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Study of anatomical variations in the azygous venous system in human cadavers
, Deepak Goyal2, Swati Bansal1, Shivi Mudgal1, Ujwaljit Singh1
* Corresponding author: Dr. Rimpi Gupta, Department of anatomy, Kalpana Chawla Government Medical College, Model Town, Karnal, India. dr.rimpigupta15@gmail.com
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Received: ,
Accepted: ,
How to cite this article: Gupta R, Budhiraja V, Swami S, Goyal D, Bansal S, Mudgal S, et al. Study of anatomical variations in the azygous venous system in human cadavers. Ann Natl Acad Med Sci (India). doi: 10.25259/ANAMS_133_2024
Abstract
Objectives
Several variations have been reported in the azygous venous system (AZV) earlier, e.g., agenesis of azygous and hemi-azygous veins (HMAZV), incomplete accessory HMAZV, and accessory HMAZV may join the HMAZV. These variations can mimic pathologies such as aneurysms, tumors, and lymphadenopathies. The possibility of the above variations should be considered while carrying out mediastinal operations. Keeping in view the above-mentioned variations & their clinical relevance, we aimed to conduct a study to observe the course, tributaries, pattern of drainage, and termination of the azygous venous system.
Material and Methods
The study was carried out on 20 human adult cadavers irrespective of age and sex. The cadavers were preserved in formalin, and standard dissection steps were followed to open the posterior mediastinum.
Results
In the present study, five cadavers (25%) showed variations in the AZV pattern. Of the five, four (20%) were type 2 (transition), which further consists of groups 2-10. Each case showed a different type of drainage pattern & fell into group 2, 6A, 7, and 4. Variations found in Case no. 5 were not reported in previous literature, hence labeled as an atypical case. No case in the present study fits into types 1 and 3. Among the five cases, case no. 3 presented a rare variation as the 8th and 10th left posterior intercostal veins (LPICVs) bifurcated and formed two anastomotic venous rings in front of the lower thoracic vertebrae. Further, 20% of the cases showed an absence of the superior intercostal vein (SICV), 10% each on the right and left sides.
Conclusion
Our study has demonstrated the possible variations in the AZV system, which are to be considered during any mediastinal surgery.
Keywords
Azygous venous system
Drainage pattern
Formation
Posterior intercostals veins
Variations
INTRODUCTION
The azygous vein (AZV) arises from the posterior surface of the inferior vena cava at the level of the first lumbar vertebra (L1) and enters the thorax through the aortic opening of the diaphragm. Here, it receives the right ascending lumbar and subcostal veins. Then it runs upward in the posterior mediastinum to the T4 level, where it loops above the root of the right lung and terminates in the superior vena cava, before the latter pierces the pericardium.
The left ascending lumbar and subcostal veins join to form the hemi-azygous vein (HMAZV), which receives blood from the lower three left posterior intercostal, esophageal, and mediastinal veins. It runs upward in front of T8, crosses to the aorta, and ends in the AZV.1 The accessory HMAZV runs downwards and drains venous blood from the 4th to 8th left posterior intercostal veins (LPICVs). It passes through the T7 vertebra to join the AZV.2 Deviations from the normal pattern of drainage have been reported earlier in literature by various authors like incomplete accessory HMAZV with posterior intercostal veins draining bilaterally into the AZV,3 pre-aortic intraserous vein,4 variant accessory HMAZV with persistent cranial segment of posterior cardinal vein5. It is essential to document such variations, as they can mimic pathologies such as aneurysms, tumors, and lymphadenopathy on imaging.6 So, these variations should be taken care of during mediastinal operations.
MATERIAL AND METHODS
The study was conducted on 20 formalin embalmed adult cadavers, irrespective of age & sex, by dissection of the posterior mediastinum.
