Cadaveric dissection of connection between accessory hemiazygos vein and left brachiocephalic vein

The azygos system of veins has many anatomical variations that can impact mediastinal and vascular interventions. While radiological reports on these are of great clinical value, this study is among the first to present a high-quality cadaveric dissection of a rare anatomical variant to supplement previously published radiologic studies. The azygos venous system consists of the azygos vein (AV), hemiazygos vein (HAV), and the accessory hemiazygos vein (AHAV), which develop from the last portion of the posterior cardinal veins. The normal anatomical configuration includes drainage of the posterior intercostal veins, vertebral vein, esophageal veins, HAV, and AHAV to an unpaired right-side AV at the level of the 8th/9th thoracic vertebra. The reported incidence of AHAV draining directly into the left brachiocephalic vein is 1–2%. An adult formalin-fixed 70-year-old female cadaver was dissected as part of a medical gross anatomy elective course. Gross documentation of a direct connection of the HAV to the AHAV with the AHAV draining into the left brachiocephalic vein. It is important to note the variations of the azygos system to avoid confusion with a potential pathology such as mediastinal masses. Understanding of the rare variant reported here could be useful in the prevention of iatrogenic bleeding from the misplacement of venous catheters and help facilitate radiological diagnosis in the incidence of venous clot formation.


Introduction
The azygos venous system consists of the azygos vein (AV), hemiazygos vein (HAV), and accessory hemiazygos vein (AHAV).The AV is formed by the right intercostal veins and ascending lumbar veins, running in the posterior mediastinum to the level of the fourth thoracic vertebra.Along its course, the azygos vein receives the HAV, AHAV, mediastinal, pericardial, esophageal, right bronchial, and right superior intercostal veins.In addition, the AV provides an important collateral circulation between the superior vena cava (SVC) and inferior vena cava (IVC).The AHAV and HAV together drain the lower eight to nine left posterior intercostal veins.Eventually, the AHAV and HAV drain into the AV via an interazygos vein at T8 and T9, respectively.The left upper four posterior intercostal veins are drained by the left superior intercostal vein into the left brachiocephalic vein (LBCV) [3,7].
The anatomical organization of the azygos system is highly variable in its origin(s), course, tributaries, and termination.This variation may be due to the complexities of venous embryological development.The azygos system and left brachiocephalic vein (LBCV) develop from the common cardinal vein system but most of the plexus regresses, leaving the right and left supra-cardinal veins during the 5th and 7th weeks of gestation.New anastomoses form after the regression of the middle common cardinal vein, giving rise to the left-side venous system.Development of the right-side venous system, in contrast, involves no appreciable change to the anterior cardinal vein with no significant formations or regressions during development [3].

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Developmental variants of the azygos system are rarely symptomatic and are often incidental findings.Yet knowledge of these variants is vital to professionals such as anatomists, surgeons, and radiologists to avoid misdiagnoses and risk of iatrogenic vascular injury due to misplacement of venous catheters or errors during spinal surgery.
We present here the first published cadaveric dissection of a rare anatomical variant of the azygos venous system, previously only documented in radiologic studies.This case offers a novel approach to viewing such a variant and its anatomical relationships.

Case presentation
During routine dissection of the posterior mediastinum in an adult formalin-fixed 70-year-old female cadaver, we encountered an anatomical variant in which the HAV is continuous with an AHAV draining directly into the LBCV (Fig. 1a,  b).During dissection, no other mediastinal pathology or anatomical variations were noted.The related structures around the azygos venous system did not exhibit any additional variants.
The AV was noted to be running superoinferiorly, close to the midsagittal plane, parallel to the thoracic vertebral bodies, and to the right of the descending aorta, draining into the SVC at the level of the fourth thoracic vertebra.On the left side of the thorax, the HAV was found to be continuous with AHAV, creating a single vein that drained the 4th to 11th posterior intercostal veins.AHAV drained directly into the LBCV with an azygos-hemiazygos anastomosis (interazygos vein) located posterior to the aorta and crossing the vertebral column obliquely at the level of the T8 vertebra (Fig. 2).

Discussion
To the best of our knowledge, the presented case is the first high-quality cadaveric dissection of a variant including both a direct HAV-AHAV connection and direct drainage of the AHAV into the left brachiocephalic vein.Several studies show radiologic evidence of these specific variants but lack supporting cadaveric examples.Anatomy textbooks do not emphasize these variations.Other cadaveric studies demonstrate the prevalence of direct HAV-AHAV connection as 15.4-33%, which is relatively low but not an uncommon developmental variation.Conversely, the drainage of AHAV into the left brachiocephalic vein is exceedingly rare as previous studies provide a rate of 1-2% [1,4].The simultaneous presence of both variations, as in our case, is of exceedingly low frequency.
Many different variants of the azygos venous system have been described in previous studies, including azygos agenesis, connection to the right atrium, or abnormal location in the thorax [6].Anson & McVay developed a classification system that divided the AV system morphology into three types: primitive (type 1), transitional (type 2), and unicolumnar (type 3) [2,8].Table 1 lists the known variations in the literature.A common trend seen from analysis of published cadaveric and radiological studies was that the left-sided venous system, HAV and AHAV, have a higher likelihood of variation.The course of development of the azygos venous system appears to be influenced to a large extent by adjacent structures [7].Though untested at present, future work should investigate whether increased levels of growth and regression of the left-side azygos system during embryogenesis leads to the broad array of variations observed in the adult azygos system, specifically of the HAV and AHAV.
Acknowledgements and understanding of azygos system variations have great clinical value in aiding proper diagnosis and intervention, including placement of left-sided intravenous catheters, radiologic diagnosis of mediastinal conditions, and avoiding confusion with thoracic pathology.Being unaware of anatomical variation increases the probability of iatrogenic injury, misdiagnosis, and inappropriate or ineffective treatments [5,9].Failure to identify common anatomical variants is a cited technical error in surgical injuries [3].Thus, radiologists are encouraged to report this finding to help plan interventions.

Conclusion
The clinical significance and risks of anatomical variants of the azygos venous system have been documented, and it is understood that acknowledgement of these variants holds great significance in clinical and surgical practice.Failure to identify variant anatomy is a commonly cited technical error in surgical injuries.Physicians should be made aware  Two longitudinal azygos lines without any connection between them 1-3% Type 2 [2,8] Further divided into five divisions according to the number of azygos-hemiazygos anastomosis 87-98% Type 2A: 12% Type 2B: 35% Type 2C: 27% Type 2D: 15% Type 2E: 12% Type 3 [2,8] Single vein located at midline draining both sides 1-10% Communication between HAV and AHAV [2,8] 15.4-33% Drainage of AHAV into left brachiocephalic vein [4] 1-2% of this possible variant and take it into consideration when diagnosing, treating, and monitoring cases involving thoracic vasculature.

Fig. 1
Fig. 1 Photograph of the left thorax in situ (A) and with the aorta resected and the esophagus retracted (B).Continuous accessory hemiazygos vein and hemiazygos vein (purple); left brachiocephalic

Table 1
Variants