The hemiazygos vein is a vessel that runs along the left side of the spine in the lower chest, collecting blood from the lower left intercostal spaces and channeling it across the vertebral column to the azygos vein on the right. It sits in a region of the body most people never think about until something goes wrong, yet it plays an outsized role in emergency circulatory detours when major veins become blocked. The vessel is also one of the most variable structures in human anatomy, which makes it both a fascinating subject for anatomists and an occasional headache for surgeons and radiologists.
Where It Sits and What It Does
The hemiazygos vein begins in the lower left chest, typically forming from the junction of the left subcostal vein and the lower left posterior intercostal veins around the level of the eleventh or twelfth thoracic vertebra. From there it ascends along the left side of the vertebral column, behind the thoracic aorta. At roughly the level of the eighth or ninth thoracic vertebra, it crosses the midline, passing in front of the vertebral body and behind the aorta and esophagus, to empty into the azygos vein on the right side. That crossover is sometimes called the hemiazygos arch.
Its primary job is venous drainage. The left lower intercostal veins, which collect deoxygenated blood from the chest wall between the ribs, feed into it. Some of that blood also arrives from small mediastinal and esophageal tributaries. Once collected, this blood is delivered to the azygos vein, which ultimately empties into the superior vena cava and returns to the heart. Think of the azygos system as a secondary highway running alongside the two big expressways of venous return, the superior and inferior venae cavae. The hemiazygos is the left-hand lane of that secondary highway.
The Accessory Hemiazygos Vein
Sitting just above the hemiazygos on the left side of the spine is a companion vessel, the accessory hemiazygos vein. This vessel typically collects blood from the middle left intercostal spaces, roughly the fifth through the eighth. It descends along the left vertebral column before crossing over to join the azygos vein at a slightly higher level than the hemiazygos, often around the seventh or eighth thoracic vertebra. In some people the two veins merge on the left side before sending a single trunk across to the azygos; in others they cross independently.
Variations in the accessory hemiazygos vein are common. In one documented cadaveric case, the accessory hemiazygos ascended rather than descended, traveling upward from the eighth thoracic vertebra to the sixth before crossing to the azygos, essentially reversing its expected direction of flow.1PubMed Central. A Unique Variation of Azygos System of Veins Another case documented the hemiazygos and accessory hemiazygos both located entirely on the left side, crossing in front of the aorta and behind the esophagus to drain into the azygos at the levels of the ninth and seventh thoracic vertebrae respectively.2PubMed Central. An abnormal course of the interazygos vein: a case report These are not just anatomical curiosities; they matter when a catheter needs to be threaded through the chest or when a surgeon encounters unexpected anatomy on the operating table.
Why the Anatomy Is So Variable
The hemiazygos vein develops from a structure called the left supracardinal vein during fetal life. Early in embryonic development, paired veins run along both sides of the spine. As the fetus grows, most of the venous drainage on the right side consolidates into the azygos vein, while the left-side remnants form the hemiazygos and accessory hemiazygos. The lower portion of the left supracardinal vein becomes the hemiazygos, and the upper portion becomes the accessory hemiazygos.3National Journal of Clinical Anatomy. Variation of hemiazygos vein with absence of accessory hemiazygos vein – a dissection study Communications between the right and left supracardinal veins form the cross-connections that later become the transverse arches linking the hemiazygos to the azygos.
Because this developmental process involves selective regression and persistence of embryonic vessel segments, even small differences in timing or signaling can produce a different adult configuration. A cadaveric study of 30 adults classified the azygos system into three broad types and found that the most common pattern, which features the expected pair of cross-connections between the left and right sides, appeared in about 90% of cases. A primitive single-column pattern appeared in roughly 3% of cases, and a unicolumnar arrangement where the left-side veins were essentially absent appeared in about 7%.4European Journal of Anatomy. Analysis of multiple variations in azygos venous system anatomy with its classification: A cadaveric study That means roughly one in ten people may have a meaningfully different arrangement from what textbooks show as “normal.”
The Hemiazygos as an Emergency Detour
One of the most clinically significant roles of the hemiazygos vein is something it does only when called upon: serving as a collateral pathway when a major vein is blocked. If the inferior vena cava becomes obstructed, whether from a blood clot, a tumor, or scar tissue, blood from the lower body needs an alternate route back to the heart. The azygos and hemiazygos system is one of the primary detours that opens up in this situation.5Clinical Anatomy. A review of venous collaterals in inferior vena cava obstruction Blood that would normally travel up the inferior vena cava instead flows through the ascending lumbar veins into the azygos and hemiazygos, which carry it upward along the spine and deliver it to the superior vena cava.
