The hemiazygos system (HV and AHV), together with the left superior intercostal vein, drains the lower eight left posterior intercostal and upper four left posterior intercostal veins (LPIVs) into the azygos and left brachiocephalic veins, respectively (Dahran & Soames, 2015).
In the third type 2A cadaver a longitudinal vein connected to the left superior intercostal vein, which then descended to drain the 4th to 9th left posterior intercostal veins terminating by joining the HV at the level of T10-T11 after crossing anterior to the 10th left posterior intercostal vein: this longitudinal vein can be considered an AHV draining into the HV.
The origin of the AHV was highly variable as it was formed at different levels from the union of the posterior intercostal veins and often communicated with the left superior intercostal vein. Table I shows the mean diameters of the origin and termination of each vein of the azygos system.
In 53 % of cadavers the left superior intercostal vein communicated with the azygos system, either directly without communicating with the AHV (7 % of cadavers) or via the AHV with or without communicating with the AV (46 % of cadavers): it drained into the left brachiocephalic vein in all cadavers.
In one type 2A a long vein was observed to communicate with the left superior intercostal vein and then descend to receive tributaries from the 4th to 9th left posterior intercostal veins: it terminated by joining the HV at the level of T10-T11 after crossing anteriorly to the 10th left posterior intercostal vein.