<unk>  is<unk>  (infolding) of the base of the skull that occurs when the top of the C2 vertebra migrates upward. It can cause narrowing of the foramen magnum (the opening in the skull where the spinal cord passes through to the brain). It also may press on the lower brainstem. This is similar to Chiari malformation. That, however, is usually present at birth.
Basilar invagination
Nasal glial heterotopia
Microlissencephaly
Lissencephaly
Schizencephaly
Hemihydranencephaly
Exencephaly
Microhydranencephaly