In Plain English...What is Chiari Malformation?
CMI is caused by the underdevelopment of the lower back of the skull (known as the posterior cranial fossa, or PCF). The PCF is abnormally small but, not really noticeable to the eye. It's only a matter of millimeters. The small PCF causes crowding and results in a downward growth of the cerebellar tonsil through the opening at the lower base of the skull (known as the foramen magnum).
Traditionally, CMI has been identified by whether or not it extends below a line drawn from the basion to the opisthion.
When this downward growth extends below the lower opening of the skull, it literally creates pressure on the brain stem (the upper part of the spinal cord). The brain stem is responsible for a vast majority of body functions and people who have this condition have all kinds of strange symptoms, ranging from headache to irritable bowel syndrome. In people with minimal CMI, there is little or no descent of the tonsils and, therefore, less pressure on the brainstem. These people may suffer horrible headaches, fatigue, and even seizures but, may not notice symptoms associated with pressure on the brainstem.
When not associated with trauma, syrinx is a buildup of fluid inside the spinal cord due to increased pressure caused by blockage and can take years to form. Ideally, there should be continuous, uninterrupted flow of CSF around the spinal cord and brain. When this flow is blocked (as with CMI), pressure builds and the fluid can eventually form "pockets" inside the spinal cord itself. Imagine using a needle to inject water into the middle of a hot dog - this is a good analogy. Syrinx can be extremely painful and can lead to permanent damage of the spinal cord.