[Aneurysms of the vein of Galen. Angiographic study and morphogenetic considerations]

The angiographic and/or anatomic study of a series of 30 cases of Vein of Galen aneurysm (which is defined as an arterio-venous fistula within the wall of the vein of Galen itself, therefore as a purely extra cerebral lesion) permits the analysis of the afferent arteries, of the efferent veins, and of the aneurysmal sac itself. The examination of the afferent arteries confirms the data from the literature, indicating that most often it consists of the abnormal hypertrophy of otherwise normally organized arteries, including the dural arteries. In particular, the involvement of distal subcallosal branches of the anterior cerebral arteries as well as the arterio-arterial maze extending above the collicular plate, fit well the normal anatomy. The venous side of the malformation nevertheless presents with more unusual features; the most striking is the non-opacification of the straight sinus and/or part of the transverse sinuses in half of the cases; also, an aberrant falcine sinus is frequently observed, which actually represents the persistence of a normal channel in the fetal anatomy. A more complex pattern was observed in two cases in which the aneurysmal sac drained into the straight sinus via a loop made of a falcine sinus, the superior sagittal sinus and then backward through a second falcine sinus. Finally, from the systematization of the afferents arteries, the aneurysmal sac appears to belong both to the (prosencephalic) velum interpositum and to the ambient cistern. Such a vessel cannot be either the true vein of Galen or an internal cerebral vein; therefore it has to be a persistent fetal vein, the median prosencephalic vein, which drains the choroid plexuses of the lateral and third ventricles between the 7th and 12th weeks, and disappears normally to be replaced by the internal cerebral veins, when the intrinsic vascularization of the neural tube develops. The adult pattern of the brain artery being attained during the 8th week, the event that led to the arterio venous fistula should have occurred some time during the 3rd month. Unfortunately, no clue was found to orient to a specific cause for the anomaly. The frequently observed venous occlusions are actually not seen in neonates but rather in older children. This, and the fact that spontaneous thromboses within the malformative veins have been observed in vivo, make likely the conclusion that venous occlusions are a secondary phenomenon only.