Chiari I malformation involves herniation of the peg-like cerebellar tonsils into the foramen magnum (>5 mm in patients <15 years old), and is associated with syringomyelia.
Chiari II malformation is more severe, involving herniation of the medulla and vermis and elongation and downward displacement of the brainstem. This malformation is virtually always associated with a meningomyelocele. Prenatal US may show crowding of the posterior fossa (“banana” sign appearance of the cerebellar hemispheres as they wrap around the brainstem). There may also be kinking of the medulla, agenesis of the corpus callosum, polygyria, hydrocephalus, and colpocephaly (disproportional enlargement of the posterior horns of the lateral ventricles).
Chiari III malformation involves herniation of contents of the posterior fossa through the occiput or upper cervical canal via a bony defect (i.e., akin to an encephalocele).