“Left-dominant” azygos system: transvertebral confluence, absent right arch, and drainage into the left brachiocephalic vein—a case description

Introduction

The azygos venous system serves two key functions: it provides the principal drainage pathway of the posterior thoracic wall and constitutes a vital collateral channel between the superior and inferior venae cavae (1). In its typical configuration, the azygos vein originates from the right ascending lumbar and subcostal veins, ascends along the right anterolateral paravertebral space, and arches over the right main bronchus to drain into the superior vena cava (SVC). The hemiazygos vein (HAV) courses superiorly on the left of the vertebral column and empties into the azygos vein at the T8–T9 level, corresponding to the eighth and ninth thoracic vertebrae. The accessory hemiazygos vein (AHV) receives the left upper posterior intercostal veins and most commonly joins the azygos vein, although direct termination into the left brachiocephalic vein (LBCV) is occasionally observed (1).

The spectrum of recognized anatomical variations includes a left-sided azygos vein, azygos continuation of the inferior vena cava, and independent drainage of the AHV into the LBCV. This last variant is documented in approximately 1–2% of the general population and typically presents as a benign, isolated finding (2). However, a fundamentally different and far rarer anomaly—the formation of a common venous trunk via transvertebral confluence of the azygos vein and HAVs—has received scant attention in the literature. This configuration functionally results in a left-sided AHV that assumes the entire azygos inflow from the right side and may be associated with agenesis of the right azygos arch, the normal terminal conduit to the SVC (2,3). To date, only sporadic cadaveric and radiological observations of comparable anomalies have been reported (4). A recent meta-analysis reaffirmed the marked heterogeneity of the azygos venous tree and underscored its potential clinical relevance; however, the transvertebral confluence pattern described herein was not reported in that systematic review (5).

Recognition of this variant holds substantial clinical significance. From a surgical and interventional standpoint, an unrecognized common azygos-hemiazygos trunk may complicate thoracic procedures, including mediastinal dissection and central venous catheterization. From a radiological perspective, the aberrant venous architecture may mimic mediastinal lymphadenopathy, a para-aortic mass, or collateral vessels secondary to SVC obstruction, potentially leading to unnecessary diagnostic interventions. We report a case of this uncommon anatomical variant, comprehensively characterized by contrast-enhanced computed tomography (CT) with three-dimensional (3D) reconstruction. The embryological origins, cross-sectional imaging features, and practical implications for both diagnostic imaging and procedural planning are discussed.

Case presentation

All procedures in this study were performed in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this article and the accompanying images. A copy of the written consent form is available for review by the editorial office of this journal.

A 51-year-old woman presented with acute chest pain of one day’s duration. She denied any history of thoracic trauma, thoracic surgery, or central venous catheterization, and her physical examination, electrocardiography, and cardiac biomarkers were unremarkable. Non-contrast chest CT incidentally revealed an anomalous azygos venous configuration, prompting further evaluation with contrast-enhanced imaging after obtaining written informed consent.

Contrast-enhanced chest CT was performed on a Philips multi-detector scanner. A 70-mL bolus of non-ionic iodinated contrast material was administered via an antecubital vein at a rate of 3 mL/s. Bolus tracking was used with the region of interest placed in the pulmonary trunk and imaging acquisition triggered at a threshold of 100 Hounsfield units. The acquisition parameters were as follows: 120 kV, 271 mA, and a slice thickness of 1 mm. Multiplanar reformations and 3D volume-rendered reconstructions were generated on a dedicated workstation.

The azygos vein originated normally from the confluence of the right ascending lumbar and subcostal veins, measuring 3.5 mm in diameter (normal azygos diameter is approximately 7–10 mm). It ascended along the right anterolateral paravertebral region, receiving the right posterior intercostal veins. However, at the T8–T9 level, the vein abruptly crossed the midline, passing posterior to the esophagus and descending thoracic aorta (DTA), and joined the left HAV. This transvertebral confluence formed a common left-sided venous trunk of approximately 8 mm in diameter. The right azygos arch was entirely absent; thus, the azygos vein effectively terminated at the confluence.

The common trunk continued superiorly as a left AHV, coursing along the left lateral border of the DTA throughout its thoracic course. At the level of the aortic arch, it curved anteriorly and superiorly over the left subclavian artery and drained into the inferior aspect of the LBCV, just proximal to its junction with the right brachiocephalic vein. The left superior intercostal veins drained into the proximal portion of this trunk, while the right superior intercostal veins entered the azygos vein before its midline crossing. No persistent left SVC, interruption of the inferior vena cava, or associated cardiac anomalies were observed. 3D volume-rendered reconstructions clearly demonstrated the anomalous pathway (Figures 1-4).

Figure 1 Axial contrast-enhanced CT image at the T8–T9 vertebral level. The azygos vein (white arrows) crosses the retromediastinal midline to join the hemiazygos vein (yellow arrows). T8–T9: thoracic vertebrae 8 and 9. AHV, accessory hemiazygos vein; CT, computed tomography; DTA, descending thoracic aorta.

