Azygos

The Azygos Veins

In our Anatomy Lecture today the subject was the Azygos system. You might have a faint recollection of it from Anatomy lectures, or it might be a totally new concept to you. The formal definition of the azygos system is "the H-shaped configuration of the azygos, hemiazygos, accessory hemiazygos veins and left superior intercostal vein" - but that sounds unnecessarily complicated what exactly does this mean? (https://radiopaedia.org/articles/azygos-venous-system)

The Azygos Vein: Etymology

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Well for starters, let's focus on the azygos vein. Derived from the Ancient Greek, the name of the vessel has an interesting history which tells you something of its anatomical structure.


1. Zygon – this is the Greek word for yoke: the structure that joins two oxen (a pair of animals)

together. So in anatomy the zygon part of the word refers to a pair.

2. a – this makes the word negative (in Greek the formal term for this is the alpha privative)

3. Vein – vessel carrying deoxygenated blood

All together

A(not) + zygon (paired) + vein = unpaired vein, so the azygos vein is an unpaired vein, which is

Different to most veins which travel symmetrically in veins. (https://www.dartmouth.edu/~humananatomy/resources/etymology/Thoracic_viscera.htm).

The Nitty Gritty:The Anatomical Details....

So now we have established that the azygos vein is unpaired - where exactly is it? And what does it do?

1. Simply put: the azygos vein carries deoxygenated blood from the abdominal walls and the back of the chest.

 It is formed from the union of the right subcostal vein and the right lumbar vein inferiorly at T12-L2 (https://radiopaedia.org/articles/azygos-vein)

It passes through the diaphragm at the aortic hiatus at the level of T12.

It drains into the Superior Vena Cava at T4. The Superior Vena Cava  takes blood back to the heart. 

Where is it?

a. If you look at the diagram to the left, you can see that the azygos vein is behind the vena cava (the vena cava is indicated by the dotted rectangle in the diagram). 

b. Behind the vena cava is the vertebral bodies (T5-L12), the anterior longitudinal ligament and the right posterior intercostal arteries (https://radiopaedia.org/articles/azygos-vein).

To the right is the: pleura, lung, right greater splanchnic nerve 

To the left is the: thoracic duct, aorta, right vagus nerve, oesophagus, trachea

 

 

2. The extra details:

The Azygos vein drains the right upper quadrant from the fourth posterior intercostal vein to the right subcostal vein.  The posterior intercostal veins contribute to the drainage of the Breast.
 (What about the others? Well the 1st posterior intercostal vein is drained by the brachiocephalic vein or the vertebral vein instead of the azygos vein. Meanwhile, the 2nd and 3rd posterior intercostal veins are drained by the superior intercostal vein instead) - Wikipedia   

The azygos vein also receives drainage from the bronchial, pericardial and oesophageal veins, giving a connection between the lung and the azygos system. Inferiorly, the prostatic plexus from the prostate can drain into the azygos vein through the internal iliac and then lumbar veins.

 

 There are extensive connections between the azygos vein and the Batson Plexus (more on this later!).

Importantly this provides a link between the breast and the lung and the spine, which is exploited by cancer. Metastasis from the breast to spinal cord is extremely common, whereas metastasis from the lung is rarer but possible.

(Why? Well this is because in the lungs, 85% of bloodflow from the lungs goes through the pulmonary vessels rather than through the bronchial and azygos system).

 

 

 

What about the Accessory Hemiazygos and the hemiazygos veins then?

On the right hand side, there are two vessels: the accessory hemiazygos vein and the hemiazygos vein.

- The hemiazygous vein is formed from the union of the left subcostal vein and left lumbar vein. It drains the top left hand side of the thorax i.e. the left upper quadrant, from the 4th to 8th intercostal vessels.

- The accessory hemiazygos vein drains the 9th intercostal vessel to the subcostal vein.

 

 

Causes of an engorged azygos vein: So what could possibly cause an engorged azygos vein? Here are a few options we came up with below - can you think of any more? ....

1. Superior Vena Cava blocked - blood instead goes through azygos, drains through IVC.

2. Right heart failure

3. Tricuspid incompetence for example because of infection (recurrent drug use), myocardial infarction, rheumatic fever, or even strep viridans/ streptococci that reside in the mouth. This is why at the dentists they ask if you have any conditions that could weaken your heart in order to give you prophylactic antibiotics. 

4. Hilar lymphadenopathy?

5. Malignancy

6. Anything that causes a mediastinal mass

A good framework for thinking up differential diagnoses is VITAMIN C.

V- vascular

I - infections

T - trauma

A - autoimmune

M - metabolic

I - idiopathic

N - neoplastic

 

C- Congenital

 

Azygos "bites"

The azygos lobe is....

a congenital variation in the right lung. It’s only seen in less than 1% of the population and is formed in embryogenesis. It’s basically when the azygos vein doesn’t move to the middle of the mediastinum where it is supposed to, and instead stays between the parietal pleura and makes an indentation in the right lung, known as the azygos fissure.

It is often confused for bulla or abscesses – so watch out for these when looking at lung imaging pictures! Chabot-Naud A, Rakovich G, Chagnon K, Ouellette D, Beauchamp G. A curious lobe. Canadian Respiratory Journal : Journal of the Canadian Thoracic Society. 2011;18(2):79-80.