Syringomyelia in Chiari malformation: relation to extent of cerebellar tissue herniation.

In 47 patients with Chiari malformations (41 with Type I, 6 with Type II) diagnosed by magnetic resonance imaging, syringomyelia was found significantly more frequently in those who had a herniation of 9 to 14 mm (9 of 16, 56%) than in those who had a smaller (2 of 15, 13%) or larger (2 of 16, 13%) herniation. Syringomyelia associated with the Chiari malformation is generally considered to be due to mechanical factors induced by altered cerebrospinal fluid hydrodynamics. On the basis of the present study, one may speculate whether these mechanical factors affect the cord most efficiently at an intermediate level of herniation, which may prove to be a risk factor for the development of syringomyelia.