Some focal cystic abnormalities of the brain may be associated with other intracranial abnormalities such as hydrocephalus, whereas others are isolated findings. Some cystic intracranial abnormalities are typically midline. These include Dandy-Walker malformation and vein of Galen aneurysm. Other cystic intracranial abnormalities are usually lateral but can occur within the midline, such as arachnoid cyst or brain tumors. In addition, complex intracranial abnormalities, such as agenesis of the corpus callosum or alobar holoprosencephaly, may have midline cystic components (Table 10-8 ).