The Anatomy of the Azygos Vein

The azygos vein is a singular blood vessel of the torso that ascends on a course just to the right of the spine. It’s part of the system that drains blood from the mediastinum (the tissues between your lungs), as well as parts of the back and abdominal walls. Arising in the mid-low back, it bends around the hilum of the lungs and passes through the diaphragm before piercing the pericardium of the heart.

Laceration (deep cut or tear) and bleeding of this internal vein is a clinical concern, and it can be affected by obstructions of surrounding veins, among other conditions. In very rare cases, congenital abnormalities affect the development of the azygos vein, something associated with some types of heart disease or asplenia (absence of the spleen).

Anatomy

Structure and Location

A larger vein, the azygos is about 0.9 centimeters in diameter. It most commonly arises at the junction of the right ascending lumbar and the right subcostal veins, in the lower back, though it can also emerge directly from the inferior vena cava (IVC) vein. It then enters the abdomen (or thorax) through the aortic hiatus, an opening in the diaphragm.

The azygos moves immediately upward, crossing through a triangular space running parallel to the spinal column called the posterior mediastinum. At the level of the T4 vertebra, between the shoulder blades, it curls above the hilum of the right lung, which is the part that attaches this organ to the lung root.

Having crested over the lung, at the level of the T4 vertebra, the azygos vein drains into the superior vena cava. It then completes its course by piercing the pericardium, which is the tissue surrounding the heart.

The azygos vein also has two tributaries (branches of the main vein):

• Hemiazygos vein: Often arising from the left ascending lumbar vein, near the kidney, this vein ascends parallel and to the left of the spinal column. Passing behind the esophagus, it crosses over to connect to the azygos vein.
• Accessory hemiazygos vein: Draining the superior left hemithorax as well as veins of the esophagus, the accessory hemiazygos vein descends to the left of the spinal column. It crosses over to join the azygos vein, or sometimes the hemiazygos vein, at the seventh vertebra.  

Anatomical Variations

Congenital abnormalities of the azygos vein are relatively common, and they’re usually asymptomatic. Among the most commonly seen such variations are the following:

• The azygos vein runs up the midline instead of to the right of the spinal column.
• The origin of the azygos can be further up than usual, causing a part of it to run behind the spinal column.
• The hemiazygos and accessory hemiazygos veins form a common trunk before joining the azygos vein.

In addition, there are a couple of other, rarer genetic variations:

• Agenesis of the azygos: This is when the azygos vein fails to develop. However, the hemiazygos and accessory hemiazygos veins are usually able to compensate for this absence.
• Azygos lobe: This occurs in 0.4 to 1% of the population. One of the precursors to the azygos vein, the right posterior cardinal vein develops too far from the center of the body. This causes a small lobe to develop in the right lung and can cause an aortic nipple, soft tissue near the aorta.
• Azygos continuation: When the development of the IVC in the womb is interrupted, the azygos vein and its tributaries become the primary pathway blood takes on its way back to the heart. This causes the vein to widen. Though generally asymptomatic, this condition is associated with heart disease and asplenia (absence of the spleen).

In addition, physical conditions, injuries, or diseases that affect blood flow can also impact the structure of this vein.

Function

Along with the hemiazygos and accessory hemiazygos veins, the azygos vein is part of what’s called the azygos system. Primarily, this system is tasked with carrying blood from the walls of the upper abdomen as well as the upper portion of the lower back (the upper lumbar region) back to the heart. It also drains the mediastinum, the tissues between the lungs.

In addition, the azygos vein can serve a critical role in cases of obstruction between the inferior and superior vena cava veins. Its central location and connections with many other veins make it a good alternative route for blood to take. The increased volume of material passing through these vessels can make them larger.

This vein may also be sought out by doctors during fluoroscopy, a type of imaging that can help doctors visualize blood flow in real time. Because of the azygos vein’s many connections to other veins in and around the heart and lungs, doctors inject it with the radioactive dye used to improve contrast with this form of imaging.

Clinical Significance

Given its location in the body and function as part of the circulatory system, diseases and conditions affecting the azygos vein can have a serious impact. This vein can be impacted by everything from endemic conditions, circulation or heart problems, or physical injury.

Laceration

Due to falls or motor vehicle accidents, the azygos vein can be cut or rupture. This can lead to pneumothorax, a pooling of blood in the pleural space (the space between the membranes that overlay the lungs). These are detected using X-ray and call for a thoracotomy, a surgical procedure in which the blood is drained through an incision in the chest.

Aneurysm

Aneurysm, a weakening and bulging of the vessel walls, can occur due to heart failure, internal bleeding, high blood pressure in the portal vein, and blockage in the inferior vena cava. This is often asymptomatic, but surgery may be necessary if there’s a risk of rupture or blood clots reaching the lungs (pulmonary embolism).

Superior Vena Cava Syndrome

When blood flow from the superior vena cava to the right atrium of the heart is obstructed, an insufficient amount from the head and neck is drained. This can cause blood flow to reverse—to move away from the heart—leading to breathing problems, lightheadedness, and swelling.

Computerized tomography (CT) imaging is used for diagnosis, and surgery or medications are used to take on superior vena cava syndrome.

Fibrosing Mediastinitis

This very rare condition causes scar tissue to form in the mediastinum, obstructing blood flow. Fibrosing mediastinitis arises most commonly due to fungal or bacterial infection, though it’s also associated with autoimmune disorders like Behcet’s disease, and other conditions.

X-ray imaging detects the presence of fibrosing mediastinitis growths. Treatments for this often asymptomatic condition range from surgery to remove scar tissue to taking medications, among other therapies.

Inferior Vena Cava Syndrome

When the inferior vena cava is obstructed, additional vessels form and the azygos can swell in size. As a result, lesions can form in the vein, and an insufficient amount of blood is able to return to the heart. Breathing problems, swelling, cognitive issues, and heart arrhythmia are among the symptoms.

Following imaging to locate the source of the obstructions, surgery or blood-thinning medications may be used to take on the condition.

Complications of Medical Devices

Since the azygos vein may be a site of catheter injection (as in cardiac catheterization)—or as a result of pacemakers placed near the heart—complications can develop such as pneumothorax or laceration of the azygos vein. Implanted devices or pieces of medical equipment left in the area can also cause abnormal tissue growths (fistulas) to form and cause blood clotting.

As with other conditions, the key to treatment is to remove the source of the problem. X-ray or CT scans are used, and surgery, among other options, may become necessary.