Chiari Malformation

What is a chiari malformation?

The term chiari malformation refers to group of anatomical abnormalities affecting the cerebellum and/or brain stem. There is a spectrum of chiari malformations classified into different types: type I, type II and type III. A type I chiari malforamtion describes a condition where a part of the cerebellum (the cerebellar tonsils) actually descends through the foramen magnum (an opening at the base of the skull containing the brain stem and spinal cord). This abnormally located cerebellar tissue can cause symptoms by putting pressure on the brain stem and spinal cord, or by blocking the normal pathways for spinal fluid circulation. By blocking the normal circulation of spinal fluid, a chiari malformation can result in the formation of a syrinx (the accumulation of spinal fluid with-in the spinal cord).

A type II chiari malformation is a more complex anomaly usually associated with spina bifida. A type II chiari malformation involves both abnormalities of the bone, cerebellum, and brain stem at the cervico-medullary junction. With a type II chiari malformation, the brainstem has an unusual shape, and a large portion of the cerebellum descends through the foramen magnum. A type III chiari malformation is an uncommon and more extreme variant associated severe neurological deficits. Other conditions sometimes associated with chiari malformations include hydrocephalus, syrinx formation, and spinal curvature (scoliosis).

What causes a chiari malformation?

Although some type I chiari malformations can be acquired following drainage of spinal fluid from the lumbar spine, this is uncommon. The vast majority of all chiari malformations are attributed to structural problems that arise during development.

How are chiari malformations discovered?

A type I chiari malformation can be associated with a spectrum of symptoms. Type I chiari malformations can be associated with severe headaches that classically are aggravated by coughing. Some patients with type I chiari malformations may note weakness in the arms and legs as well as a variety of sensory disturbances. Some patients will even note difficulty walking, talking or swallowing.

A type II chiari malformation may be associated with severe difficulty breathing and swallowing. Some patients with type II chiari malformations may experience arm weakness and even quadraparesis.

If a chiari malformation is suspected, the diagnosis is confirmed with an MRI of the brain and spinal cord. An MRI is the best way to demonstrate the anatomic anomalies associated with these malformations.

How are chiari malformations treated?

Type I chiari I malformations with mild or no symptoms may be observed. However, surgery is the only method to treat the symptoms associated with both type I and II chiari malformations. The surgery aims to relieve the pressure placed on the brainstem and spinal cord by the low lying cerebellar tissue. Almost all patients with a type II chiari malformation have hydrocephalus (the accumulation of cerebrospinal fluid in the brain itself) requiring the placement of a shunt.


A) Sagittal T1 weighted MRI of the brain demonstrating the protrusion of the cerebellar tonsils below the foramen magnum (indicated by the white line). This is a type I Chiari malformation.

B) Intra-operative photo demonstrating the cerebellar tonsils.      

C) Post-operative sagittal T1 weighted MRI of the brain demonstrating the new position of the cerebellar tonsils above the foramen magnum (indicated by the white line).