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Brain & Nervous System Health Center

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Chiari Malformation

Chiari malformations are structural defects in the cerebellum. That's the part of the brain that controls balance.

Some people with Chiari malformations may have no symptoms. Others may have symptoms such as:

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Chiari malformations affect females more often than males.

Scientists once believed that Chiari malformations occurred in only 1 in every 1,000 births. But the increased use of diagnostic imaging techniques such as CT scans and MRIs suggests that the condition may be much more common.

Accurate estimates are difficult to make. That's because some children who are born with the condition either never develop symptoms or don't develop symptoms until they reach adolescence or adulthood.

Causes of Chiari Malformations

Chiari malformations are usually caused by structural defects in the brain and spinal cord. These defects develop during fetal development.

Due to genetic mutations or a maternal diet that lacked certain nutrients, the indented bony space at the base of the skull is abnormally small. As a result, pressure is placed on the cerebellum. This blocks the flow of the cerebrospinal fluid. That's the fluid that surrounds and protects the brain and spinal cord.

Most Chiari malformations occur during fetal development. Much less commonly, Chiari malformations can occur later in life. This can happen when an excessive amount of cerebrospinal fluid is drained away because of:

  • injury
  • infection
  • exposure to toxic substances

Types of Chiari Malformations

There are four types of Chiari malformations:

Type I. This is by far the most commonly observed type in children. In this type, the lower part of the cerebellum -- but not the brain stem -- extends into an opening at the base of the skull. The opening is called the foramen magnum. Normally, only the spinal cord passes through this opening.

Type I is the only type of Chiari malformation that can be acquired.

Type II. This is usually only seen in children born with spina bifida. Spina bifida is the incomplete development of the spinal cord and/or its protective covering.

Type II is also known as "classic" Chiari malformation or Arnold-Chiari malformation. In type II Chiari malformation, both the cerebellum and the brain stem extend into the foramen magnum.

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