RESULTS
Out of 20 cadavers, variations in the anatomy of human azygous venous system were found in the 5 cases. For each case, these variations are described in the Table 1 [Figures 1-5].
| Variation observed | Right | Left |
|---|---|---|
| Case 1 (see Figure 1) | A normal drainage pattern was observed |
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| Case 2 (see Figure 2) | Normal drainage pattern except the fourth PICV, instead of joining the SICV, drained into the AZV. |
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| *Case 3 (See Figure 3) |
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| Case 4 (Figure 4) |
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Case no 5 (see figure 5) |
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*This case is unique as two LPICVs bifurcated, connecting the three (8th to 10th) LPICVs & forming two anastomotic venous rings in front of the lower thoracic vertebrae (see Figure 3). AZV: Azygous vein, HMAZV: Hemi-azygous vein, LPICVs: Left posterior intercostal veins, SICV: Superior intercostal vein, PICVs: Posterior intercostal veins.
- A continuous venous line on left side of vertebral column which drained into the azygous vein at T9 by a single transverse channel.
- Left superior intercostal vein (SICV) was absent. Hence, second to fifth left posterior intercostal veins (LPICVs), drained into the accessory hemi-azygous vein. The sixth & seventh LPICVs formed a common trunk which joined the accessory hemi-azygous vein & drained into the azygous vein at T6 level. Eighth LPICV instead of draining into accessory hemi-azygous vein united with the hemi-azygous vein & formed a common trunk which drained directly into the azygous vein at T8 level.
- Right superior intercostal vein was absent. Hence, 2 to 11 right posterior intercostal veins (PICVs) were draining directly into the azygous vein. The 8th left posterior intercostal veins (LPICV) bifurcated, the main division of which drained into the azygous vein while the lower one communicated with 9th LPICV & drained into the hemi-azygous vein forming an anastomotic venous circle in front of T9. The l0th LPICV also bifurcated, main division of which drained into the hemi-azygous vein, while the another division ascended to communicate with 9th LPICV. This is again forming a venous circle in front of T10. This case is unique as two PICVs on left side bifurcate, connecting the three (8th to 10th) PICVs & forms two anastomotic venous rings in front of lower thoracic vertebrae.
- Superior intercostal vein (SICV) was absent, hence 2nd to 11th Posterior intercostal veins (PICVs) were draining into the azygous vein. On left side, a continuous venous line was present which drained into the right azygous vein by two transverse channels at the level of disc between T6-T7 & T9-T10 Levels respectively.
- There was no Superior intercostal vein (SICV) on left side. 2nd-7th Posterior intercostal veins (PICVs) drained into accessory hemi-azygous vein which then drains into azygous vein by two transverse channels at the level of T6-7 & T7-8. Then 8-11th PICVs drained directly into the azygous vein. There was no hemi-azygous vein.
DISCUSSION
The AZV system develops as the right and left azygous venous lines on both sides of the aorta. The right one evolves into the longitudinal part of AZV. The persistent cephalic part of the right posterior cardinal vein partially forms the arch of the azygous. The left one develops into an array of vertically aligned veins. The upper set is called accessory or superior HMAZV, which drains the 4th to 6th posterior intercostal spaces. The lower one, named inferior HMAZV, arises as the left ascending lumbar vein & drains the lower intercostal spaces. Both these veins connect to AZV with transverse veins, which pass behind the aorta. These veins are usually short, as AZV lies anterior to the thoracic vertebrae. The 2nd and 3rd LPICVs retain their connection to the left posterior cardinal vein, the greater part of which disappears, but a small part adjoining the common cardinal vein persists and contributes to the formation of the left SICV along with the caudal part of the left anterior cardinal vein.
Any deviation from the normal formation or pattern of drainage of veins leads to variations, which are not rare.
In the present study, observations were made according to Anson BJ & McVay CB,1984.7 They classified the AZV system into three main types and 11 subtypes.
Type 1: This is the primitive form where AZV (right side) and superior and inferior AZVs (left side) lie parallel to each other. There is only one subtype, i.e., group 1 for this type. Only one case in Anson’s study belonged to this subtype, whereas none of the cases in the present study belonged to this type.
Type 2 (transition): It has subgroups 2-10. There are numerous anastomoses between AZV & HMAZV. The number of transverse anastomoses increases from groups 2-5 & they have a continuous left venous line. Vertical bending is seen from groups 6-10, with a decrease in the number of transverse anastomoses.
