Arachnoid cyst resulting in tonsillar herniation and syringomyelia in a patient with achondroplasia. Case report.

Achondroplasia has been associated with varying degrees of cervicomedullary and spinal compression, although usually in the pediatric population. Large arachnoid cysts have also been found to result in tonsillar herniation and syringomyelia. The authors present the case of a patient with achondroplasia who presented with symptoms of foramen magnum compression and syringomyelia, and who was subsequently found to have a large posterior fossa arachnoid cyst. This 38-year-old woman with achondroplasia presented with an 8-month history of headache and numbness of the hands and fingers. Admission magnetic resonance (MR) imaging of the head and spine revealed a large arachnoid cyst in the posterior cranial fossa, a 6-mm tonsillar herniation consistent with an acquired Chiari malformation, and a large cervicothoracic syrinx. The patient was treated using suboccipital craniectomy, C-1 laminectomy, fenestration of the arachnoid cyst, and decompression of the acquired Chiari malformation with duraplasty. Surgical decompression resulted in improvement of the presenting symptoms, adequate decompression of crowding at the foramen magnum, and resolution of the syrinx. Although there was only partial reduction in the retrocerebellar cisternal space on follow-up MR imaging, no residual symptoms were related to this.