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).
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.
See the symptoms list for more details.
Click here to see an example of
CMI in which the cerebellar tonsils do NOT descend below the level of
the foramen magnum.
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.
