Chiari Malformations

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What are Chiari malformations?

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

CM 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. This can put pressure on your brain stem and spinal cord and block the flow of cerebrospinal fluid (CSF)—the clear liquid that surrounds and cushions the brain and spinal cord. 


Some people with a CM do not show symptoms. Symptoms may change for some individuals, depending on the compression of the tissue and nerves and on the buildup of CSF pressure. If you have a CM, your 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

    Who is more likely to get Chiari 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 CM. The disorder also can be caused later in life if spinal fluid is drained excessively from the spine either due to traumatic injury, disease, or infection. This is called acquired or secondary CM.  Primary CM is much more common than secondary CM. 

    Classifying Chiari malformations

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

    • CM Type I – the most common form – happens when the lower part of your 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.       
    • CM 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.
    • CM 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.
    • CM Type IV involves 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 CMs?

    • Hydrocephalus is an excessive buildup of CSF in the brain.  A CM can block the normal flow of this fluid and cause pressure within your 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 CM 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 CM Type II 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 your spinal cord's central canal.  The growing syrinx destroys the center of the spinal cord and presses on your nerves, resulting in pain, weakness, and stiffness. You may lose the ability to feel extremes of hot or cold, especially in your 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 and treated?

    Diagnosing CMs

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

    You, as well as many people with a CM, may not have symptoms and your malformation is discovered only during the course of diagnosis or treatment for another disorder.  The doctor will perform a physical exam and check your memory, cognition, balance (functions controlled by the cerebellum), touch, reflexes, sensation, and motor skills (functions controlled by the spinal cord).  Your doctor also may order imaging tests to diagnose a CM or show hydrocephalus or bone abnormalities that might be associated with the disorder.

    Treating CMs

    Treatment depends on your symptoms and their severity. Talk with your doctor about your symptoms and how they affect you, as well as what treatments you may need.  CMs that do not show symptoms and do not interfere with your 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.  You may need more than one surgery to treat the condition.

    The most common surgery to treat CM is posterior fossa decompression, which creates more space for the cerebellum and reduce pressure on the spinal cord and 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 (a procedure called a 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) to divert the CSF 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).

    Tethered cord syndrome can be treated by surgically untethering the spinal cord and releasing tension on it.

    What are the latest updates on Chiari malformations?

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

    Genetic studies. The PI3K-AKT signaling pathway is important in normal brain development.  Mutations can cause many brain malformations and the 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 CM 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 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 magnetic resonance imaging-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 CM.  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.  

    For research articles and summaries on Chiari malformation, search PubMed, which contains citations from medical journals and other sites.

    Learn About Clinical Trials
    Clinical trials are studies that allow us to learn more about disorders and improve care. They can help connect patients with new and upcoming treatment options.

    How can I or my loved one help improve care for people with Chiari malformations?

    Consider participating in a clinical trial so clinicians and scientists can learn more about Chiari malformations. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

    All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.

    For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with CM at

    Where can I find more information on Chiari malformations?

    Information is available from the following organizations:

    Order publications from the NINDS Catalog
    The NINDS Publication Catalog offers printed materials on neurological disorders for patients, health professionals, and the general public. All materials are free of charge, and a downloadable PDF version is also available for most publications.