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Chiari Type I Malformation
The brain sits in a cavity surrounded by bone. The brain stem normally is positioned in a funnel-like cavity above the spinal cord. Chiari malformation occurs when the posterior fossa is formed improperly. Instead of sitting in its proper space, the brain stem is displaced downward into the funnel, putting pressure on this part of the brain and spinal cord. This displacement also obstructs the normal flow of cerebrospinal fluid (CSF). In some cases, the brain stem appears shrunken and may stick to the spinal cord. This downward displacement of brain tissue, specifically the cerebellar tonsils, is called Chiari malformation. Although a form of Chiari malformation occurs in children, the Chiari I malformation usually causes problems in adults.
Some people with Chiari I malformation also have a syrinx or syringomyelia. This is a long cavity containing CFS within the spinal cord. This condition is not always associated with Chiari I malformation and may also be associated with spinal cord trauma or tumors. The cause of a syrinx is unknown. There is some evidence that the normal flow of CSF is blocked because the cerebellar tonsils have descended into the posterior fossa. This blockage causes the CFS to flow abnormally, creating an abnormal pathway in the spinal cord. |
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The cause of Chiari malformation is unknown. It may occur during early embryo development of the brain stem and spinal cord. An abnormally small posterior fossa forces the brain stem downward. People with Chiari I malformation often have no symptoms until the enter their 30s or 40s. The malformation occurs slightly more often in women than men.
