MR imaging of an extreme case of cerebellar ectopia in a patient with Chiari II malformation.

A 12-year-old boy who had been born with a meningomyelocele that was closed surgically 3 days after his birth had preoperative workup for a planned anterior release and posterior spinal instrumentation and fusion for worsening lumbosacral scoliosis. The patient had a history of developmental delay and learning disabilities and bowel and bladder incontinence. Physical examination showed thoracolumbar scoliosis with a curvature of 135 ° to the left. The lower extremities were spastic without evidence of any movements, deeptendon reflexes were absent, and sensation was absent below the level of T6. MR of the spine and hindbrain showed cerebellar tonsillar herniation posteriorly and to the right of the cervical cord extending down to approximately the C3 level. From C3 to T3 on the left, an intradural extramedullary mass was present, which was not causing a mass effect on the atrophied spinal cord (Fig. 1 A). This mass was dorsal and to the left of the atrophic lower cervical and thoracic cord (Fig . 1 B) and did not enhance with gadopentetate dimeglumine. ACT myelogram confi rmed this finding. MR of the brain showed absence of the entire left cerebellar hemisphere; the right cerebellar hemisphere was present in the posterior fossa, and the cerebellar tonsil was slightly ectopic (Fig . 1 C). The patient had suboccipital craniectomy, cervical laminectomy, and intraspinal exploration to identify the cervical mass. At surgery, it was confirmed that the right cerebellar tonsil was ectopic as shown by MR; on the left side, attenuated vessels of vertebral artery origin coursed caudal ly, not in continuity with cerebellar tissue , but supplying the intradural extramedullary mass in the midcervical region. Biopsy of this mass showed that it was consistent with cerebellum. The vessels presumably arose from the posterior inferior cerebellar artery, and the mass represented a heterotopic cerebellar hemisphere.