Dandy Walker syndrome In this condition there is enlargement of the fluid-filled cavity of the brain known as the fourth cerebral ventricle. It expands due to the pressure of the fluid inside it and forms a large sac-like structure which presses on other parts of the brain. This is a type of hydrocephalus. It is caused by a malformation of the brain known as Dandy Walker malformation. The holes through which the cerebro-spinal fluid usually drains do not open during the development of the fetus (this is known as aqueductal stenosis). The fluid continues to be made by the special lining of the ventricles and so the pressure builds up. Head enlargement may be evident especially at the back of the skull and a prominent brow may also develop. This malformation is often associated with other abnormalities of the brain and sometimes of other organs of the body. In a few of the reported cases there have been hereditary factors but in the rest there has either been no cause identified or it has been associated with another abnormality or syndrome. Operations on the brain are usually successful and a valve may need to be inserted to relieve the pressure. Without a successful operation mental handicap is very likely to occur and there may be a residual problem after operation especially if there are other abnormalities of the brain.