The azygos vein is located along the right side of the thoracic vertebral column, and it carries deoxygenated blood from areas of the chest and abdomen to the heart. Unlike most veins and arteries, it does not have a corresponding vein on the left side of the body, thus explaining its name, which means "unpaired" in Greek. Veins that perform a similar function can be found on the body's left side, but these are considered tributaries and thus only parts of this vein system rather than equivalent body structures. Some of these tributaries on the left side are the hemiazygos vein and the posterior intercostal veins. Blood in azygos vein empties into the superior vena cava, which then carries it to the heart to be recirculated.
Although many veins and arteries in the body have a similar structure and function in all individuals, the azygos vein system can be configured differently from person to person. Sometimes the vein system feeds different areas of the abdomen, In some cases, it not only serves the abdominal and thoracic walls, it also receives blood from bronchial and gonadal veins. Structurally, this system is different in various species of mammals, as well. Humans, dogs and cats do not have paired azygos veins, but cows, sheep and other ruminants do.
Sometimes the thoracic and cervical veins can become compromised so that they do not function as well as they should. This is referred to as chronic cerebrospinal venous insufficiency (CCSVI), which can lead to decreased brain function as well as stenosis, or abnormal narrowing, of the azygos vein. Overall narrowing or blocking of the veins that drain the upper body causes deoxygenated blood to pool in the brain, causing edema, and it slows the delivery of oxygenated blood back to the brain, depriving the brain of much-needed oxygen.
Paolo Zamboni, an Italian vascular surgeon and scientist, described CCSVI in 2008 and cited research indicating that it is very common in patients who have multiple sclerosis and that it might cause many of the wide range of symptoms associated with this devastating disease. Zamboni's studies included close observation and testing of the azygos vein system in subjects with multiple sclerosis. Some doctors and scientists believe that further study and treatment of CCSVI and stenosis of the azygos vein system could result in new, more effective treatments for multiple sclerosis.