Vein of Galen malformation is a large intracranial arteriovenous shunt that develops during the 1st trimester of pregnancy.

The origin is failure of the embryonic median vein of prosencephalon to obliterate. Multiple arteries feed directly to a large median venous sac resulting in a low resistance intracranial circulation. Most studies report an equal male to female preponderance. It is a rare congenital anomaly (<1/25,000 live births), although the exact incidence is not known. The genetic basis is heterogeneous. Diagnosis may occur in the 3rd trimester of pregnancy if routine ultrasound is undertaken at this stage. Post-natal diagnosis is usually early due to the development of cardiac failure soon after birth. Blood flows preferentially through the low-resistance intracranial AVM causing volume overload and failure of the right ventricle with pulmonary hypertension, and multi-organ failure. Neonates can be shocked at presentation and may need to be intubated on NICU with inotropic support. In less severe cases, post-natal diagnosis is made later with hydrovenous features, namely mild cardiac failure, failure to thrive, increased head circumference and/or developmental delay.