Median prosencephalic vein

The median prosencephalic vein is a precursor to the vein of Galen, and usually regresses before birth. If an arterio-venous fistula exists, it balloons out to become the 'aneurysmal' vein of Galen malformation. This occurs since the vein is thin-walled, lies free within the subarachnoid space, and is unable to accommodate the arterial flow.