Chiari Malformation Fact Sheet

What are Chiari malformations?

Chiari malformations are structural defects in the base of the skull and cerebellum, the part of the brain that controls balance.  Normally the cerebellum and parts of the brain stem sit above an opening in the skull that allows the spinal cord to pass through it (called the foramen magnum).  When part of the cerebellum extends below the foramen magnum and into the upper spinal canal, it is called a Chiari malformation (CM). 

Chiari malformations may develop when part of the skull is smaller than normal or misshapen, which presses on the brain and forces the cerebellum to be pushed down into the spinal canal.  The pressure on the cerebellum and brain stem  may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF)—the clear liquid that surrounds and cushions the brain and spinal cord. 


What are the symptoms of a Chiari malformation?

Some individuals with CM may not show any symptoms.  Symptoms may change for some individuals, depending on the compression of the tissue and nerves and on the buildup of CSF pressure.  Symptoms may include:

  • Headache, especially after sudden coughing, sneezing, or straining
  • Neck pain
  • Hearing or balance problems
  • Muscle weakness or numbness
  • Dizziness
  • Difficulty swallowing or speaking
  • Vomiting
  • Ringing or buzzing in the ears (tinnitus)
  • Curvature of the spine (scoliosis)
  • Insomnia
  • Depression
  • Problems with hand coordination and fine motor skills 
  • Difficulty swallowing
  • Excessive drooling, gagging, or vomiting
  • Breathing problems
  • Difficulty eating and an inability to gain weight


What causes these malformations?

CM Is most often caused by structural defects in the brain and spinal cord that occur during fetal development.  This is called primary or congenital Chiari malformation.  CM can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to traumatic injury, disease, or infection.  This is called acquired or secondary Chiari malformation.  Primary Chiari malformation is much more common than secondary Chiari malformation. 


How are CMs classified?

Chiari malformations are classified by the severity of the disorder and the parts of the brain that protrude into the spinal canal.

  • Chiari malformation Type I – the most common form – happens when the lower part of the cerebellum (called the cerebellar tonsils) push into the foramen magnum.  Normally, only the spinal cord passes through this opening.  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 II—also called classic CM – involves both the cerebellum and brain stem tissue pushing into the foramen magnum.  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 (see below).  A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening.  Symptoms of Type II usually appear during childhood and are generally more severe than in Type 1.  It can cause life-threatening complications during infancy or early childhood, and treating it requires surgery.  The term Arnold-Chiari malformation is specific to Type II malformations.               
  • Chiari malformation Type III – the most serious form –has some of the cerebellum and the brain stem stick out, or herniate, through an abnormal opening in the back of the skull.  This can also include the membranes surrounding the brain or spinal cord.  Symptoms of this very rare form of CM appear in infancy and can cause debilitating and life-threatening complications.  Babies with Type III can have many of the same symptoms as those with Type II but can also have additional severe neurological defects such as seizures and mental and physical delays.                 
  • Chiari malformation Type IV involved an incomplete or underdeveloped cerebellum (a condition known as cerebellar hypoplasia.  In this rare form of CM, the cerebellum is in its normal position but parts of it are missing, and portions of the skull and spinal cord may be visible. 


What other conditions are associated with Chiari malformations?


  • Hydrocephalus is an excessive buildup of CSF in the brain.  A CM can block the normal flow of this fluid and cause pressure within the head that can result in mental impairment and/or an enlarged or misshapen skull.  Severe hydrocephalus, if left untreated, can be fatal.  Hydrocephalus can occur with any type of Chiari malformation, but is most commonly associated with Type II. 
  • Spina bifida is the incomplete closing of the backbone and membranes around the spinal cord.  Individuals with Type II Chiari malformation usually have myelomeningocele, and a baby’s spinal cord remains open in one area of the back and lower spine.  The membranes and spinal cord protrude through the opening in the spine, creating a sac on the baby’s back.  This can cause neurological impairments such as muscle weakness, paralysis, and scoliosis. 
  • Syringomyelia is a disorder in which a CSF-filled tubular cyst called a syrinx, forms within the spinal cord’s central canal.  The growing syrinx destroys the center of the spinal cord and presses on nerves, resulting in pain, weakness, stiffness , and may include a loss of the ability to feel extremes of hot or cold, especially in the hands. . 
  • Tethered cord syndrome occurs when a child’s spinal cord abnormally attaches to the tissues around the bottom of the spine – preventing  the spinal cord from moving freely within the spinal canal.  As a child grows, the disorder worsens and can result in permanent damage to the nerves that control the muscles in the lower body and legs.  Children who have a myelomeningocele have an increased risk of developing a tethered cord later in life.
  • Spinal curvature is common among individuals with syringomyelia or CM Type I.  The spine either may bend to the left or right (scoliosis) or may bend forward (kyphosis).


How are Chiari malformations diagnosed?

Currently, no test is available to determine if a baby will be born with a Chiari malformation.  Since CMs are associated with certain birth defects like spina bifida, children born with those defects are often tested for malformations.  However, some malformations can be seen on ultrasound images before birth.

Many people with Chiari malformations have no symptoms and their malformations are discovered only during the course of diagnosis or treatment for another disorder.  The doctor will perform a physical exam and check the person's memory, cognition, balance (functions controlled by the cerebellum), touch, reflexes, sensation, and motor skills (functions controlled by the spinal cord).  A doctor also may order imaging tests to diagnose a Chiari malformation or show hydrocephalus or bone abnormalities that might be associated with the disorder.


