Purpose: Chiari malformation type I (CMI) is characterized by deformed hindbrain. This study aimed to quantitatively evaluate the alterations in position of hindbrain after Posterior fossa decompression (PFD), and to identify the factors associated with syrinx resolution in pediatric patients with CMI.
Methods: Eighty-seven patients, aged from 5 to 18 years, who underwent PFD for CMI between September 2006 and September 2012 were retrospectively reviewed. On mid-sagittal MR images, the position of medulla oblongata and cerebellum was quantitatively evaluated preoperatively and at follow-up. The maximal syrinx/cord (S/C) ratio and syrinx length were also measured. Significant improvement of syrinx was defined as a more than 20 % decrease in maximal S/C ratio or length on follow-up MRI.
Results: Neurological deficits were found in 51 of the 87 patients preoperatively and 37 (72.4 %) of them obtained improvement of their symptoms at the last visit. Overall, upward shifting of the tip of cerebellar tonsil was observed in 66 (75.9 %) patients at the last follow-up. Moreover, the mean longitudinal distance of the tip of cerebella tonsil changed from 16.47 ± 5.00 to 13.89 ± 4.38 mm (P < 0.001) at final follow-up. Significant syrinx resolution was noticed in 79 (90.8 %) cases. Pointed cerebellar tonsils were found in 85 (97.7 %) of our patients preoperatively and 78 (91.8 %) of them acquired round cerebellar tonsils after PFD. The improvement of maximal S/C ratio was significantly correlated with upward shifting of the tip of cerebellar tonsil (P = 0.023).
Conclusions: Following PFD for CMI, position and morphology of the cerebellar tonsil could revert to normal in most of the pediatric patients, and the upward shifting of the tip of cerebellar tonsil is significantly correlated with syrinx improvement. From this study, PFD without shunting may be effective for syrinx secondary to CMI in pediatric population.