Noonan syndrome with occipito-atlantal dislocation and upper cervical cord compression due to C1 dysplasia and basilar invagination

A 11-year-old female with Noonan syndrome presented with occipito-atlantal dislocation and upper cervical cord compression due to C1 dysplasia and basilar invagination. Computed tomography (CT) of the cervical spine showed dysplasia of the C1 posterior arch and bilateral dislocation of the occipito-atlantal joints. Dynamic lateral radiography revealed no instability at the occipito-atlantal joints. CT also demonstrated basilar invagination. The tip of the odontoid process extended above the Chamberlain line by 9 mm and the McGregor line by 10 mm. Whole spinal radiography showed no scoliosis. C1 laminectomy was performed with instrumented occipito-C2 fixation. The postoperative course was uneventful, and magnetic resonance imaging revealed sufficient decompression of the upper cervical cord at 2 months after surgery. CT demonstrated solid bony fusion between the occipital bone and C2 at 8 months after surgery. Cervical neuraxial malformations are rare in patients with Noonan syndrome.