A Meta-Analysis and Systematic Review of Anatomical Variations in the Drainage Pattern of the Azygos Venous System

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%.