Figure 2 Coronal reformatted CT image. The left-sided common trunk (white arrows) ascends along the left lateral margin of the DTA and terminates in the LBCV (black circle). BCT, brachiocephalic trunk; CT, computed tomography; DTA, descending thoracic aorta; LBCV, left brachiocephalic vein; LCCA, left common carotid artery; LSA, left subclavian artery; LSV, left subclavian vein; RBCV, right brachiocephalic vein; RCCA, right common carotid artery; RSA, right subclavian artery; SVC, superior vena cava.

Figure 3 Sagittal reformatted CT image. The left-sided trunk (white arrows) ascends along the left border of the descending thoracic aorta, then turns anteriorly to join the LBCV (black circles). The vessel does not arch over the left subclavian artery. CT, computed tomography; DTA, descending thoracic aorta; LBCV, left brachiocephalic vein; LSA, left subclavian artery.

Figure 4 Three-dimensional volume-rendered CT reconstruction. The right azygos vein crosses the midline to join the HAV, forming a left-sided common trunk (previously termed the accessory hemiazygos trunk) that drains into the LBCV. The right azygos arch is absent. AV, azygos vein; CT, computed tomography; DTA, descending thoracic aorta; HAV, hemiazygos vein; LAHT, left accessory hemiazygos trunk; LBCV, left brachiocephalic vein; LSA, left subclavian artery; LSV, left subclavian vein; RBCV, right brachiocephalic vein; SVC, superior vena cava.

The variant was an incidental finding. CT excluded pulmonary embolism, aortic dissection, and other acute pathology. The diagnosis was based on five imaging features: (I) normal right-sided azygos origin; (II) transvertebral course at the T8–T9 level; (III) formation of a common left-sided trunk ascending along the left mediastinum; (IV) termination in the LBCV; and (V) complete agenesis of the right azygos arch. No invasive confirmation was necessary. The patient was informed of the benign nature of the finding, and no variant-specific follow-up was planned.

Discussion

We report a case of an uncommon azygos variant characterized by transvertebral confluence of the azygos vein and HAVs, formation of a common left-sided trunk draining into the LBCV, and complete agenesis of the right azygos arch. This configuration is fundamentally distinct from the isolated termination of the AHV into the LBCV—a recognized variant with a reported prevalence of 1–2% (1)—and from the classic left-sided azygos vein, which originates and courses entirely on the left (6). In the present case, the azygos vein arose normally on the right before crossing the midline, resulting in the transposition of the entire right-sided azygos inflow to the left. Anatomical and radiological series consistently confirm the extreme rarity of such midline crossover variants (2-5).

Embryologically, this anomaly likely reflects persistence of an intersupracardinal anastomosis at the midthoracic level, accompanied by regression of the cranial segment of the right supracardinal vein and preferential drainage via the primitive left common cardinal vein (7,8). The azygos system derives from the supracardinal veins: the right supracardinal vein normally forms the azygos vein, while the left supracardinal vein gives rise to the hemiazygos and AHVs. Under the proposed mechanism, retained communication between these bilateral channels diverts right-sided flow across the midline, resulting in a “left-dominant” azygos system. The caliber discrepancy—3.5 mm at the azygos origin versus approximately 8 mm in the common trunk—is consistent with this flow redistribution and mirror-image regression of the right-sided arch.

From a diagnostic imaging perspective, failure to recognize this variant may carry tangible consequences. On axial CT, the left para-aortic trunk may mimic mediastinal lymphadenopathy, a para-aortic mass, or venous collaterals secondary to SVC obstruction. The abrupt midline crossing of the azygos vein should not be misinterpreted as a pathological mediastinal vascular connection, nor should the absence of the right arch be mistaken for acquired occlusion. Multiplanar and 3D volume-rendered reconstructions, as employed in the present case, are essential for tracing vessel origin, course, and termination, and for confidently distinguishing such variants from pathological conditions (9). Dedicated CT studies have shown that congenital azygos anomalies remain underrecognized and may frequently mimic disease (10).

The clinical relevance of this variant extends across multiple disciplines. For the cardiothoracic surgeon, an unrecognized large left-sided venous trunk may pose a risk of inadvertent injury and major hemorrhage during mediastinal dissection, particularly in the context of oncologic resections (11,12). For the interventional radiologist and anesthesiologist, drainage into the LBCV represents a potential site of central venous catheter malposition during left-sided access; misplacement into accessory hemiazygos or left-sided azygos systems has been reported (13,14). The variant may also alter mediastinal venous drainage patterns relevant to thoracoscopic procedures (15).

This case expands the recognized spectrum of azygos arch agenesis. Unlike previously reported isolated accessory hemiazygos terminations, it is characterized by transposition of the entire azygos inflow across the midline (1). It also differs from the classic left-sided azygos vein, in which the vessel originates, ascends, and terminates entirely on the left (3,6). In the present case, the origin was conventionally right-sided, with midline crossover occurring at the T8–T9 level. Complete agenesis of the right azygos arch further distinguishes this configuration and underscores the embryological continuum of right supracardinal vein regression.

Several limitations merit acknowledgment. This is a single-case report, precluding estimation of population prevalence. Dedicated CT venography with a delayed venous phase was not performed; nevertheless, the standard contrast-enhanced protocol provided sufficient opacification for definitive anatomical characterization.

Acknowledgments

None.

Footnote

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2026-1072/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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