Venous embryology is one of the most neglected parts in basic medicine by the clinicians and a lack of the basic knowledge in embryology often misguides clinical management of anomalous venous condition mostly developed during the embryogenesis, including ‘truncular’ venous malformation.
The truncular venous malformation (VM) represents an embryologically defective vein occurred during the vascular trunk formation period in the‘later stage’of the embryonic development; the developmental arrest involves the already formed, established venous trunk to varying degrees. These often result in immature or incomplete vessels (aplasia or hypoplasia), or as hyperplastic vessels (hyperplasia).
Truncular VM lesions therefore, present either as a hypoplastic or hyperplastic lesion causing obstruction or dilatation (e.g. internal jugular vein aneurysm; azygos vein stenosis), depending on the various defective conditions of the vessel trunk. Intraluminal defects within the vein (e.g. vein webs or membrane) result in stenosis or obstruction as well.
Truncular VM lesions present with more serious hemodynamic consequences in general due to the direct involvement of the lesion with the truncal venous system (e.g. avalvulosis, marginal veins, popliteal vein aneurysm, IVC stenosis/occlusion), compared to extratruncular lesions.
As a consequence, chronic venous congestion and hypertension due to venous reflux or occlusion results in more tissue and organ damage along the venous capacitance; membranous/focal/segmental stenosis of suprahepatic inferior vena cava, known as Primary Budd-Chiari Syndrome, has such a profound impact on the liver resulting in severe portal hypertension due to hepatic venous outlet obstruction in addition to chronic venous insufficiency affecting the lower extremities .
The cerebrospinal venous circulation is not exempt from truncular VMs. Cerebrospinal VMs also carry the potential risk of long-term chronic venous hypertension to the brain resulting in various clinical conditions/illnesses such as chronic cerebrospinal venous insufficiency (CCVI).
An example of CCVI, internal jugular vein valve incompetence (IJVVI) has been postulated to be a cause of transient global amnesia (TGA). IJVVI is diagnosed when retrograde jugular vein flow is detected by extracranial duplex ultrasound during Valsalva maneuver.
It is believed that IJVVI may produce transient mesiotemporal ischemia by venous congestion. This mechanism requires a patent venous pathway from the affected IJV through the transverse sinus, confluence, straight sinus, and vein of Galen into the basal vein of Rosenthal and into the internal cerebral veins.
Truncular VM lesions causing stenosis along the internal jugular, innominate, superior vena cava, and azygos vein system, have also been suspected to contribute to the development or exacerbation of multiple sclerosis.
Therefore, proper understanding of the evolutional as well as involutional process of embryonic development of the venous system is essential for the appropriate assessment of these truncular VM lesions as a potential cause of various conditions involving CCSVI.
By the beginning of the fourth week of gestation, an extensive network of blood vessels is formed throughout the embryonic body; the evolution and development progresses to form the venous system; the portion of the body cephalad to the developing heart (head, neck, upper torso & upper limbs) drains through the ‘bilateral anterior cardinal veins’ also known as the precardinal veins.
Paired anterior cardinal veins soon develop an anastomosis to form the brachiocephalic (innominate) vein allowing blood to drain from the ‘left anterior cardinal vein’ into the ‘right anterior cardinal vein’ as an evolving process.
The portion of the left anterior cardinal vein distal (cephalad) to the anastomosis, becomes the ‘left internal jugular vein’ and joins the ‘left subclavian vein’.
The distal (cephalad) portion of bilateral precardinal (anterior cardinal) veins become the bilateral internal jugular veins and the blood from the left internal jugular vein passes through the left brachiocephalic veins instead and directly into the superior vena cava (SVC).
The left anterior cardinal vein proximal (caudal) to the brachicephalic anastomosis regresses/atrophies with the terminal segment of the left posterior cardinal vein.
On the right side, the proximal part of the right anterior cardinal vein remains forming the SVC with the right common cardinal vein; the rest of the cardinal veins form the arch of azygos vein, SVC and its tributaries.
Superior Vena Cava (SVC): The proximal part of the right precardinal/anterior cardinal vein forms the SVC with the right common cardinal vein. These veins are also involved in the formation of the arch of azygos vein together with the proximal segment of the right postcardinal/posterior cardinal vein.
Azygos Vein: The right supracardinal vein remains as ‘azygos vein’ together with the cephalad part of the right posterior cardinal vein to form the arch of azygos vein. The left supracardinal vein becomes the hemiazygos vein and accessory azygos vein.
Due to the complex development and various stages of evolution and involution of multiple paired embryonic veins, several variations and anomalies could develop easily as a result where embryological connections persist alone or in conjunction with aplasia or hypoplasia of normally developing channels. These variations involve the great veins from the common cardinals, primitive jugular, and anterior and posterior cardinal veins.
For example, the likelihood of SVC variable anatomy is relatively high due to the involvement of three different embryonic vein segments. A left sided SVC or double SVC may develop from a ‘persistent’ left common cardinal and precardinal veins. This is the best known intrinsic defect to cause varying degree of stenosis and obstruction together with another condition of venectasia and aneurysm causing venous dilatation.
In view of such profound impact of a relatively simple truncular venous malformation such as a venous web at the hepatic venous outlet causing portal hypertension, a similar condition involving the head and neck venous system may cause CCSVI and may be involved in the development or exacerbation of multiple sclerosis.