Chiari Malformations

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What are Chiari malformations?

Chiari malformations (CM) are caused by problems in the structure of the brain and skull. In Chiari malformations, the lower part of the 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) where the spinal cord connects to the brain. When part of the cerebellum extends below this opening and into the upper spinal canal, it is called a Chiari malformation. 

CM may develop when part of the skull is smaller than normal or misshapen, which can put pressure on the cerebellum, 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. 


The types and severity of symptoms depend on the extent to which the tissue and nerves are compressed and on the buildup of CSF pressure.  Some people with CM do not show symptoms.  People with CM may not have symptoms initially but may develop them later in life.  CM symptoms can include:

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

CM is associated with and can cause other neurological conditions, including hydrocephalus (an excessive buildup of CSF in the brain), spina bifida (the incomplete closing of the backbone and membranes around the spinal cord), syringomyelia (a CSF-filled cyst known as a syrinx in the spinal cord), and tethered cord syndrome (in which the spinal cord movement is restricted by the tissue around the bottom of the spine).  

Who is more likely to get Chiari malformations?

CM is often caused by problems in the development of the brain and spinal cord that occur during fetal development. This is called primary or congenital CM. It is also possible for CM to develop later in life, if too much spinal fluid is drained from the spine due to traumatic injury, disease, or infection. This is called secondary or acquired CM. Congenital CM is much more common than acquired CM. 

Classifying Chiari malformations

CMs are classified by how severe they are and which parts of the brain protrude into the spinal canal.

  • CM Type I – This is the most common form. It occurs when the lower part of your cerebellum (called the cerebellar tonsils) pushes into the opening between the skull and spinal cord (foramen magnum). Normally, only the spinal cord passes through this opening.  Type I CM is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition. 
  • CM Type II - CM Type II 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 body below the spinal opening.   It is the CM type most associated with hydrocephalus.  The term Arnold-Chiari malformation is specific to CM Type II.
  • CM Type III – CM Type III is a critical condition characterized by some of the cerebellum and the brain stem sticking out, or herniating, through an opening in the back of the skull.  Membranes surrounding the brain or spinal cord may also herniate.  Symptoms of this very rare form of CM appear in infancy and can cause debilitating and life-threatening complications.  Babies with CM Type III can have additional severe neurological defects such as seizures and mental and physical delays.
  • CM Type IV – A very rare condition, CM Type IV involves an incomplete or underdeveloped cerebellum (a condition known as cerebellar hypoplasia).  While the cerebellum is in its normal position, parts of it are missing, and portions of the skull and spinal cord may be visible. 

How are Chiari malformations diagnosed and treated?

Diagnosing CM

Currently, no test is available to determine if an infant will be born with CM. Since CMs are associated with congenital conditions like spina bifida, children born with other developmental problems are often tested for malformations. Some malformations can be seen on ultrasound images before birth.

To diagnose CM, the doctor will perform a physical exam and check your memory, cognition, balance, touch, reflexes, sensation, and motor skills.  The doctor may order imaging tests to diagnose CM or show hydrocephalus or bone abnormalities that might be associated with the disorder.

Treating CM

Treatment depends on your symptoms and their severity. Talk with your doctor about your symptoms, how they affect you, and what treatments you may need. CMs that do not show symptoms and do not interfere with daily life may only need regular monitoring by a physician and periodic imaging. Medications can ease headache and pain. 


In many cases, surgery is the only treatment available to ease symptoms or stop ongoing damage to the central nervous system. Surgery can improve or stabilize symptoms in most people. A person 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 to reduce pressure on the spinal cord and restore the normal flow of CSF.  In posterior fossa decompression, a surgeon will make an incision at the back of the head and remove 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 in what’s called a 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.

Other conditions associated with CM, including tethered cord syndrome and hydrocephalus, may be treated with surgery. Hydrocephalus is usually treated with a shunt (tube) to help drain the excess fluid (CSF) and relieve pressure.

Patients with CM Type II often have myelomeningocele, which requires surgical closure of the skin as a fetus or newborn to prevent CSF leakage from the myelomeningocele.  Surgical treatment of the hydrocephalus is often needed, but less frequently when the myelomeningocele is closed as a fetus than as an infant. Infrequently, CM Type II causes life-threatening complications during infancy or early childhood and must be treated with posterior fossa decompression surgery.   

What are the latest updates on Chiari malformations?

NIH is the leading supporter of biomedical research in the world. NINDS is exploring several avenues to advance research on CM and related conditions. While several components of the National Institutes of Health (NIH) sponsor research on CM, NINDS is the primary organization at NIH that funds research on CM and other brain and spinal conditions. 

NINDS helped develop Common Data Elements for CM that define standard CM clinical trial research terms. The CDEs create a common language for describing patient symptoms, imaging findings, and outcomes of CM treatment.

Genetic studies

Changes in genes can cause many brain malformations and the brain overgrowth that may accompany CM, hydrocephalus, and other brain disorders.  NINDS scientists are looking for gene changes that could act through the PI3K-AKT genetic signaling pathway, which is essential for normal brain development and other related pathways. This research may lead to new diagnostic tests and better treatment options for CM and other developmental brain disorders. 

Brain mechanisms

Signals in the brain at the midbrain-hindbrain (MHB) boundary tell the brain how to develop the cerebellum and other parts of the brain properly.  However, how these brain regions are initiated, formed, and maintained is poorly understood. NINDS scientists are studying zebrafish embryos 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 how different growth factors are expressed during the development of the brain, skull, spine, and spinal cord.  Interference with normal gene function through gene variations 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, leading to earlier disease detection and better treatment.  Researchers are also working on establishing new MRI-based CM biomarkers (signs that can help diagnose and monitor a disease) to help doctors decide what type of surgery may be the best treatment option.


Little is known about the long-term effects of having surgery to treat syringomyelia on a person’s 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.  

A recent NIH-funded research study examined the outcomes one year after surgery for Chiari I malformation and syringomyelia in children treated with posterior fossa decompression surgery. The researchers compared the outcomes of study participants who just had bone removed with the outcomes for study participants who also had opening and expansion of the dural membrane (duraplasty) to make more space at the foramen magnum. Patients with duraplasty had fewer and less severe symptoms after surgery, a smaller syrinx, and a lower chance of needing another surgery. Complication rates were greater in the duraplasty group early after surgery, but comparable by six months after surgery.

NINDS continues to support research on these and other areas to advance our understanding of Chiari malformations and how to best diagnose, treat, and prevent them.

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 CM. Clinical research uses human study participants to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

All types of study participants 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.