Marsupial cava

Marsupial cava, also known as preaortic iliac venous confluence, is a rare anatomical variant in which the confluence of the common iliac veins is located anterior to the right common iliac artery and aortic bifurcation.

Terminology

The term marsupial cava is in reference to the normal anatomy of marsupials who have an anteriorly placed iliac vein confluence .

Clinical presentation

Marsupial cava is usually an incidental finding, however, the unusual anatomy can complicate procedures in the retroperitoneum, particularly involving the infrarenal aorta and common iliac arteries.

Gross anatomy

Several types of marsupial cava have been described with variations in the location of the common iliac veins and its tributaries :

• both common iliac veins are posterior to their respective arteries and then cross over the right common iliac artery to form a preaortic confluence
• left common iliac vein passes anterior to the aortic bifurcation and the right common iliac vein follows a posterior course along the right common iliac artery
• right common iliac veins follow an anterior course along the right common iliac artery that forms a confluence with the left common iliac vein anterior to the aortic bifurcation 

Development

The anatomic relationship of the inferior vena cava, its tributaries and the aorta is the result of complicated formation and regression of anastomoses between three paired embryological venous channels (posterior cardinal, supracardinal and subcardinal veins). Between the sixth and tenth week of gestation, these channels form circumumbilical venous rings around the primitive common iliac arteries on each side. Normally, the dorsal portion of the ring persists and the ventral limb regresses resulting in an iliac confluence behind the right common iliac artery . Marsupial cavae result from the anomalous persistence of the ventral limb and regression of the dorsal limb.

Practical points

Recognition of this rare anatomical variant in computer tomography is important in planning retroperitoneal and vascular procedures.

Detection of a marsupial cava in the setting of abdominal aortic aneurysms can be made difficult because of flattening and compression of the IVC and its tributaries by the aneurysm . The radiologist and surgeon should be acquainted with this variant so that it can ideally be recognized pre-operatively.

Differential diagnosis

It is also important to not confuse this anatomical variant with lymphadenopathy or other pericaval masses especially on non-contrast-enhanced scans .