Chiari I malformation is an uncommon congenital (present at birth) disorder in which the Posterior Fossa (rear cranial cavity) is not large enough and, as a result, causes the lower part of the Cerebellum (referred to as the “Tonsils” of the brain) to protrude down into the spinal canal. This results in tissue compression and hinders normal spinal fluid flow. Although the Chiari (pronounced kee-ar'-ee) I malformation can cause problems in childhood, many people with this disorder do not do not develop symptoms until adulthood.

The Chiari I malformation occurs more often in women than in men though the exact cause is still unknown. However, it has been suggested that the malformation occurs during early embryonic development where an abnormally small posterior fossa results in the tonsils (and sometimes the lower part of the brainstem) being pushed downward through the opening at the bottom of the skull.

People with Chiari malformation who previously were asymptomatic (having no symptoms) may begin to experience symptoms as they grow older. Symptoms may include headache, dizziness, double vision, unusual eye movements, pain or numbness in the face or extremities, or ringing in the ears.