July 31st, 2009 -- For several years now, researchers have been
using MRIs to measure skull base dimensions of Chiari patients in what is known as
morphometrics. It was from this type of research that emerged the theory that a
significant number of Chiari cases - if not a majority - are caused by an
underdevelopment of the skull in the occipital area (Figure 1). In other words, it
is not that the brain is too big for the skull, but rather that the skull is too
small for the brain.
While the evidence is fairly strong that there is something of interest in the posterior fossa region of Chiari patients, as is often the case, the devil is in the details. Researchers have used various methods to measure the posterior fossa volume and in some cases have found that indeed on average Chiari patients tend to have smaller posterior fossas than normal. However, this is not true for every patient and other studies have contradicted this finding, with one showing that only Chiari men had smaller posterior fossas, and another finding that only patients with both Chiari and syringomyelia had smaller posterior fossa regions. Because of this, researchers are still exploring different ways to analyze the relative size of this skull region. Some have tried using a volume ratio of skull to brain tissue, while others have turned to measuring specific bone segments as opposed to general volume measurements.
Figure 1: Occipital Bone and Posterior Fossa (from Conquer Chiari: A Patient's Guide)
This is the approach that a group of French researchers took in using morphometrics to look for skull undergrowth in a group of 17 adult Chiari patients as compared to a group of healthy controls. They published their findings recently on-line in the Journal of Neurosurgery. All the Chiari patients had at least 5mm of herniation and were clearly symptomatic. The controls were taken from people who had had MRIs associated with migraines but were found to not have any herniation. Based on previously published morphometric studies, the researchers decided to focus on 4 skull measurements and 3 brain tissue measurements (Figure 2 and Figure 3).
Figure 2: Skull Measurements Used in Study:
a) basiocciput, FM) foramen magnum (FM), b) supraocciput,
c) tentorial angle
Figure 3: Brain Measurements Used in Study:
d) hindbrain length, e) cerebellum length, f) tonsillar herniation
The researchers found that of the four skull measurements, only
the basiocciput differed significantly between Chiari patients and the controls
(Table 1). Specifically, for the Chiari group the average length of this bone was
19.4 mm compared to 25.7 mm for the controls. Interestingly, the brainstem and
cerebellum lengths didn't differ between the two groups, supporting the idea
that the brains of Chiari patients are of normal size. Also of note was that the
fact that the length of tonsillar herniation did not correlate to any other
measurements, providing even more evidence that the size of a Chiari malformation
is not a good indicator of the crowding or symptoms.
In discussing their findings, and the entire idea of a small posterior fossa, the authors make an interesting point; namely that if the skull abnormalities found with Chiari are due to a defect in the early embryo, why is there such a range in the age when symptoms become apparent? Why isn't everyone symptomatic at a very early age? One possibility, as the authors note, is that there are actually different mechanisms which can lead to a mismatch between the skull and the brain. For example, some skull sutures don't fully close until late teens or adulthood, which could push back symptoms. This would imply that in some cases Chiari is actually a dynamic phenomenon, and there are case studies which support this view.
However, another possibility is that crowding alone doesn't always lead to symptoms. For example, perhaps over time scar tissue and adhesions build up which then lead to symptoms. Of course a wild card in all of this speculation is what role physical trauma plays in sparking symptoms, which is currently not well understood.
Morphometrics is a promising area of Chiari research, but in order to really have an impact results from these types of studies must be synthesized into some type of useful theory.
-- Rick Labuda