we listen - we know your symptoms are real.


December 17, 2014














A+ | Reset | A-

pdf downloadChiari I malformation

Chiari type I, the most common, affects both children and adults. The condition begins with the underdevelopment of the fetal skull forming during pregnancy. During childhood, the brain continues to grow and the skull hardens. However, the small size or shape of the Chiari skull is mismatched to the size of the brain. Thus, a crowding of the brainstem, cerebellum, and tonsils occurs. Crowding pushes the tonsils out of the skull through the opening (foramen magnum) where the spinal cord exits (Fig. 3A). The tonsils put pressure on the spinal cord, blocks CSF flow, and results in the Chiari signs and symptoms. Sometimes a fluid-filled cyst (syrinx) develops within the spinal cord.

posterior fossa

Figure 3A. The posterior fossa (orange color) is formed by the occipital bone, which has a large hole (foramen magnum) for passage of the spinal cord.

chiari side view

Figure 3B. The posterior fossa is too small causing the cerebellar tonsils to herniate through the skull into the spinal canal.

Chiari I malformation is seen on MRI scans in people of all ages. Its incidence was earlier estimated to affect 1 in every 1,000 births. Now with increasing use of diagnostic imaging, physicians note that Chiari may be far more common. Patients typically seek medical attention in their 20s and 30s. Three times more women than men are affected. Genetic studies show that Chiari may cluster in some families.

Bone deformity

The skull is formed from eight bones that fuse together along suture lines. Of interest in Chiari are the occipital bone (where the skull and spinal column connect) and the top two spinal bones (C1 and C2 vertebrae). In the middle of the occipital bone lies the foramen magnum, an opening where the spinal cord connects to the brainstem.

Inside the skull are three areas: anterior fossa, middle fossa, and posterior fossa. The posterior fossa contains the brainstem, cerebellum, and cranial nerves IV-XII (Fig. 3A).   

A variety of bone abnormalities can occur in patients affected by Chiari:

• The posterior fossa may be smaller than normal. If too small, the effects can be crowding of the brainstem and cerebellum, as well as herniation of the tonsils through the foramen magnum (Fig. 3B).

• Sometimes the occipital bone is misshaped or thickened.

• Basilar invagination is a condition where the top of C2 (odontoid) pushes upward into the foramen magnum. This defect can narrow the foramen magnum and crowd the brainstem and cerebellum causing Chiari-like symptoms.

• Scoliosis is a bone deformity that causes curvature of the spine. There is a high rate of scoliosis associated with Chiari and syringomyelia, especially in children.

• Ehlers-Danlos syndrome (EDS) is a connective tissue disorder that may increase the incidence and severity of Chiari. EDS causes joint hypermobility and loose/unstable joints. In an area called the craniocervical junction, strong ligaments attach the C1 and C2 vertebrae to the skull, allowing movement of the head. For someone with both Chiari and EDS, extra testing and precautions are taken to ensure the connection between the spine and skull is intact. Spinal fusion surgery may be needed to support the neck and skull.

Cerebellum herniation

The cerebellum is the lower part of the brain located in the posterior fossa. On the underside of the cerebellum are two tonsils (Fig. 4). The cerebellum primarily coordinates body movement. It maintains muscle tone and balance. The cerebellum is also involved in cognitive functions (e.g., attention, language), memory, and learning. Signs of cerebellum problems include loss of coordination, unstable walking (gait), trouble with speech, and difficulty with eye movement and swallowing.

In Chiari, the cerebellar tonsils are stretched as they push through the foramen magnum into the spinal canal (Fig. 5). This results in compression of the brainstem and spinal cord. The extent and progression of this tonsillar herniation can be seen on MRI. 

normal anatomy chiari I
Figure 4. (left) Normal anatomy of the cerebellar tonsils. Figure 5. (right) In Chiari, the tonsils are pushed through the foramen magnum into the spinal canal.

Brainstem compression

Acting as a relay center, the brainstem connects the cerebrum and cerebellum to the spinal cord. The brainstem performs many automatic functions such as breathing, heart rate, body temperature, wake and sleep cycles, digestion, sneezing, and coughing. Ten of the 12 cranial nerves originate in the brainstem.

In Chiari, compression of the brainstem and cranial nerve nuclei can occur. Patients may experience problems with sleeping (pons), breathing (medulla), swallowing (CN IX), facial pain or numbness (CN V), hearing loss (CN VIII), irregular heart beat (X), and digestion (X).

CSF flow

Figure 6. Cerebrospinal fluid (CSF) circulates through the ventricles inside the brain to the subarachnoid spaces surrounding the brain and spinal cord.

Cerebrospinal fluid blockage

Cerebrospinal fluid (CSF) is a clear, watery-like liquid that flows within and around the brain and spinal cord. CSF helps keep the brain buoyant and cushions it from injury. This fluid is produced inside the ventricles by the choroid plexus and is constantly being absorbed and replenished.

The CSF flows through the ventricles and out into the space between the brain and skull (subarachnoid space) and down into the spinal canal (Fig. 6). A large amount of CSF lies at the back of the cerebellum in an area called the cisterna magna. CSF drains from the fourth ventricle into the cisterna magna. From there it then circulates down into the spinal canal and around the spinal cord. CSF also flows from the fourth ventricle down the central canal inside the spinal cord.

As the heart beats, CSF flows into the brain. This is normally balanced by CSF then flowing from the brain into the spinal compartment. In a Chiari malformation, this balanced flow is disrupted. The obstructed CSF begins to force its way like a water hammer through the foramen magnum. Pushing the tonsils down even farther, it exerts pressure on the brainstem. The increasing pressure compromises normal functions of the brain and/or spinal cord and a myriad of symptoms occur.

Excess CSF can collect and enlarge either the ventricles in the brain (hydrocephalus), or form a cyst in the spinal cord (syringomyelia).


updated: 1.2012
reviewed by: John M. Tew, MD, Nancy McMahon, RN
Mayfield Clinic, University of Cincinnati Department of Neurosurgery