Two concurrently occurring rare anatomic variants of the accessory hemiazygos vein

A 36-year-old female presented to the emergency department with intermittent chest pain for three weeks. She endorsed worsening shortness of breath and chest tightness on the day of admission. Initial laboratory investigations revealed an elevated D-dimer, prompting CT angiography (CTA) of the chest to rule out pulmonary embolism.

A CTA was obtained and showed no evidence of pulmonary embolism. Incidentally, the accessory hemiazygos vein was noted to have an aberrant anatomical course. The first variant involved the proximal accessory hemiazygos vein, which was found posterior to the aorta coursing upwards at the level of the aortic arch before draining into the left brachiocephalic vein. The second variant involved the distal accessory hemiazygos vein, which was directly connected to the hemiazygos vein creating a single vascular structure. In the vast majority of people, these two normal anatomic variants are not present.

The posterior thoracic wall is drained through a complex system of posterior intercostal veins, each eventually draining into larger veins. One of these veins is the accessory hemiazygos vein which is traditionally found in the posterior mediastinum running along the thoracic vertebral column. At this location, the accessory hemiazygos vein will receive tributaries from the left posterior 4^th to 7^th, and sometimes 8^th, posterior intercostal veins and left bronchial veins. The hemiazygos vein receives the left 9^th to 11^th posterior intercostal veins and left subcostal vein [1,2]. The accessory hemiazygos vein and hemiazygos vein will eventually separately drain into the azygos vein at the level of the T8-T9 vertebrae [3]. On the right, the 4^th to 11^th posterior intercostal veins simply drain into the azygos vein [1,2].

However, it is possible to have multiple anatomical variants of the azygos venous system. These anatomic variants are best assessed on contrast-enhanced imaging. Specifically, in this case, two accessory hemiazygos vein variants were highlighted. One variant is the result of the proximal accessory hemiazygos vein draining directly into the left brachiocephalic vein (figure 1,2); the chance of this variant occurring has been suggested to be around 1-2% [4]. Another variant involves the connection between the accessory hemiazygos vein and the hemiazygos vein forming a common venous pathway (figure 3,4); the chance of this occurring has not been previously reported, but it is considered very rare [5]. The incidence of both anatomical variants presenting simultaneously, as depicted in our case, is unknown. This is only the second reported case of both anatomical variants occurring simultaneously on imaging [5]. Additionally, it has been reported rarely in cadaveric dissections [6].

Accessory hemiazygos vein variants present asymptomatically but can put patients at risk of complications during surgical procedures. Therefore, awareness and identification of these variants is imperative during preoperative radiological investigations. This is especially true for spinal surgery involving the posterior mediastinum, as damage to an unidentified accessory hemiazygos vein variant could lead to haemorrhage or postoperative hematomas. Presence of a variant could also present as a risk during venous catheter placement, especially one placed into the left subclavian vein; if present, the guide wire could travel into the anatomical variant connecting the left brachiocephalic vein and the accessory hemiazygos vein. The hemiazygos system, when dilated secondary to one of the above variations, could be mistaken for something other than a venous system, such as an abnormal lymph node, aneurysm, or tumour, which could alter the suggested diagnosis made by the radiologist [6,7].

The accessory hemiazygos vein can present in multiple different asymptomatic variations. This case demonstrated two concurrent anomalies found incidentally on CTA. Due to the surgical complications that could occur with the presence of a variant, radiologists should be aware of the different anatomical variations of the accessory hemiazygos vein.

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