Vein of Galen malformation is most commonly diagnosed during the neonatal period when an infant presents with rapid heart failure. Sometimes the malformation can be diagnosed from an antenatal ultrasound from around 25 weeks of gestation but this is uncommon. Some babies with vein of Galen malformation never develop heart failure, these children may present with an abnormally enlarged head called hydrocephalus (water on the brain), which could also cause the child to have some developmental delay. Older children can present with a large head and developmental delay or learning difficulties. These children may also have seizures and may also suffer a stroke.

Once an infant presents with heart failure, investigations begin to determine the cause of the heart failure. An echocardiogram, a type of ultrasound, is usually carried out . The echocardiogram gives doctors pictures of the heart to help work out the cause of the heart failure. A transfontanelle ultrasound is usually also carried out to help diagnose this condition.

MRI image of patient with a vein of Galen malformation. Image courtesy of Dr Laughlin Dawes, MB, BS.

The following tests may be used once a diagnosis has been made. These tests are used to gather more information and to provide images of the malformation which then can be used to make decisions about treating the condition.

• CT scan (computed tomography) – A CT scan is an advanced imaging technique which uses x-rays to produce detailed cross-sectional pictures of the body. CT shows the organs much better than standard x-rays and is a very common and important examination technique. It can be used to detect blood in the brain and the anatomy of the vessels.
• MRI (magnetic resonance imaging) – An MRI scan produces detailed cross-sectional images of the head and other parts of the body. These images are acquired without using x-rays. MRI uses two naturally occurring forces to generate images: magnetic fields and radio waves. Through a complex process, a computer processes signals received from atoms in the body into a series of 50–200 single images for each examination. These images are then recorded onto large sized films for interpretation.
• Angiogram – This is an important test in the diagnosis of vein of Galen malformation. An angiogram involves fluoroscopy, which enables an interventional neuroradiologist to study a person's blood vessels and organs. The interventional neuroradiologist will insert a small tube, called a catheter, into the blood vessel and then inject a special x-ray dye/contrast. The x-ray dye/contrast makes the vessels visible under x-rays. This will allow the interventional neuroradiologist to observe how the blood travels through the blood vessels of the brain.

If left untreated, a vein of Galen malformation can cause problems with mental development, retardation or in most cases death.