Chiari Malformations

INTRODUCTION

Chiari malformations are a heterogeneous group of disorders that are defined by anatomic anomalies of the cerebellum, brainstem, and craniocervical junction, with downward displacement of the cerebellum, either alone or together with the lower medulla, into the spinal canal.

ANATOMIC ASPECTS

Description — Chiari malformations are congenital conditions that are defined by anatomic anomalies of the craniocervical junction with downward displacement of the cerebellar structures. Chiari malformations are often associated with spinal cord cavitations (ie, syringomyelia). In most cases of Chiari malformations, the posterior fossa is small, and neural elements are crowded and impacted at the foramen magnum.

Classification — Chiari malformations were first described by John Cleland in 1883. They were later classified by Hans Chiari in 1891, into four groups.

●Chiari type I malformation (CM-I) is characterized by abnormally shaped cerebellar tonsils that are displaced below the level of the foramen magnum.

●Chiari type II malformation (CM-II), also known as Arnold-Chiari malformation, is characterized by downward displacement of the cerebellar vermis and tonsils, a brainstem malformation with beaked midbrain on neuroimaging, and a spinal myelomeningocele.

●Chiari type III malformation (CM-III) is rare and combines a small posterior fossa with a high cervical or occipital encephalocele, usually with displacement of cerebellar structures into the encephalocele, and often with inferior displacement of the brainstem into the spinal canal.

●Chiari type IV malformation (CM-IV) is now considered to be an obsolete term that describes cerebellar hypoplasia unrelated to the other Chiari malformations.

●Other subtypes (not widely used) have since been defined. These include the Chiari 0 malformation, characterized by anatomic aberration of the brainstem (posterior pontine tilt, downward displacement of the medulla, low-lying obex) but with normally placed cerebellar tonsils, and the Chiari 1.5 malformation, which is a CM-II like malformation without spina bifida. Both of these subtypes show crowding at the foramen magnum.