A vein of Galen malformation starts during early prenatal development, as early as the first trimester. In normal prenatal development, a large vein forms at the base of the brain and is eventually replaced by a true vein of Galen. When a child has a vein of Galen malformation, high-pressure, oxygen-rich blood from the arteries flows directly into the initial vein, preventing development of the actual vein of Galen.
Without the vein of Galen, oxygen-rich blood is unable to flow slowly through capillaries that deliver this blood to surrounding tissues. Since there is also no slowing of the blood flow, it rushes quickly away through the vein, causing a constant rush of blood to the heart and lungs. The heart and lungs have to work extra hard to keep up with the volume of blood, which may result in congestive heart failure or pulmonary hypertension. These conditions are often life-threatening.