POSTERIOR-FOSSA VOLUME AND RESPONSE TO SUBOCCIPITAL DECOMPRESSION IN PATIENTS WITH CHIARI-I MALFORMATION

SMALLER POSTERIOR FOSSA (PF) volume has been suggested to be one of th e mechanisms responsible for tonsillar herniation through the foramen magnum in patients with Chiari I malformation (CM I). Although previou s radiological analyses of the cranial anatomy have suggested a smalle r PF volume in patients with CM I, the relationship of the PF volume t o decompressive surgery has not been reported. We have measured the ra tio of PF volume to supratentorial volume (PF ratio [PFR]) in 20 patie nts with CM I and 20 control patients by retrospectively studying thei r magnetic resonance images with a computerized image analyzer. The me an PER in patients with CM I (with or without syringomyelia) was signi ficantly smaller than for those in the control group (15.6 +/- 1.9 ver sus 17.5 +/- 1.2, P = 0.0008). Although PFR did not correlate with the extent of tonsillar herniation in patients with CM I, it did directly correlate with their age, i.e., younger patients with CM I (but not c ontrol patients) had smaller PFRs. All but three patients responded bo th clinically and radiographically to decompressive surgery. Those pat ients who did not benefit from surgical intervention had normal PFRs. We conclude that: 1) PFRs are smaller in most patients with CM I; 2) a smaller PF may be a primary cause of tonsillar herniation; 3) patient s with CM I who have smaller PFRs tend to develop symptoms earlier tha n those with normal values; 4) patients with smaller PFRs tend to resp ond better to suboccipital decompression.