Hindbrain herniation: an unusual occurrence after shunting an intracranial arachnoid cyst. Case report.

The case is presented of a prepontine arachnoid cyst in a 28-year-old woman. She was treated by craniotomy, stereotactic aspiration, internal shunting, and finally by insertion of a cystoperitoneal shunt. Eight months later she developed an asymptomatic Arnold-Chiari type I malformation. The pathogenesis of this unusual hindbrain herniation is explained on the basis of the differential pressure between two compartments and cephalocranial disproportion.