A semi-quantitative approach to variation of the azygos vein course.

The azygos vein (AV) is typically described (illustrated) as ascending vertically on the right of thoracic vertebrae. Most thoracic vein studies have focused on tributary patterns, but some have noted more leftward AV courses. This study statistically documents variation in AV course independent of tributary patterns. A more statistical approach to the probable position of AV at different vertical levels may aid clinicians in locating and assessing it in clinical contexts. The AV course was exposed in 84 cadavers by removing overlying viscera between the aortic hiatus and tracheal bifurcation. Subjectively, non-pathological specimens were digitally photographed in anterior view. For each photo, a scaled grid was used to mark the horizontal position of the AV center at each of five vertical levels. The summated numerical distributions showed the following: ∼5% of the AVs ascended on the right side (classical) position, ∼30% did not cross the midline, ∼70% included part or all of their course left of the midline, and ∼14% reached the extreme left side. Based on this data, the modal AV course (1) begins at, or to the right of, the midline, (2) deviates leftward, (3) crosses the midline below mid-level, (4) reaches a leftward maximum at about 3/5 of its course, (5) then deviates rightward (often only reaching the midline at the uppermost level). In several noticeable cases, the leftward maximum was associated with large connections to left-side veins, suggesting a possible tension mechanism exerting traction on the AV over time.