Vein of Galen Arteriovenous Malformations

Embryology

After closure of the two ends of the neural tube in the fourth week of development, the amniotic fluid no longer supplies the developing central nervous system . During the choroidal stage of embryological development, the MPV provides venous drainage of the anterior cerebral and choroidal arteries during the 6th through 11th weeks of gestation . During continued development, the venous drainage of the choroid plexus is assumed by the paired internal cerebral veins, leading to regression of the MPV in an anterior-to-posterior fashion, with only the most caudal segment of the MPV contributing to the origin of the vein of Galen (VOG) .

In patients with VOGM, the MPV does not undergo its normal regression, with concomitant development of the deep venous system; thus, formation of the normal internal cerebral veins, basal veins, VOG, and straight sinus often does not occur . Rather, persistent fetal venous drainage patterns are often seen , including infratentorial drainage via anterior and lateral pontomesencephalic veins, persistent falcine sinuses, and persistent occipital sinuses. The venous outflow often eventually joins the normal transverse sinus pathway leading to distal venous drainage . Thus embryologically speaking, a more appropriate name for VOGM would have been median vein malformation.

Anatomy and Classification

VOGM, which involves a fetal vein normally absent from postnatal anatomy, as described earlier, should be distinguished from aneurysmal dilatation of the VOG. The latter consists of an anatomically normally developed VOG draining a deep brain arteriovenous malformation (AVM), resulting in enlargement of the vein, as can occur in any brain AVM.

Additionally, the persistent MPV occupies a potential space absent in the normal anatomy, bounded from above by the inferior pial surface of the fornix, and from below by the roof of the third ventricle . Therefore VOGM represents an anatomically extrapial abnormality, as opposed to the pial compartment occupied by brain AVMs, including those involving the mature VOG.

The two most commonly used classification schemes for VOGM are by Lasjaunias and Yasargil.

The Lasjaunias classification is morphological, dividing VOGM into choroidal and mural subtypes. The choroidal subtype has supply from multiple choroidal arterial feeders that converge in a complex network of dysplastic vessels that resembles an AVM nidus, onto the MPV, typically along its anterior-superior wall. In contrast, the mural subtype has a direct fistula in the wall of the MPV, potentially as morphologically straightforward as a single-hole fistula.

The Yasargil classification scheme includes four subtypes, although only three of the four correspond to true VOGM. Yasargil type I has a direct arteriovenous shunt between the arterial feeders and the MPV; this corresponds to the Lasjaunias mural subtype. Yasargil type II has arterial feeders converging onto a nidus prior to draining into the MPV; this corresponds to the Lasjaunias choroidal subtype. Yasargil type III is a hybrid of types I and II with both direct fistulous inflow and interspersed nidal vascular anatomy. Type IV is not a VOGM, but rather a pial AVM with venous drainage into a dilated but mature VOG.

Ultimately, these classification schemes provide a framework to assess each individual case. In considering the anatomy of a VOGM, there should be a thorough description of the arterial supply, the location and structure of the arteriovenous connection, the pathologic venous anatomy (caliber and morphology of the MPV), and the drainage pathway of the deep and infratentorial brain. All these factors should then be taken into consideration, in the context of the clinical presentation and prognosis, to construct a treatment plan.

Arterial supply is most often derived from the choroidal supply, such as the medial and lateral posterior choroidal arteries and occasionally the anterior choroidal artery and the pericallosal artery . Prenatally, the choroidal arterial supply is intimately related to the diencephalic and mesencephalic arteries, and this may be an explanation for the common involvement in VOGM of the thalamostriate perforators from the basilar bifurcation and the proximal segments of the posterior cerebral arteries .

The presence of a VOGM and the embryologic MPV, in most cases, precludes the presence of normal deep venous drainage anatomy . However, it is certainly possible for the normal deep venous system to develop and be utilized for drainage, even in the presence of a VOGM, although angiographic visualization of this normal venous drainage pattern may be obscured by the prominent arteriovenous shunting . Deep venous drainage patterns figure prominently in constructing a safe treatment strategy.

In addition to the course of the venous drainage, the presence of venous outflow restriction is a second important consideration in prognostication and treatment planning. Venous outflow restriction can reduce the quantity of arteriovenous shunting, but may also result in intracranial venous hypertension. The abnormal anatomy eventually will flow into a point of normal venous anatomy (such as the torcular or sigmoid sinus), and venous outflow restriction balances with arteriovenous shunting to create a pattern of clinical presentation. Thus, for example, progressive stenosis at the sigmoid–jugular transition point may herald irreversible global neurological injury and functional decline due to venous hypertension, despite the resulting decreased flow through the malformation.