How are Chiari malformations treated?

CMs that do not show symptoms and do not interfere with a person’s activities of daily living may only need regular monitoring by a physician with diagnostic imaging.  Medications may be prescribed to ease headache and pain. 


In many cases, surgery is the only treatment available to ease symptoms or halt the progression of damage to the central nervous system.  Surgery can improve or stabilize symptoms in most individuals.  More than one surgery may be needed to treat the condition.

The most common surgery to treat Chiari malformation is posterior fossa decompression, which creates more space for the cerebellum and relieves pressure on the spinal cord and should help restore the normal flow of CFS.  It involves making an incision at the back of the head and removing a small portion of the bone at the bottom of the skull (craniectomy).  In some cases, the arched, bony roof of the spinal canal, called the lamina, may also be removed (spinal laminectomy).

In some instances the surgeon may use a procedure called electrocautery to remove the cerebellar tonsils, allowing for more free space.  These tonsils do not have a recognized function and can be removed without causing any known neurological problems.

Infants and children with myelomeningocele may require surgery to reposition the spinal cord and close the opening in the back.  This surgery is most effective when it is done prenatally (while the baby is still in the womb) instead of after birth and restores the cerebellum and brain stem to a more normal alignment.

Hydrocephalus may be treated with a shunt (tube) system that drains excess fluid and relieves pressure inside the head.  A tube that is surgically inserted into the head drains the excess fluid into either the chest cavity or the abdomen so it can be absorbed by the body.  Hydrocephalus may also be treated surgically using third ventriculostomy, in which a small hole is made at the bottom of the third ventricle (brain cavity) and the CSF is diverted there to relieve pressure.  Similarly, in cases where surgery was not effective, doctors may open the spinal cord and insert a shunt to drain a syringomyelia or hydromyelia (increased fluid in the central canal of the spinal cord).


What research is being done?

The National Institute of Neurological Disorders and Stroke (NINDS) is the primary organization at the National Institutes of Health (NIH) that funds research on Chiari malformations and other brain and spinal conditions.  NHI is the leading supporter of biomedical research in the world. 

Genetic studies. Mutations in the PI3K-AKT signaling pathway can cause brain overgrowth that may accompany CM, hydrocephalus, and other brain disorders.  NINDS scientists are looking for other gene mutations that could act through PI3K-AKT signaling and additional pathways., which may lead to new diagnostic tests and better treatments options for Chiari malformations and other developmental brain disorders. 

Brain mechanisms. Certain signals at the midbrain-hindbrain (MHB) boundary tell the brain to properly develop the cerebellum and other parts of the brain.  However, how these brain regions are initiated, formed, and maintained is not well understood.  NINDS scientists are studying zebrafish embryos in order to gain a better understanding of how the MHB forms.  This will provide valuable insights into human brain development, particularly the cerebellum.  Other investigators are studying the expression of different growth factors on the development of the brain, skull, spine, and spinal cord.  Interference with normal gene function through gene mutation or environmental factors may influence the development of CM. 

Neuroimaging tools. Other NINDS-funded studies aim to develop improved brain imaging methods, such as amplified MRI, to help doctors diagnose CM earlier and inform brain surgery, leading to earlier disease detection, diagnosis, and better treatment.  Researchers are also working on establishing new MRI-based CM indicators, or biomarkers (signs that can help diagnose and monitor a disease), to help doctors decide what type of brain surgery may be the best treatment option.

Surgery. Little is known about the effect of surgery to treat syringomyelia over time on muscle strength, pain level, and overall function.  NINDS scientists are examining individuals who have syringomyelia or a related condition such as Chiari l malformation.  By recording more than 5 years of symptoms, muscle strength, general level of function and MRI scan findings from individuals who receive standard treatment for syringomyelia, researchers hope to obtain more information about factors that influence its development, progression, and relief of symptoms.  


Where can I get more information?

For more information on neurological disorders or research programs funded by the National Institute of Neurological Disorders and Stroke, contact the Institute's Brain Resources and Information Network (BRAIN) at:

P.O. Box 5801
Bethesda, MD 20824

Information also is available from the following organizations:

American Syringomyelia & Chiari Alliance Project
P.O.  Box 1586
Longview, TX  75606-1586

Bobby Jones Chiari and Syringomyelia Foundation
29 Crest Loop
Staten Island, NY 10312

March of Dimes Foundation
1550 Crystal Drive, Suite 1300
Arlington, VA 22202

National Organization for Rare Disorders (NORD)
55 Kenosia Avenue
Danbury, CT  06810-1968
Toll-free voicemail: 800-999- 6673

Spina Bifida Association of America
1600 Wilson Boulevard, Suite 800
Arlington, VA 22209


Online text updated May 20, 2021

"Chiari Malformation Fact Sheet", NINDS, Publication date June 2017.

NIH Publication No. 17-NS-4839

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Prepared by:
Office of Neuroscience Communications and Engagement
National Institute of Neurological Disorders and Stroke
National Institutes of Health
Bethesda, MD 20892

NINDS health-related material is provided for information purposes only and does not necessarily represent endorsement by or an official position of the National Institute of Neurological Disorders and Stroke or any other Federal agency. Advice on the treatment or care of an individual patient should be obtained through consultation with a physician who has examined that patient or is familiar with that patient's medical history.

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