Dandy-Walker syndrome; marked dilatation of 4th ventricle: The Dandy-Walker syndrome is a complex malformation of posterior fossa structures. This may be associated with atresia of the foramina of Magendie and/or Luschka. According to one theory the failure of these foramina to open during embryonic development leads to enlargement of the fourth ventricle, with secondary compression of the cerebellum. The latter phenomenon results in failure of normal development of the cerebellum, accounting for the frequent small size of the cerebellum, as demonstrated in this slide. The large cyst in this slide is actually an enlargement of the fourth ventricle and not separate from it. The third ventricle and lateral ventricles are markedly enlarged secondarily.