That was the question posed at a scientific conference, or to be more accurate, "What percent of hindbrain hernias are asymmetrically descended?"  The answer wasn't obvious, according to Dr. R. Shane Tubbs, of the University of Alabama Birmingham,  so he and his colleagues, Dr. Wellons and Dr. Oakes at UAB, decided to study the question and reported their results in the October, 2002 issue of the journal Pediatric Neurosurgery.

While recent research has shown that the size of a malformation - measured in millimeters of descent into the spinal area - does not predict severity of symptoms, there has been little focus, historically, on the shape (in regards to symmetry) of the tonsils.  Among the Chiari public, the MRI image most often used to describe a Chiari malformation (aka tonsillar ectopia), is a side view; in which it is not even obvious that there is a distinct right and left tonsil of the cerebellum.  An image from a different angle, however, clearly shows the right and left tonsils.

So the questions the team from UAB wanted to answer were how often are the tonsils descended the same length, how often is either the right or left tonsil descended further than the other, and in cases where one tonsil is further descended, can it be linked to any symptoms.

To answer these questions, the team retrospectively reviewed the preoperative MRI's of 42 pediatric Chiari I patients they had operated on over the previous 9 years.  From the images, they were able to measure the amount of descent for the right and left tonsil of each patient.  The patients records were reviewed to note preoperative symptoms and surgical outcome.

They found that in 64% of the cases, the right tonsil was descended further than the left, in 13% of the cases the left tonsil was descended further than the right, and in 23% of the cases, the tonsils were symmetrically descended.  The average difference between the amount of descent of the two tonsils was 3.73 mm.  Is this even enough to notice during surgery?  "At operation symmetry is appreciated," notes Dr. Tubbs.  "However, prior to our examination, this was not examined thoroughly or dwelled upon.  We now appreciate this, and I would say most cases have some degree of asymmetry, although the discrepancy is often minimal."

As for symptoms and surgical outcome, six of the patients (18%) had symptoms - such as weakness or pain - that occurred only on the side of greater tonsillar descent.  The surgical outcome did not seem to depend on the symmetry of descent.  Perhaps the most surprising finding was that 95% of the patients who had a syrinx in addition to the Chiari malformation had a right tonsil that was descended lower than the left.

Dr. Tubbs, however, believes it is too early to speculate on the significance of this and points out that larger studies are needed to confirm or deny the syringomyelia observation.  Let's hope the surgeons and researchers from UAB continue to address this interesting topic.