Arnold-Chiari (kee-AHR-ee) malformation is a congenital structural defect in the cerebellum where the brain connects with the spinal cord.

With this condition, the cerebellum protrudes through the bottom of the skull and into the spinal canal, disrupting the flow of protective cerebrospinal fluid. The disorder is classified according to four grades, or types, and can be life-threatening.

While the exact cause is unknown, Chiari malformations may have a genetic component. Additionally, it may result from infection, lack of proper nutrition and/or exposure to harmful substances during fetal development. It is typically brought to light after a child begins exhibiting developmental delays and the parent seeks medical care.

Chiari malformations are defined by the areas of the brain that are involved in protruding into the spinal canal, and according to their severity. Of the four types of Chiari malformations, Arnold-Chiari falls into the second type.

The term Arnold-Chiari was coined in 1907 and named for Hans Chiari and Julius Arnold, the pathologists who first described the condition.

The terminology of Chiari malformation is fluid, and its description is not universal. Some use the term Arnold-Chiari, one of the four types of Chiari, interchangeably with Chiari.

Symptoms

• Decreased strength and/or sensation in the arms and legs
• Balance and coordination problems
• Neck stiffness or pain
• Headache
• Dizziness or vertigo
• Rapid eye movement
• Swallowing difficulty
• Respiratory problems

Types of Chiari Malformations

Type I: The lower part of the cerebellum, minus the brain stem, reaches into the base of the skull. It is the most common form of Chiari malformation, and may be present without symptoms.

Type II: This is defined as the “classic” type of Chiari malformation, and is also called Arnold-Chiari malformation. In this form of the condition, the cerebellum and the brain stem extend into the opening at the base of the skull (called the foramen magnum). This is most often seen in those children born with spina bifida.

Type III: This is a severe yet relatively rare form of the condition in which the involved parts of the brain actually herniate (protrude) through an abnormal opening of the back part of the skull. It can cause severe and life-threatening neurological problems.

Type IV: This type is also very rare, and involves an incomplete or undeveloped cerebellum (known as cerebellar hypoplasia). Portions of the cerebellum are missing, and parts of the skull and spinal cord may be visible.

Causes

Chiari malformations are usually congenital (developed in utero) and present at birth. Often, however, the cause is unknown. Other disorders which affect the brain and spine can lead to Chiari malformations. These include spina bifida, hydrocephalus, scoliosis and syringomyelia (a cyst in the spinal cord). A Chiari malformation may possibly be hereditary, since it can run in families. However, no gene has yet been identified as a cause.

Most Chiari malformations occur during development of the fetus. However, they can occur later in life due to disruption of the cerebrospinal fluid (CSF). This can be caused by injury, infection or exposure to toxic substances or happen following brain or spinal surgery.

Diagnosis

A Chiari malformation is often diagnosed with the use of MRI to evaluate the relevant structure. Cine MRI may additionally be used. This is a specific type of MRI to observe the flow of CSF, and to check for abnormality. X-rays or CT scans may also be done. While there is no single objective test, doctors will combine these scans with a person’s symptoms and a neurological exam to make the diagnosis.

Treatment

Surgery is the usual treatment for Chiari malformations that cause symptoms. The most common surgical treatment is called a posterior fossa decompression. The posterior fossa is a small space in the skull located near the brainstem and cerebellum. This surgery allows more room for the section of the cerebellum that has herniated, and to relieve the pressure on the brain. It is also hoped that the surgery restores the normal flow of cerebrospinal fluid (CSF).

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