The second anatomic displacement related to downward central incisural herniation is a clockwise rotation of the visual pathway relative to the plane of the long axis of the visual pathway.
The third anatomic displacement related to downward central incisural herniation is downward displacement of the optic chiasm relative to the entrance of the sella that is estimated by the plane of the tuberculum sella, the plane that establishes the position of the diaphragm sella and also delineates the intracranial surface of the central skull base.
These anatomic relationships between the cervicomedullary junction and the cerebellar tonsils relative to the plane of the foramen magnum allow independent assessment of secondary displacement of the brainstem versus the caudal poles of the cerebellar tonsils in the context of central incisural herniation.
Exclusionary criteria included hydrocephalus, acute global brain swelling, and pseudotumor cerebri as seen on MRI, all of which can also produce central incisural herniation.
A simple grading system for downward central incisural herniation was created by assessing the mammillary bodies' relationship to the incisural line.
The normal visual pathway angle is initially positive, 7.42 degrees; however, downward central incisural herniation causes a progressive clockwise radial shift measuring -0.68 degrees (IH grade 1), diminishing to -1.19 degrees (IH grade 2) and finally to -9.71 degrees (IH grade 3).
Evidence of central incisural downward herniation was evident in every case of symptomatic IH.
