Chiari Malformation

Medically Reviewed by Carol DerSarkissian, MD on November 19, 2022
5 min read

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:

  • dizziness
  • muscle weakness
  • numbness
  • vision problems
  • headaches
  • problems with balance and coordination

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.

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

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.

Type III. This is the most serious form of Chiari malformation. It involves the protrusion or herniation of the cerebellum and brain stem through the foramen magnum and into the spinal cord. This usually causes severe neurological defects. Type III is a rare type.

Type IV. This involves an incomplete or undeveloped cerebellum. It sometimes is associated with exposed parts of the skull and spinal cord. Type IV is a rare type.

In addition to spina bifida, other conditions sometimes associated with Chiari malformations include:

Hydrocephalus. An excessive buildup of cerebrospinal fluid in the brain.

Syringomyelia. A disorder in which a cyst develops in the spinal cord's central canal.

Tethered cord syndrome. A progressive disorder in which the spinal cord attaches itself to the bony spine.

Spinal curvature. This includes conditions such as:

Chiari malformation is associated with a wide range of symptoms which vary by type.

Type I Chiari malformation usually causes no symptoms. Most people with the condition don't even know that they have it unless it is incidentally discovered during a diagnostic imaging test.

But if the malformation is severe, type I may cause symptoms such as:

  • Pain in the lower back of the head into the neck; it usually develops quickly and intensifies with any activity that increases pressure in the brain, such as coughing and sneezing.
  • Dizziness and problems with balance and coordination
  • Swallowing difficulties
  • Sleep apnea

Most children born with type II Chiari malformation have hydrocephalus. Older children with type II Chiari malformation may develop head pain associated with:

Some of the most common symptoms are linked to problems with the function of nerves in the brain stem. These include:

  • weakness of vocal cords
  • swallowing difficulties
  • breathing irregularities
  • serious changes in the function of nerves in the throat and tongue

If a Chiari malformation is suspected, a doctor will perform a physical exam. The doctor will also check functions controlled by the cerebellum and spinal cord. These functions include:

  • balance
  • touch
  • reflexes
  • sensation
  • motor skills

The doctor may order diagnostic tests, such as:

  • X-ray
  • CT scan
  • MRI

An MRI is the test most often used to diagnose Chiari malformations.

If Chiari malformations cause no symptoms and do not interfere with activities of daily living, no treatment is necessary. In other cases, medications can be used to manage symptoms such as pain.

Surgery is the only treatment that can correct functional defects or stop progression of damage to the central nervous system.

In both type I and type II Chiari malformations, the goals of surgery are to:

  • Relieve pressure on the brain and spinal cord
  • Re-establish normal fluid circulation through and around the area

In adults and children with Chiari malformations, several types of surgery can be performed. These include:

Posterior fossa decompression surgery. This involves the removal of a small portion of the bottom of the skull and sometimes part of the spinal column to correct the irregular bony structure. The surgeon also may open and widen the dura. That's the firm covering of the brain and spinal cord tissues. This creates additional space for the cerebrospinal fluid to circulate.

Electrocautery. This uses high-frequency electrical currents to shrink the lower part of the cerebellum.

Spinal laminectomy. This is the removal of part of the arched, bony roof of the spinal canal. This increases the canal's size and reduces pressure on the spinal cord and nerve roots.

Additional surgical procedures may be needed to correct conditions associated with Chiari malformations such as hydrocephalus.

Surgery usually results in a significant reduction of symptoms and a prolonged period of remission. According to Children's Hospital in Boston, which specializes in the treatment of Chiari malformations, surgery virtually eliminates symptoms in 50% of pediatric cases. Surgery substantially reduces symptoms in another 45% of cases. Symptoms are stabilized in the remaining 5%.