Our discussion of anatomy is best begun by demonstrating the key attributes of a normal brain as it differs from one with CMI. A normal brain mri is shown at the right. Notice the generous amount of CSF space posterior to the cerebellum. Also, notice that the tonsils do not impede flow at the foramen magnum. There is ample space for CSF around the cerebellar tonsils and behind the cerebellum.
Interestingly, though, if a line is drawn from the basion to the opisthion, the cerebellar tonsils do appear to show some downward growth (although, here, they do not cross the level of the foramen magnum). This gives foundation to the argument that tonsillar descent is not a valid means of diagnosing CMI. The "redefinition" of CMI must be kept in mind - CMI is not a condition that is defined by the level of tonsillar descent. It is defined by a lack of CSF flow posterior to the cerebellum.
A case of CMI (0mm) is shown below. Here, there is an obvious lack of CSF flow space behind the cerebellum. The cerebellar tonsils do not descend below the level of the foramen magnum, nor do they impede flow. If comparing tonsillar descent alone, little difference can be seen between this case and the one above. However, there is a significant difference. This case was diagnosed as CMI and treated with decompression surgery at the Chiari Institute in NY. The patient's symptoms resolved immediately following surgery.