The same principle applies when the superior vena cava is blocked, a condition known as superior vena cava syndrome. In that scenario, blood from the upper body that cannot pass through the superior vena cava can reverse course through the azygos and hemiazygos veins, traveling downward to reach the inferior vena cava instead. In one documented case of superior vena cava obstruction, collateral formation was observed through the azygos and hemiazygos veins, which joined inferiorly to form a single vessel draining into the inferior vena cava, while multiple left intercostal veins formed communications with the azygos vein to reroute blood flow.6PubMed Central. The Aftermath of Obstruction: Decoding Collateral Pathways in Superior Vena Cava Syndrome
Other collateral routes exist, including pathways through the internal mammary veins, the lateral thoracic veins, and the vertebral venous plexus.7PubMed. Where there is blood, there is a way: unusual collateral vessels in superior and inferior vena cava obstruction But the azygos-hemiazygos pathway tends to be the most prominent, especially in chronic obstructions of the lower inferior vena cava.8Clinical Anatomy. A review of venous collaterals in inferior vena cava obstruction When radiologists see a noticeably enlarged hemiazygos vein on a chest scan, it can be a clue that something else in the venous system is blocked and the body is compensating.
The Hemiazygos and Portal Hypertension
The hemiazygos vein also plays a role in a different kind of circulatory rerouting: the portosystemic shunting that occurs in liver disease. When the portal vein, which carries blood from the gut to the liver, faces elevated pressure due to cirrhosis or other causes, blood seeks alternative escape routes. One of the better-known consequences is esophageal varices, swollen veins in the lower esophagus that can rupture and bleed dangerously.
These varices drain predominantly into the azygos vein, but a study of 155 patients with portal hypertension found that the hemiazygos vein served as the predominant drainage pathway for esophageal varices in about 8% of cases.9PubMed. Collateral pathways of the left gastric vein in portal hypertension While the azygos side dominates, awareness that the hemiazygos can also be carrying high-flow variceal blood matters when interventional radiologists plan procedures like transjugular intrahepatic portosystemic shunts or embolization of bleeding varices.
Congenital Anomalies Involving the Hemiazygos
Some of the most dramatic variations involving the hemiazygos vein are congenital conditions where other major veins fail to form properly during fetal development. In a condition called left-sided inferior vena cava with hemiazygos continuation, the segment of the inferior vena cava that normally runs through the abdomen is absent on the right side. Instead, the inferior vena cava forms on the left and continues upward as the hemiazygos vein, which eventually crosses to the azygos and reaches the heart that way. In some cases, this is paired with a persistent left superior vena cava, creating a highly unusual venous circuit where all major venous return runs through left-sided channels.
These anomalies are associated with a developmental pattern called left isomerism, where the normal left-right asymmetry of the body’s organs is disrupted. While the venous rerouting itself does not usually cause symptoms on its own, it creates serious challenges if heart surgery is ever needed. The inferior vena cava is not in its expected location, making standard venous cannulation for cardiopulmonary bypass impossible. The hepatic veins, which normally drain through the inferior vena cava, instead empty directly into the right atrium, requiring a different cannulation strategy. In patients requiring single-ventricle repair, knowing about hemiazygos continuation can change the entire surgical approach.10BMJ Case Reports. Hemiazygos continuation of isolated left-sided inferior vena cava into persistent left superior vena cava: rare association of left isomerism The anomaly can also predispose to deep vein thrombosis in the lower limbs because of venous stasis through the longer, less efficient drainage pathway.
In one particularly complex case, a hemiazygos vein was found draining into a persistent left superior vena cava, which in turn emptied into a significantly dilated coronary sinus at the back of the heart. The hemiazygos in that individual was collecting blood from a wide territory, including the ninth through eleventh right posterior intercostal veins, the right subcostal vein, and the fifth through eleventh left posterior intercostal veins.11PubMed. A rare variation of the hemiazygos vein draining into the persistent left superior vena cava The entire normal drainage pattern was essentially reorganized, with the hemiazygos serving as the central trunk for nearly all posterior chest wall drainage.
