Hindbrain herniation: A review of embryological theories.

Herniation of the hindbrain outside of the setting of intracranial hypertension, trauma, and brain tumors is an uncommon phenomenon with estimated incidence of less than 1%. In the late 1890's, Hans Chiari, a German pathologist, classified hindbrain herniation into three forms. This classification was then extended to include six types. We reviewed the current literature for the proposed embryological theories as well as the potential genetic mutations/syndromes associated with the hindbrain herniation or Chiari malformation. The review is illustrated by a unique cadaver with Chiari type I malformation (i.e. herniation of the cerebellar tonsils through the foramen magnum). Finally, it seems that no single theory could explain all forms of the Chiari malformation, and that this malformation might be a heterogeneous entity.