Arachnoid Cyst Resulting in Tonsillar Herniation and Syringomyelia in a Patient With Achondroplasia

Case Report

Andrew M. Bauer, B.S.; Diane M. Mueller, N.D., R.N., C.-F.N.P.; John J. Oró, M.D.


Neurosurg Focus. 2005;19(5) 

In This Article

Abstract and Introduction

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.

Achondroplasia, the most common form of bone dysplasia and the best-known form of congenital dwarfism, is caused by a disturbance of endochondral bone formation.[10,16,19,20,24] Neurological symptoms are identified in as many as 47% of patients with achondroplasia. Symptoms in children include psychomotor delay, hypotonia, feeding and sleep disorders, apnea, macrocrania with or without hydrocephalus, and compressive spinal syndromes.[19] Cervicomedullary compression is also common in the pediatric population[20,24] and is almost always diagnosed and treated before adulthood. In the adult population of patients with achondroplasia, spinal stenosis with compression of the spinal cord results in many radicular pain syndromes.[2,15,19] The neurological manifestations can be severe, with extreme cases resulting in death due to hydrocephalus or compression of the brainstem. Patients in all age groups with achondroplasia generally have a small cranial base, whereas the rest of the skull is normal in size. This size discrepancy may predispose patients to anomalous formation of the posterior fossa and foramen magnum, thus creating a smaller than normal posterior fossa.

Syringomyelia has been associated with many intracranial and spinal anomalies. One theory is that syringomyelia results from obstruction of CSF flow at the level of the foramen magnum and the outlet of the fourth ventricle.[1] Obstruction of CSF flow may lead to an increased pulsatile pressure within the spinal canal and may force CSF into the spinal cord.[18] This obstruction is most commonly caused by the Chiari malformation and crowding of the posterior fossa. The Chiari malformation has been traditionally defined as downward herniation of the cerebellar tonsils through the foramen magnum.[14] The average delay in diagnosis of a Chiari malformation is 5 years after the onset of symptoms, leaving ample time for the development of other complications such as syrinx.[5]

Intracranial and posterior fossa mass lesions such as tumors or arachnoid cysts may also result in tonsillar herniation. Arachnoid cysts are benign collections of CSF within an anomalous arachnoid enclosure that account for approximately 1% of intracranial mass lesions.[6] There have been several reports of large arachnoid cysts in the posterior fossa associated with syrinx.[4,9,21,23] We describe the case of a patient with achondroplasia who presented with an 8-month history of headache and neurological symptoms. Brain MR imaging revealed a large arachnoid cyst in the posterior fossa that was causing a 6-mm tonsillar herniation and a cervicothoracic syrinx.