Why Surgeons and Radiologists Care
The clinical stakes of hemiazygos anatomy extend well beyond academic interest. Peripherally inserted central catheters, pacemaker leads, and other devices that are threaded through the venous system can end up in unexpected locations when the azygos system is abnormal. A catheter tip intended for the superior vena cava might follow a persistent left superior vena cava down into the coronary sinus, or a pacemaker lead might end up in the hemiazygos rather than the right atrium. Knowledge of variant anatomy helps prevent misplacement, which can damage vascular structures or cause hemorrhage.12PubMed. A rare variation of the hemiazygos vein draining into the persistent left superior vena cava
The reverse scenario also arises: when the normal venous routes are blocked, the hemiazygos system can become the path of last resort for device delivery. In a case of complete superior vena cava occlusion, clinicians successfully implanted transvenous pacemaker leads through the accessory hemiazygos vein, threading them across to the azygos and down into the heart.13PubMed Central. Transvenous lead implantation via the accessory hemiazygos vein in superior vena cava occlusion This creative use of the collateral pathway underscores why understanding hemiazygos anatomy is not just academic trivia for interventional cardiologists and electrophysiologists.
Connections between the accessory hemiazygos vein and the left brachiocephalic vein have also been documented in cadaveric dissection. These rare communications create additional pathways that could be encountered during central line placement or thoracic surgery. Awareness of such variants helps prevent iatrogenic bleeding and supports accurate radiological diagnosis when unexpected vascular structures appear on imaging.14PubMed. Cadaveric dissection of connection between accessory hemiazygos vein and left brachiocephalic vein
When the Hemiazygos Mimics Something Worse
An enlarged or aneurysmal hemiazygos vein can cause diagnostic confusion on imaging. Hemiazygos aneurysms are rare, but when they occur they can look remarkably like an enlarged lymph node or even a mediastinal tumor on a standard chest CT scan. Because routine CT scans are not always timed to capture the venous phase of contrast enhancement, the hemiazygos may not be well opacified, making a dilated vein difficult to distinguish from a solid mass.
In one reported case, a hemiazygos vein aneurysm was not diagnosed preoperatively because the azygos system was not enhanced on the standard chest CT. Only delayed contrast-enhanced imaging, which gives the venous system time to fill with contrast dye, can reliably confirm whether a suspicious structure is connected to the azygos venous system and is therefore vascular rather than a tumor.15PubMed Central. Hemiazygos vein aneurysm mimicking an enlarged lymph node The lesson from such cases is that when a radiologist spots an unexplained rounded structure near the left side of the spine in the posterior mediastinum, a dilated hemiazygos vein should be on the list of possibilities. Delayed-phase imaging or MRI can usually sort it out without the need for a biopsy or surgery.
This kind of mimicry is not limited to aneurysms. Any cause of hemiazygos enlargement, whether from collateral flow due to vena cava obstruction, portal hypertension, or congenital anomaly, can produce a soft-tissue density near the spine that raises concern on imaging. The key differentiator is almost always contrast enhancement timing: if the structure lights up on delayed venous-phase imaging and connects to the azygos system, it is a vein, not a node or mass.
Comparative Anatomy Across Species
The azygos system is not unique to humans. Comparative anatomical studies have examined the azygos venous system in species including monkeys, dogs, cats, rats, and rabbits.16PubMed Central. A comparative study of the azygos venous system in man, monkey, dog, cat, rat and rabbit The system varies considerably across mammals. In some species, the azygos vein is predominantly left-sided rather than right-sided. Dogs, for example, tend to have a single azygos vein on the right, while in cats the azygos often courses on the left before crossing over. These interspecies differences reflect differing patterns of regression and persistence of the same embryonic supracardinal veins that produce human variations, suggesting that the developmental program underlying the azygos system is evolutionarily ancient but loosely constrained. The high variability seen in humans fits a broader mammalian pattern: the posterior thoracic venous system is simply less tightly regulated during development than, say, the aorta or pulmonary arteries.
For veterinary surgeons, the same practical concerns that matter in human medicine apply. Placing a central venous catheter or performing thoracic surgery in a dog or cat requires awareness that the azygos and hemiazygos anatomy may not match the textbook for that species, just as it may not in a human patient. The fundamental lesson is consistent across species: the azygos system is a flexible, variable network, and the hemiazygos vein is its most unpredictable element.