RESEARCH Int J Anat Var Vol 14 No 1 Nov 2021 35 A Meta-Analysis and Systematc Review of Anatomical Variatons in ihe Drainage Patern of the Azygos Venous System Mihretu Dessie Jegnie, Girma Seyoum Jegnie MD, Seyoum G. A meta-analysis and systematic review of anatomical variations in the drainage pattern of the azygos venous system. Int J Anat Var. 2021;14(1):35-38. ABSTRACT Introduction: The azygos venous system drains blood from the upper lumbar region of the abdomen and thoracic wall. It includes the azygos vein laying on the right and its left side counterparts, the hemiazygos, and accessory hemiazygos veins. The azygos vein receives all the right side posterior intercostal veins. The hemiazygos and accessory hemiazygos veins empty into the azygos vein after receiving the left side posterior intercostal veins. The azygos venous system shows variable drainage pattern in the posterior thorax. Objective: The main aim of this systematic review and meta-analysis was to determine the pooled prevalence of each of the azygos venous system drainage pattern types. Method: PubMed, Scopus, Science Direct, and Google Scholar were searched extensively. The prevalence of each drainage pattern and other relevant data were extracted and organized in a Microsoft Excel spreadsheet. Stata/SE 14 software was employed to estimate the pooled prevalence of the four different types of azygos venous system drainage patterns. Heterogeneity was checked by a forest plot. Funnel plots and Egger’s test were done to assess publication bias. Subgroup analysis was done based on the geographic location of studies. Result:A total of nine articles conducted using 588 human cadaveric specimens were included in the meta-analysis and systematic review. Type II (Transitional) was the most prevalent drainage pattern of the azygos venous system with a prevalence of 81.83% (95% CI: 66.71, 96.95). The other variant types were: Type I (Primitive or Embryological) (10.85%), Type III (Unicolumnar) (3.41%), and atypical (7.13%). Conclusion: A considerable variability in the drainage pattern of the azygos venous system was noted. Transitional type was the most prevalent drainage pattern of the azygos venous system with a prevalence of 81.83%. Key Words: Azygos Venous System; Drainage Pattern; Anatomical Variation; Meta- Analysis INTRODUCTION T he azygos venous system in the thorax is responsible primarily for draining venous blood from the thoracic wall to the superior vena cava (SVC). The term azygos means “unpaired” and describes the asymmetry in this venous system. The azygos vein (AV) on the right, as well as the hemiazygos vein (HV) and accessory hemiazygos vein (AHV) on the left, make up the azygos system of veins. Both the AV and the HV are formed from the lumbar veins ascending from the abdomen uniting with the subcostal vein [1]. On the right, the AV is continuous, collecting blood from the right intercostal veins before arching over the root of the lung to join the SVC. It receives blood from the bronchial, pericardial, and right superior intercostal veins; the posterior right intercostal veins from the fifth to eleventh intercostal spaces; the superior phrenic, mediastinal, and esophageal veins; and the vertebral venous plexuses [2]. Apart from the drainage of the structures in the posterior mediastinum, the azygos systems of veins serve as a collateral circulation by forming cavo-caval and porto-caval junctions during caval vein occlusion and portal hypertension [3-5]. The azygos system veins vary greatly in their mode of origin, course, tributaries, anastomoses, termination, and drainage pattern. An effort has been done to classify the different variable arrangements of the azygos venous system by different researchers. Two of the most commonly accepted classification schemes by many anatomists are the work of Anson B.J. [6] and Seib G.A. [7]. In his study of 100 cadavers, Anson B.J. classified the azygos venous system vertical and horizontal connections into 3 types (primitive or embryological, transient, unicolumn) and 11 groups (Table 1). Similarly, Seib G.A. classified the azygos venous system pattern into three types (double column, transitional, and single column). However, unlike to Anson B.J., Seib G.A. divided the three types into 21 groups. Type I (Primitive or Embryological): It is a primitive or embryological form consisting of two separate veins lying parallel to each other in the posterior mediastinum, being anterior and lateral to the vertebral column. These Department of Anatomy, College of Health Sciences, Addis Ababa University, Addis Ababa, Ethiopia Correspondence: Dr. Mihretu Dessie Jegnie, Department of Anatomy, College of Health Sciences, Addis Ababa University, Addis Ababa, Ethiopia.Telephone +251 918 277 105; E-mail:mihretuj@gmail.com Received: Sep 17, 2020, Accepted: Jan 22, 2021, Published: Jan 29, 2021 OPEN ACCESS This open-access article is distributed under the terms of the Creative Commons Attribution Non-Commercial License (CC BY-NC) (http:// creativecommons.org/licenses/by-nc/4.0/), which permits reuse, distribution and reproduction of the article, provided that the original work is properly cited and the reuse is restricted to noncommercial purposes. For commercial reuse, contact reprints@pulsus.com Group Description 1 Type I (Two separate veins lying parallel to each other in the posterior mediastinum) 2 There is one midline horizontal connection between the right and left trunk, usually at the level of T8. 3 There is one horizontal connection between the right and left trunk above T8. 4 There are two horizontal connections between the right and left trunk, superior horizontal connection at T8 and inferior horizontal connection below the level of T8. 5 There are three to five horizontal connections between the right and left trunk. 6 The trunk of the left side is broken once. This group is again divided into i) Group 6A, where there is a connection between accessory hemiazygos vein and left brachiocephalic trunk vein and ii) Group 6B, where there is no connection between accessory hemiazygos vein and left brachiocephalic vein. 7 There are two breaks in the left trunk. 8 There is one break in the left trunk, above and below the break; posterior intercostal veins are joined by collaterals. 9 There are 5 breaks in the lower part of the left trunk, and two single veins, in the lower part formed by joining of 11th and 12th intercostal vein and 10th and 9th intercostal vein. 10 There are 5 breaks in the lower part of the left trunk 11 Type III (It has a single azygos vein that lies on the anterior surface of the vertebral column) TABLE 1 Anson B.J. classification of the azygos venous system drainage pattern.