A chiari malformation (CM) is when part of the skull is too small or misshaped, causing brain tissue to be pressed into the spinal canal.
Chiari malformation Type I
Type I happens when the lower part of the cerebellum (called the cerebellar tonsils) extends into the foramen magnum. Normally, only the spinal cord passes through this opening. Type I – which may not cause symptoms – is the most common form of CM. It is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition. Adolescents and adults who have CM but no symptoms initially may develop signs of the disorder later in life.
Chiari malformation Type II
Individuals with Type II have symptoms that are generally more severe than in Type 1 and usually appear during childhood. This disorder can cause life-threatening complications during infancy or early childhood, and treating it requires surgery.
In Type II, also called classic CM, both the cerebellum and brain stem tissue protrude into the foramen magnum. Also the nerve tissue that connects the two halves of the cerebellum may be missing or only partially formed. Type II is usually accompanied by a myelomeningocele – a form of spina bifida that occurs when the spinal canal and backbone do not close before birth. A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening.
Not all Chiari malformations require surgery.
To learn more, visit National Institute of Neurological Disorders and Stroke.