Chiari malformation is considered a genetic condition, though there are also acquired forms. The condition develops due to a structural defect in the base of the skull and cerebellum, which causes part of the cerebellum to extend into the upper spinal canal. The condition affects about one in 1,000 people, though most cases are asymptomatic.

Development of a Chiari Malformation

Chiari malformations develop from a defect that causes a misshapen skull or one that is smaller than normal. Symptoms can arise from excess pressure on the brain or impeding the flow of cerebral spinal fluid, which acts as a cushion, circulates chemicals and nutrients, and removes waste products from the brain.

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Skull defects leading to Chiari malformation have various causes. The most common is primary Chiari malformation, which occurs during fetal development due to either genetic mutation or a lack of necessary vitamins and minerals. Acquired secondary forms of Chiari malformation occur later in life as a result of infection, disease, surgery that leads to excessive loss of spinal fluid, or trauma, and are less common.

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Chiari Malformation Type I

Chiari malformation Type I occurs when the bottom of the cerebellum extends into the foramen magnum, where the spinal cord passes. This is the most common type and may not cause any symptoms, though symptoms can appear later in adulthood. An individual may not be diagnosed until these symptoms develop, or the issue is discovered accidentally.


Chiari Malformation Type II

Chiari malformation Type II or Arnold-Chiari malformation is more severe than Type I. Symptoms usually appear during childhood and can be life-threatening. Both the cerebellum and brain stem protrude into the foramen magnum. The person may also have a nerve deformity between the two halves of the cerebellum. Type II usually appears with myelomeningocele, a form of spina bifida.


Chiari Malformation Types III and IV

Chiari malformation Types III and IV are very rare and severe. Type III is the most serious form: the brain stem and some of the cerebellum herniate through an abnormal opening in the base of the skull. Symptoms appear in infancy and are debilitating and life-threatening. In Type IV, the cerebellum is in the right location but underdeveloped or incomplete.



Symptoms of Chiari malformation depend on the type and severity. The hallmark sign is a headache after straining, coughing, or sneezing. Infants may have difficulty swallowing, excessive drooling, gagging, weak cry, arm weakness, breathing problems, or developmental delays. Other symptoms include neck pain, dizziness, balance problems, tinnitus, and difficulty swallowing.

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For some individuals, Chiari malformation is a progressive disorder leading to excess fluid accumulating in the brain or hydrocephalus. This is usually corrected with shunt placement to divert excess fluid to another part of the body. Other complications are syringomyelia, in which a cyst or cavity forms within the spinal cord. The cyst can destroy the central portion of the cord and cause nerve damage. This can lead to chronic pain, difficulty walking, numbness in the extremities, and stiffness in the back, arms, and legs.

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Although primary Chiari malformation is congenital, there is no prenatal test for it. Because it often does not cause any symptoms until later in life, the condition is usually diagnosed during testing or treatment for another disorder. For example, a spinal x-ray may show an abnormality that warrants further testing. In this case, or if the individual is symptomatic, an MRI can confirm the diagnosis.

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A posterior fossa decompression is the most common surgical intervention for Chiari malformation. This procedure creates more space for the cerebellum and relieves pressure on the brain and spinal cord. In some cases, this surgery is enough to relieve symptoms. If more room is needed, the surgeon may also remove the cerebellar tonsils and add a patch to further expand the space.

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Other Possible Interventions

Other interventions depend on the type and severity of the Chiari malformation. Infants with myelomeningocele may need surgery to reposition the spinal cord and close the defect. Often, a surgeon will perform this procedure while the baby is still in the womb, as this is more effective. While hydrocephalus is commonly treated with a shunt, another option is third ventriculostomy. A small hole is made in the bottom of the third ventricle that allows the flow of cerebrospinal fluid, relieving pressure.


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