Herniation of cerebellar tonsils following supratentorial shunt placement.

Acquired Chiari 1 following ventriculoperitoneal shunting is an extremely unusual event. We report the case of an 8-year-old boy who presented with clinical and radiological signs of cerebellar tonsil herniation shortly after the placement of a cystoperitoneal shunt. Quantitative analysis of posterior fossa volumes (PFV) revealed that the patient had a smaller posterior fossa than age-matched normal controls. This abnormality, expressed as a decreased ratio between the posterior fossa and the supratentorial cavities (PFR), had already been present when the preoperative MRI was done. Our results suggest that preexisting structural abnormalities in the posterior fossa may constitute an important factor in the development tonsillar herniation following supratentorial shunts.