What is the Chiari (Kee-AR-ee) Malformation?
The Chiari Malformation (also called the Arnold Chiari Malformation) is a developmental anomaly at the base of the brain which results in the downward displacement of some of the brain’s structures into the spinal canal. It is characterized by a small or misshapen posterior fossa (the compartment in the back of the skull where the brain normally joins to the spinal cord). It is the largest of many such openings in the skull base. These openings serve as passageways through which nerves leaving the brain and blood vessels going to and from the brain can pass. When a Chiari Malfunction occurs there is a reduction in cerebrospinal fluid pathways and a protrusion of the cerebellar tonsils through the bottom of the skull (foramen magnum) into the spinal canal. The tonsils would normally be round but often become elongated as they protrude down the spinal canal. Diagnosis can be difficult because not all patients will have the classical sign of deeply herniated tonsils.
