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 sometimes referred to as Chiari 0, and has become a controversial subject. Some surgeons believe that operating on Chiari 0 type patients is beneficial, while others remain skeptical.
Unfortunately, the way Chiari is defined now causes problems for patients in two ways. Some patients, with significant herniations and symptoms, are told that Chiari is no big deal, lots of people have herniations, and that it is benign. At the other end of the spectrum, Chiari 0 patients are told they don't have Chiari because there is only a little herniation, and their symptoms are dismissed as being due to something like stress, or emotional problems.
This limited usefulness of tonsillar herniation in defining Chiari has led many experts to base their diagnoses not solely on MRI's, but rather on a combination of MRI, cine MRI, symptoms, neurological exam, and their own experience and judgment. It has also led researchers to look beyond the herniation measurement for more useful metrics which might define the Chiari condition.
Along those lines, several research studies over the years have focused on the size of the posterior fossa region in Chiari patients. The posterior fossa is the area in the back of the skull, near the bottom where the cerebellum and brain stem are situated. Research has shown that in general, Chiari patients have smaller posterior fossa's than average people. This has led to the theory that Chiari is not a disorder of the brain, but rather a result of the skull being too small for a normal sized brain.
Now, a team from the Allegheny Neuroscience Institute in Pittsburgh, led by Dr. Sekula and Dr. Jannetta, have extended this line of research and shown that so-called Chiari 0 patients tend to have small posterior fossa's, similar to Chiari I patients. They published their work in December, 2005 in the journal Cerebrospinal Fluid Research.
Specifically, they looked at 22 patients who were suffering from Chiari like symptoms (see Table 1) and compared their MRI's to 25 people with no evidence of Chiari or syringomyelia (they had been treated for trigeminal neuralgia). The Chiari 0 group [Ed. note: the authors did not use the term Chiari 0 in their paper, they referred to the group as Chiari-like] was comprised of 15 women and 7 men. On average they had been experiencing symptoms for more than 7 years, and none had been treated surgically.
The patients were given a complete exam, and filled out both a symptom check-list and a personality assessment. The personality assessment was designed to identify anyone with emotional problems or who was depressed. In addition, the group underwent MRI's which were then reviewed by a physician who did not know their diagnosis.
Based upon the results from other studies, the team decided to look at nine different measurements in the posterior fossa region (see image below).
They found that in the Chiari 0 group, 4 of the nine measurements
were significantly different from the control group (see Table 2). The clivus, the
basiocciput, and the basisphenoid were all smaller in the Chiari group, and the
tentorial angle was steeper. Interestingly, the most significant difference
between the two groups was the clivus. An article in the December issue of Chiari
& Syringomyelia News (Tracing The Origins
Of Chiari) reported on a study which hypothesized that Chiari is due to a
problem in the development of this exact structure It should also be noted that
the size of the cerebellum was not significantly different between the two groups.
While the authors believe that their findings indicate that the current definition of Chiari may be too restrictive, they are cautious about the surgical implications until more research is done. At this time, they do not recommend surgery for patients with minimal herniation, but suggest observation instead.
The findings from one study are not likely to eliminate the controversy surrounding Chiari 0, but it is certainly a start.