Chiari I malformation is a congenital condition in which part of the brainstem herniates through the base of the skull. Normally the cerebellar tonsils reside inside the skull. For unknown reason in a Chiari I malformation, the cerebellar tonsils hang lower. This can compress other parts of the brainstem and impair flow of cerebrospinal fluid (CSF). In severe cases, a cyst (called a syrinx) can form in the spinal cord.
Various symptoms may arise from a Chiari I malformation. Many people with the condition do not have any symptoms at all.
Some patients with low lying cerebellar tonsils fit the imaging characteristics (MRI) of a Chiari I malformation. Additional testing can be performed to determine if the CSF flow is impaired. If the CSF flow is normal, these patients may not require surgery. If the CSF flow is impaired, and symptoms are present, surgery may be an option.