Neuroimaging is a key tool in evaluating and diagnosing those disorders of the craniocervical junction known as “Chiari malformations.” Although linked by their name, these are very different disorders, linked by a need to image the entire brain and the entire spine for proper evaluation. The Chiari I malformation is a disorder in which a small bony posterior fossa results in the herniation of the cerebellar tonsils through the foramen magnum; as a result, the cerebellar tonsils and cervicomedullary junction are compressed, and CSF flow through the foramen magnum is impeded, putting the patient at risk for syringohydromyelia. Proper imaging evaluation is essential to differentiate this condition from tonsillar herniation due to intracranial hypotension (from CSF leaks) or intracranial hypertension (idiopathic or from masses/hydrocephalus) in which treatment is quite different. The Chiari II malformation results from intracranial hypotension in utero due to chronic CSF leakage at the site of an open neural tube defect (myelomeningocele). The myelomeningocele may be repaired prenatally or soon after birth. The many forebrain and hindbrain malformations that result from the chronic intracranial hypotension in utero are described. The definition of the Chiari III malformation is in transition but is most easily described as herniation of upper cervical spinal cord and hindbrain structures (combined cephalocele and myelocele) through a dorsal low occipital and dorsal upper cervical bony defect. Optimal imaging evaluation for these disorders is discussed.
