About Chiari Malformations

What is Chiari malformation type 1?

Chiari malformation type 1 is a condition in which a small portion of the brain descends into the spinal column, where it can impede the flow of cerebrospinal fluid (CSF). Symptoms of the disorder, which can be progressive and disabling, usually don’t appear until late childhood or early adolescence. Chiari malformation type 1 is typically treated with surgery that restores the free flow of CSF by providing more room in the back of the brain. Lucile Packard Children’s Hospital Stanford is a Chiari malformation type 1 Destination Center on the West Coast. 

Understanding the condition

The thick bundle of nerves connecting the brain to the spinal cord passes through a funnel-like opening at the base of the skull called the foramen magnum. The CSF, which nourishes, maintains and protects the brain, also flows through the foramen magnum on its route toward the lower parts of the central nervous system. Just above the foramen magnum is an area of the skull called the posterior fossa, which holds the cerebellum, the part of the brain that regulates balance and coordination. If the posterior fossa is too small, small extended pieces of the cerebellum, called the cerebellar tonsils, may be squeezed down through the foramen magnum and into the spinal column.  Chiari malformation type 1 is diagnosed if the cerebellar tonsils drop more than 5 mm below the foramen magnum.


Although the cranial defect that causes Chiari malformation type 1 may be present before birth, symptoms typically take years to emerge. Common ones include headaches in the back of the head that worsen with straining, neck pain, dizziness, difficulty swallowing, loss of coordination and fine motor skills, depression, muscle weakness, fatigue, and numbness in the hands, arms, or legs. Patients also sometimes experience problems with balance, sleep, hearing and vision. Symptoms vary from patient to patient, and they can change over time depending on the intensity and location of the points of pressure.

When brain tissue blocks the flow of CSF, a pressure differential between the skull and the spinal column can cause a hammer-like effect similar to the shock waves that knock through water pipes when flow is quickly turned on and off. Such pressure waves can cause the formation of a kind of CSF-filled cyst, called a syrinx, in the central canal of the spinal cord.

Because so many Chiari malformation type 1 symptoms are also associated with other disorders, the condition can be tricky to identify. Diagnosis starts with x-ray images of the bone abnormalities underlying Chiari malformation type 1 and a magnetic resonance image (MRI) showing the cerebellar tonsils descending into the spinal column.


Chiari malformation type 1 is uncommon, diagnosed in fewer than 1 in 1,000 children. However, the cranial anomaly that leads to the condition may be more common than diagnosis suggests. If brain tissue passes down through the foramen magnum in a way that neither damages brain tissue nor significantly obstructs the flow of CFS, the anomaly itself may never be diagnosed. In other words, many cases of Chiari malformation type 1 may be asymptomatic, or symptoms may never get severe enough to prompt a diagnosis. In most cases, though, symptoms gradually worsen and, if left untreated, eventually become debilitating.

Once considered an intractable and progressive disorder, today Chiari malformation type 1 can usually be effectively treated with surgery that creates more room for the cerebellum and restores the flow of CFS from the skull down into the spinal column.

Other types of Chiari malformation

While type 1 is by far the most common type of Chiari malformation diagnosed in children, there are several other distinct types of Chiari malformation.
Chiari malformation type 2 is much less common and usually accompanies a form of open spina bifida called myelomeningocele, a birth defect in which a baby’s spinal cord fails to develop normally. In Chiari malformation type 2, both cerebellar tissue and brain stem tissue extend down through the foramen magnum.

Chiari malformation type 3 is quite rare and is the most disabling form of Chiari malformation. Like type 2, part of the cerebellum and brain stem protrude through the foramen magnum, but part of the brain’s ventricular system also moves down into the spinal cord. This rare form of Chiari malformation always causes serious neurological disability.

In Chiari malformation type 4, which is also extremely rare, the cerebellum is underdeveloped or parts of it are sometimes missing altogether.

Association with other conditions

Chiari malformations are frequently associated with other medical conditions. If the normal flow of CSF through the foramen magnum is impeded, it can cause a buildup of CSF in the brain, which can cause hydrocephalus. (Link to hydrocephalus page.) Hydrocephalus occasionally accompanies type 1 but is more commonly associated with type 2. Curvature of the spine is another condition that commonly occurs with type 1.

The formation of CSF-filled cysts in the center of the spinal cord, a condition known as syringomyelia (sear-IN-go-my-EEL-ya), may also be associated with Chiari malformation type 1. As they grow, these cysts can damage the nerve fibers that connect the brain to the body’s extremities. Different symptoms may emerge depending on the location and extent of the cyst and may include weakening of the arms and legs; incontinence; stiffness in the shoulders, back, arms, or legs; and chronic pain.

Packard Children’s is a Destination Center for Chiari malformation type 1

Lucile Packard Children’s Hospital Stanford is a West Coast destination site for families seeking treatment for children and young adults with Chiari malformation type 1. Surgical approaches to the disorder continue to evolve, and our team passionately pursues the safest, least invasive ways to attain the best possible outcomes. Our pediatric Chiari malformation type 1 experts have decades of experience treating this disorder and its associated conditions. Our dedicated, world-class pediatric neurosurgeons are backed by anesthesiologists, neurologists, sleep doctors, integrated pain specialists and nurses, all of whom are specifically trained to treat children with these disorders. The specificity of that training makes all the difference.

From diagnosis to surgery and into long-term follow-up care, we take a team-based approach to treating young patients. Our multidisciplinary group of top-tier experts works closely with every patient’s family to provide continuing education and guidance in order to ensure the best possible outcome and maximize the overall well-being of our young patients.


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