Arnold-Chiari MalformationSyringomyeliaForamen MagnumPlatybasiaDecompressive CraniectomyCranial Fossa, PosteriorEncephaloceleDecompression, SurgicalCerebrospinal Fluid PressureSubarachnoid SpaceOccipital BoneSpinal CanalCerebrospinal FluidMagnetic Resonance ImagingArachnoidCranial Nerve DiseasesArteriovenous MalformationsCervical VertebraeBrain Hemorrhage, TraumaticVascular MalformationsMeningomyeloceleCraniotomy