Had mri scan to check for cci / chiari malformation (eds / severe headaches) I'm confused by one sentance "the cerebellar tonsils are situated at the entrance to the foramen magnum but do not obstruct it" - what does that mean? Chiari 0 possibility?

Basics. This means there is no evidence by this one study that suggests an obstruction of CSF flow by the classical herniation of cerebellar tissue that defines most Chiari cases. The new label class "O" is defined by clinical features of Chiari with this pattern. Other CSF flow studies or clinical information would be needed to designate it as such. In most this is just a normal MRI. .