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April 19, 2018

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normal anatomy

chiari I

Figure 1. (left) Normal anatomy of the cerebellum.

Figure 2. (right) In a Chiari I malformation, the posterior fossa is too small causing the cerebellar tonsils to herniate through the skull into the spinal canal. The tonsils block the flow of CSF (blue) and may cause fluid buildup inside the spinal cord, called a syrinx.

Chiari (pronounced key-AR-ee) malformation is a condition in which the lower part of the brain, called the cerebellum, herniates through the skull and down into the spinal canal (Fig. 1 and 2). The herniated tissue compresses the brainstem and blocks the normal flow of cerebrospinal fluid (CSF). The blockage can then cause a buildup of fluid in the spinal cord (syringomyelia) or in the brain (hydrocephalus). Chiari is often misdiagnosed because of the wide variety of bony and soft tissue abnormalities that compress the cervical spinal cord, brainstem, or cranial nerves, resulting in a wide array of possible symptoms. Symptoms include headache, neck pain, dizziness, arm numbness or weakness, sleep problems, fatigue, etc. Headache in the back of the head that worsens with coughing, sneezing, or straining is a hallmark symptom.

Chiari affects children and adults. Treatment options depend on the type of malformation and severity of symptoms. If symptoms are mild, regular monitoring and medications can be effective. However, symptoms typically progress and worsen over time. Surgery may be recommended to remove a part of the skull bone and create space for the cerebellum and brainstem. An accurate diagnosis and prompt treatment are important to prevent permanent injury to the nervous system. A neurosurgeon with expertise in Chiari can use imaging studies to confirm the diagnosis and recommmend a treatment for each unique Chiari case.

Types of Chiari malformations

Chiari type I, the most common, affects both children and adults (Fig. 2). Because the back of the skull is too small or deformed, a crowding of the brainstem, cerebellum, and tonsils occurs. As the tonsils push out of the skull opening (foramen magnum), they press onto the spinal cord and block CSF flow. Chiari I sometimes is found with a fluid-filled cyst (syrinx) in the spinal cord. Symptoms, which may not appear until late childhood or adulthood, include severe headache, neck pain, imbalance, dizziness, swallowing problems, numbness in the hands, depression, fatigue, and sleep problems.

Chiari type 0, a newly identified form of Chiari, describes the absence (or a “zero” herniation) of the tonsils below the foramen magnum. Yet Chiari 0 includes the presence of both symptoms and a syrinx in the spinal cord. This new type is under study and controversial.

Chiari type II is present at birth and affects infants. It occurs with the birth defect myelomeningocele, a form of spina bifida. When the spinal canal does not close before birth, some of the spinal cord protrudes like a sac from the baby’s back. Both the brainstem and tonsils are pulled down into the spinal canal to block CSF flow in the brain and causing hydrocephalus. Symptoms can include trouble swallowing and gagging, high-pitched breathing, weak cry, arm weakness, and developmental delays. This type is correctly called Arnold-Chiari malformation.

Chiari type III affects infants and is a rare but severe herniation that involves the cerebellum. It can develop with the birth defect encephalocele, a fluid-filled sac at the back of the baby’s neck.

Chiari type IV affects infants. This rare and often fatal malformation occurs when the cerebellum does not develop properly.

updated: 1.2016
reviewed by: Andrew Ringer, MD, John M. Tew, MD, and Nancy McMahon, RN
Mayfield Clinic, University of Cincinnati Department of Neurosurgery