Chiari Malformation

A Chiari malformation is a growth abnormality where brain tissue in the lower back of your skull extends into your spinal canal (the base of your skull).

It happens because of a structural problem like a smaller-than-expected skull size. If you don’t have enough room in your skull, part of your brain, specifically your cerebellum, will grow downward where there’s extra space in an opening at the base of your skull — called the foramen magnum.

Your cerebellum is the part of your brain that helps with your muscle movements, posture, balance, speech and coordination. It causes symptoms that affect the function of this part of your brain, like loss of balance.

This growth abnormality causes your brain to press against the base of your skull. It blocks the flow of cerebrospinal fluid. This fluid cushions your brain and spinal cord, circulates nutrients and chemicals, and removes waste products.

What are the types of Chiari malformation?

There are five types of Chiari malformations including:

• Chiari malformation type 0: Type 0 is very rare. In this type, there are little to no parts of your cerebellum in the hole at the base of your skull (foramen magnum) but there is crowding at that level. Symptoms occur because of an abnormal flow of cerebrospinal fluid near the base of your skull.
• Chiari malformation type I: Type I occurs when the lower part of your cerebellum extends into the hole (foramen magnum) at the base of your skull. Your spinal cord normally passes through this hole. Type I is the most common form of Chiari malformation. People with type I may not have any symptoms or they may not develop symptoms until they’re adolescents or adults.
• Chiari malformation type II: Type II occurs during brain and spine development in the uterus. With a type II malformation, your cerebellum and brain stem grow abnormally and cause pressure inside of your skull. Type II usually happens with a severe form of spina bifida called myelomeningocele. Myelomeningocele occurs when the backbone and spinal canal don’t close properly before a baby is born. Surgery closes the spine after birth, but paralysis can occur.
• Chiari malformation type III: This is rare and the most severe of the Chiari malformations. It occurs when some of your cerebellum and brain stem extend through an abnormal opening in the back of your skull (not the foramen magnum). In addition, some of the membranes that surround your brain or spinal cord extend through the opening. This malformation results in swelling at the back of your skull called an encephalocele. Type III causes severe, life-threatening symptoms and they appear during childhood. Children with type III have neurological problems, learning delays and seizures. It often requires surgery.
• Chiari malformation type IV: Chiari malformation type IV is rare and happens when your cerebellum itself is underdeveloped or has parts missing. This type is severe and usually life-threatening in infants.

How common is a Chiari malformation?

Studies suggest that Chiari malformations occur in about 1 in every 1,000 people in the United States. Because some people don’t have any symptoms or don’t show them until adolescence or adulthood, the condition may be more common.