LA = Ascending lumbar vein, AC = Anastomotic venous circle, 1-11 = Posterior intercostal veins, 12 = Subcostal veins

DISCUSSION

Azygos system of veins is subjected to wide range of variations. Knowledge of its variations is especially important for cardiothoracic surgeons and radiologists. Arslan et al. have reported a case of total absence of azygos vein.[2] Anomalies of azygos vein are often associated with anomalies of superior and inferior venae cavae. A case of absence of azygos vein with the presence of a left superior vena cava has also been diagnosed through computed tomography.[3] Absent left and right superior vena cava and azygos continuation of inferior vena cava have also been reported.[4] Abnormal location of the azygos vein might cause difficulties to surgeons. An anomalous azygos vein covering the sympathetic chain has been reported. Such a vein might pose a significant risk in endoscopic thoracic sympathectomy.[5] Azygos vein usually lies on the right side of the vertebral column, but it may be shifted to the midline or the left of midline as age advances.[6] Hence, the midline position of the azygos vein in the current case was possibly due to old age.

Hemiazygos and accessory hemiazygos veins can also be absent of show variation in their formation, course, termination or tributaries. A ladder pattern of hemiazygos veins has been reported.[7] Transverse segment of hemiazygos vein crossing in front of the vertebral column is called “interazygos vein.” An anomalous course of interazygos vein, between the esophagus and descending thoracic aorta has been reported.[8] The absence of hemiazygos vein and presence of preaortic interazygos vein has also been reported.[9] Our literature survey also revealed a case where the hemiazygos vein was absent with the presence of right and left azygos veins.[10] Another very rare variation associated with the azygos system of veins is opening of all pulmonary veins into the azygos vein. One such case has been diagnosed by echocardiographic technique.[11]

Current case is unique because of the following reasons; (1) to the best of our knowledge, there is no report on the venous circle formed in front of the vertebral column by the bifurcated hemiazygos vein. (2) There is no report on anastomotic venous circles on the right side of lower thoracic vertebrae. (3) Usually, the hemiazygos vein descends down on the left side of the vertebral column from 4^th to 8^th thoracic vertebra. However in the current case, it ascended to the 6^th thoracic vertebra from the level of the 8^th thoracic vertebra. Apart from knowing the normal anatomy of azygos system of veins, its possible variations such as the one being reported here, have to be kept in mind during clinical procedures such as cardiothoracic surgery, echocardiography, and other radiological techniques. The variations of azygos system may lead to iatrogenic bleeding or faulty radiological diagnosis. Idiopathic azygos aneurysms might mimic mediastinal masses and have to be confirmed radiographically.[12] Misplacement of venous catheters can happen when there are variations of azygos veins, especially when the terminal parts of the veins are dilated. The misplacement might lead to severe bleeding.[13] The knowledge of variation of the azygos system is also important during the surgery to repair the esophageal atresia.[14] It is possible to identify the gross anomalies like azygos continuation of inferior vena cava[15] and absence of superior vena cava.[16] However the less frequent, uncommon variations might lead to confusions during echocardiography or any other procedures.

CONCLUSIONS

The knowledge of unique variations of azygos system, being reported here is of importance to radiologists performing venous cannulations, cardiac echography, and other vascular radiology procedures. It is also important to cardiothoracic surgeons to minimize the chances of iatrogenic hemorrhage.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.