Chiari I anatomy after ventriculoperitoneal shunting: posterior fossa volumetric evaluation with MRI.

Introduction: Cephalocranial disproportion was said to be responsible for Chiari I malformation after ventriculoperitoneal shunt. We aimed to evaluate if the volumetric characteristics of Chiari I after a ventriculoperitoneal shunt was due to a general volumetric reduction and if it is restricted to the posterior fossa.

Results: Our results show that the posterior fossa volume, cisternal, clival length, and posterior cranial fossa volume ratio were reduced in the shunted group compared to the controls (p<0.05). Cerebellar and supratentorial volumes were similar between both groups. Craniocaudal extent, inferior, and superior tonsillar herniations were greater in the shunted group than control (p<0.05). The frontal occipital horn ratio in both groups was within normal range.

Discussion: Chiari I anatomy after a ventriculoperitoneal shunt could develop in children and we propose a "posterior cranial fossa disproportion" rather than a "cephalocranial disproportion."