Split Azygos Vein: A Case Report

The azygos venous system, which comprises the azygos, hemiazygos, and accessory hemiazygos veins, assists in blood drainage into the superior vena cava (SVC) from the thoracic cage and portions of the posterior mediastinum. Routine dissection of a fresh-frozen cadaveric specimen revealed a split azygos vein. The azygos vein branched off the inferior vena cava (IVC) at the level of the second lumbar vertebra as a single trunk and then split into two tributaries after forming a venous plexus. The right side of this system drained into the SVC and, inferiorly, the collective system drained into the IVC. Variant forms in the venous system, especially the vena cavae, are prone to dilation and tortuosity, leading to an increased likelihood of injury. Knowledge of the anatomical variations of the azygos vein is important for surgeons who use an anterior approach to the spine for diverse procedures.


Introduction
The inferior vena cava (IVC) is the largest vein in the human body. Its principal function is to return venous blood from the abdomen and lower extremities to the right atrium of the heart [1]. Developmental patterning of the IVC consists of three paired embryonic veins: subcardinal, supracardinal, and postcardinal. These veins go through recurrent phases of regression and progression, forming various anastomotic connections and ultimately giving rise to the final, mature vena cava system [2].
Anatomically, the azygos vein originates from the connection of the right subcostal vein and right ascending lumbar veins at the T12 vertebral level [3]. It enters the thorax through either the aortic hiatus or posterior to the right crus of the diaphragm, ascending along the anterolateral surface of the right thoracic vertebra [3]. Ultimately, it courses superiorly, uniting with the superior vena cava (SVC) at the level of the fourth thoracic vertebra. The main and most numerous tributaries draining into the azygos vein are the posterior intercostal veins. Superiorly, the azygos drains into the SVC. In view of the complexity of the various embryological stages of the IVC, numerous variant conditions can be attributed to defects in the three paired embryonic veins. In this report, we discuss a unique case of a variant azygos vein.

Case Presentation
A split azygos vein was found during routine dissection of the fresh-frozen cadaveric specimen of a Caucasian male who had been 46 years old at the time of death. The azygos vein branched off the IVC at the level of the second lumbar vertebra and ascended medially, forming a small venous plexus and splitting into two tributaries between the T12 and L1 levels of the vertebral column. The right tributary drained into the SVC while the left drained from the intercostal veins ( Figure 1).

Embryological development
Embryological development of the IVC begins between four and eight weeks after conception and entails the recurrent regression and appearance of paired embryonic veins: subcardinal, posterior cardinal, and supracardinal [1,4]. During embryonic development, these three paired veins go through continual regression and anastomosis until the mature IVC is formed. The first major embryonic event is the regression of the paired umbilical veins, which drain blood from the placenta, and the left vitelline vein, which drains blood from the yolk sac [4]. This gives rise to the hepatic segment of the IVC, formed from the persistence of the right vitelline vein. Around the 35th day of gestation, the appearance of the subcardinal veins and the associated intersubcardinal anastomosis progressively take over the task of draining venous blood from the lower half of the body, previously done by the posterior cardinal veins [5]. The connection between the right subcardinal vein and the hepatic segment of the IVC emerges, forming the suprarenal and hepatic portions of the IVC, respectively. Around the seventh week of gestation, the supracardinal veins emerge and take over venous drainage of the lower half of the body from the subcardinal veins [5].  Azygos vein variants can be classified into three types -type 1: two vertical veins with no connection points; type 2: an anastomotic connection dorsal to the aorta between the azygos and hemiazygos systems; and type 3: a single azygos vein located at the midline [6]. Studies by Seib (1934), Anson and McVay (1984), Kutoglu et al. (2012), and Dahran and Soames (2016) indicate that type 2 is the most common azygos venous system variant while type 1 is the rarest [6][7][8][9].
Nayak and Soumya [10] have described a reversed azygos system with the azygos shifted to the left side and draining into the brachiocephalic vein and the accessory and hemiazygos system shifted to the right side of the body. Badagabettu et al. [11] have reported a hemiazygos vein with multiple linkages to the azygos vein across the midline. Neither of the cases reported by these authors or others was similar to our case, where two more or less vertical venous chains converged into a single vein inferiorly to drain into the IVC and, just prior to this, formed a circular venous plexus.
A venous plexus and split azygos vein were observed in the present case. This variant could have resulted from abnormal regression and/or persistence of the subcardinal and supracardinal veins, and intersubcardinal anastomosis during the fourth to eighth weeks of gestation. This variant is seemingly uncommon due to the plexiform nature of its thoracic parts and the drainage into the IVC of this collective system.

Clinical significance
Anatomical variations in the venous system, especially the vena cava, are of utmost importance to surgeons and interventionalists. For example, there can be thromboembolic complications in the variant vena cava, increasing the risk of pulmonary embolism [5]. Potential problems arise when clinicians reviewing medical images misdiagnose a doubled IVC as mediastinal lymphadenopathy or a para-aortic mass, leading to the unnecessary risk associated with a biopsy [12]. Since minimally invasive vascular approaches, especially via the venous system, are on the rise currently, anatomical venous variants are now even more important to the clinician than ever before. Moreover, surgically, such an anatomical variation, as demonstrated in the present case, could result in catastrophic hemorrhage if not recognized either with preoperative imaging or during surgical approaches. Therefore, a thorough knowledge of their potential variations can improve patient care [13][14][15].