Chiari 1 Malformation

History: 15 year old boy with headache and arm numbness.

Chiari 1 malformation: Sagittal T2-weighted image at the foramen magnum demonstrates descent of the tip of the cerebellar tonsils (green arrow) below a line between the basion and the opisthion (red line). A cervical syrinx is also noted (yellow arrow).

Chiari 1 malformation: Axial T2WI at the foramen magnum demonstrates ectopia of the cerebellar tonsils (yellow arrows) and effacement of the subarachnoid space.

Chiari 1 malformation: Axial T2WI of the cervical spine shows a syrinx (yellow arrow ).

This is a case of a Chiari 1 malformation, which is is defined as caudal extension of the cerebellar tonsils 5 mm below the foramen magnum. Syringomyelia (syrinx) is seen in one-half of the patients, and it is not associated with other neurulation malformations.

There are three main types of Chiari malformations:

Type I involves the extension of the cerebellar tonsils into the foramen magnum, without involving the brain stem. This is the most common Chiari malformation and is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition.

Type II, also called classic CM, involves the extension of both cerebellar and brain stem tissue into the foramen magnum, and is usually accompanied by a myelomeningocele. A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening.

Type III is the most serious form of CM. The cerebellum and brain stem protrude, or herniate, through the foramen magnum and into the spinal cord and the herniated cerebellar tissue can enter an occipital encephalocele. The meninges or spinal cord can also protrude through an abnormal opening in the back or skull.