ACM.medullary spur

Notice the changes to the cerebellum, medulla, and spinal cord in this image * Cerebellar peg: Protrusion of vermis and hemispheres through the foramen magnum (90%) results in craniocaudal elongation of cerebellum behind the spinal cord * Medullary kink: The medulla is kinked inferiorly (75%) and lies dorsal to the spinal cord, which is unable to descend because of competent dentate ligaments * Towering cerebellum or vermian pseudotumor: The cerebellar hemispheres and vermis also extend above the incisura of the tentorium * Corners of the cerebellum are wrapped around the brainstem, pointing anteriorly and laterally * Tubelike elongated fourth ventricle: The fourth ventricle is elongated craniocaudally, narrowed transversely, and decreased in anteroposterior diameter * The cerebellopontine cistern and the cisterna magna are obliterated. * The combined displacements of the spinal cord, medulla, pons, and cerebellum form a cascade of herniations, each of which compresses all of the tissue in front of it, displacing them anteriorly. * In older patients, a wide subarachnoid space may be seen behind a vermis that is deeply grooved at the level of cervical nerve C1. The vertebral artery frequently loops on itself within the cervical canal and passes caudally to the level of cervical nerve C3. The possible cause of this groove may be the pulsatile effect of the vertebral artery on the adjacent cerebellum.