January 20, 2006 -- Defining exactly what Chiari is is not an
easy task. Because Hans Chiari first described the condition in
terms of the cerebellar tonsils herniating out of the skull,
most people define Chiari as tonsillar herniation of greater
than 3mm-5mm. Herniation is defined (or measured) as the
distance below the foramen magnum, or the opening at the bottom
of the skull through which the brain and spine meet.  While this
definition is still widely used today (potentially to the harm
of many patients), research has continued to show that the
amount of tonsillar herniation is not related to severity of
symptoms or clinical outcome. In fact, some people have
significant herniations, greater than 3mm, with no symptoms. In
fact, one study examined over 12,000 MRI's (Elster et al.) and
found that over 30% of the people who had herniations greater
than 5mm were symptom free.  Conversely, some people exhibit
classic Chiari-type symptoms with little to no herniation.
Having symptoms with no herniation is some