Type 3: It consists of one AZV in the midline anterior to the thoracic vertebrae. There is only one subtype (Group 11), which is seen in 1% of all cases.
None of the cases in the present study belonged to this type.
Kutoglu et al.,8 (2012), Dahran N & Soames R, (2016)9 and Mohanty et al.10 (2022) used the classification of Anson BJ & McVay CB,19847 in their studies. Table 2 shows the findings of these studies and those of the present study.
| Study | No. of cadavers | Type 1 | Type 2 | Type 3 |
|---|---|---|---|---|
| Anson and McVay7 (1984) | 100 | 1% | 98% | 1% |
| Kutoglu et al.8 (2012) | 48 | 2.1% | 92% | 2.1% |
| Dahran & Soames9 (2016) | 30 | 3.3% | 86.7% | 10% |
| Mohanty et al.10 2022 | 20 | 5% | 85% | 10% |
| *Present study | 20 | ---- | 95% | - |
*One cadaver out of 20 (5%) was an atypical case, which did not fit into any of the above classifications.
In the present study, Case 1 belongs to Type 2, group 2, as the hemiazygos and accessory HMAZVs formed a single vein that drained into AZV by a single transverse channel. So, 1 out of 20 cadavers fall into this group. In Anson’s study, 27.1% cases belonged to type 2, group 2. Kutoglu et al.8 (2012) found 14 cases out of 48 cadavers where single retro-aortic transvertebral anastomoses exist between right & left azygous venous lines.
Case 2 of our study belonged to type 2 Group 6A as the HMAZV showed vertical bending, i.e., it drained directly into the AZV. There is only one transverse channel at T6 level through which the accessory HMAZV opens into AZV.
Case 3 of our study partially belonged to type 2 group 7, as the HMAZV showed vertical bending, i.e., it drained directly into the AZV, and one of the LPICVs (8th) was draining directly into the AZV instead of draining into the accessory HMAZV.
Moreover, the present case is rare because two LPICVs bifurcated (8th& 10th), connecting the three LPICVs (8th to 10th), and two anastomotic venous rings were formed in front of the lower thoracic vertebrae.
Case 4 of our study partially belonged to type 2 group 4 as the hemiazygos & accessory HMAZVs formed a continuous venous channel that drained into the AZV by two transverse channels at the level of discs between T6-7 & T9-10, respectively.
Case 5 of our study did not belong to any of the above classifications, so we labeled it as an atypical case of azygous venous system variation.
CONCLUSION
AZ forms collateral circulation in conditions like occlusion of the superior vena cava & in increased portal venous pressure. The knowledge of variations is important to radiologists and surgeons performing echography, venous cannulation, and various other radiology procedures for reducing the risk of iatrogenic hemorrhage. Such conditions may lead to anomalies like aneurysms, lymphadenopathy, etc., and hence should be taken care of during mediastinal operations.
Acknowledgement
The authors are grateful to the voluntary donors who wished to donate their body after death to the Anatomy department of our institute and also, to the family of deceased who helped in fulfilling their wish. Secondly, authors are thankful to the dissection hall attendants who helped the authors in dissection. Last but not the least, special acknowledgment to Dr Amanjot Lamba who helped in labelling the figures of the variant anatomy of azygos venous system and technical part of the manuscript
Authors’ contributions
RG: Conceptualized the study, conducted the anatomical dissections, and provided critical revisions of the manuscript; VB: Provided overall guidance, verified data accuracy, approved the final version, and ensured the integrity of the research process; SS: Assisted in cadaver preparation, specimen documentation, and proofreading of the final draft; DG: Contributed to manuscript drafting. SB: Reviewed relevant anatomical and clinical literature and contributed to manuscript editing; SM: Collected and organized morphometric data, prepared figures and photographs, and assisted in literature review; US: Performed statistical analysis, interpreted findings, and contributed to the Results and Discussion sections.
Ethical approval
Institutional Review Board approval is not required since the present study is on cadavers which have been received under body donation programme after getting consent from family of the deceased for teaching and research purpose.
Declaration of patient consent
Patient’s consent not required as there are no patients in this study.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
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