There are two levels of this anomaly, in which part of the brain protrudes into the spinal cord. Chiari I Malformation is relatively simple and is not associated with other congenital brain malformations. In this condition, peg-like cerebellar tonsils are displaced downward through the foramen magnum (the opening at the bottom of the skull) into the upper cervical spinal canal. Chiari II malformation is a complex anomaly with skull, dura, brain, spine, and spinal cord manifestations, which usually presents in early childhood or in the neonatal period. This disorder is usually associated with the spinal defect myelomeningocele.

Symptoms: Chiari malformation is characterized by pain, especially headache in the back of the head, aggravated by coughing and straining. Weakness is also prominent, especially in the hands. Other symptoms include neck, arm, and leg pain, numbness, loss of temperature sensation, unsteadiness, double vision, slurred speech, trouble swallowing, vomiting, and tinnitus (ringing in the ears). In Chiari II malformation, these symptoms are often accompanied by difficulties swallowing and breathing, especially inhaling.

Diagnosis: MRI is the diagnostic test of choice for Chiari I malformation, since it easily shows the tonsillar herniation as well as syringomyelia, which occurs in 20-30 percent of cases.

Treatment: These conditions are surgically treated by removing bone in the region of tonsillar herniation. Chiari II malformation typically requires inserting a shunt to reduce the herniation.