Ventral compression in adult patients with Chiari 1 malformation sans basilar invagination: cause and management

Abstract

Background: A small subset of patients with adult Chiari I malformation without basilar invagination (BI) and instability show ventral cervicomedullary distortion/compression and have symptoms pertaining to that. The cause of this ventral compression remains speculative. Additionally, it is unclear if these patients would require ventral decompression with posterior fusion or only posterior decompression would suffice.

Methods: Sixteen adult patients with Chiari I malformation with significant ventral cervicomedullary compression, in the absence of BI, were included in the study. Atlantoaxial dislocation (AAD) was excluded in these patients by flexion-extension craniovertebral junction X-rays and computed tomography (CT). Their clinical profile, especially symptoms pertaining to cervicomedullary compression, i.e. dysphagia, dysarthria and spasticity, were graded. The ventral cervicomedullary compression (VCMC) was quantified using pBC2 (maximum perpendicular distance to the basion-infero posterior point of the C2 body) on sagittal magnetic resonance imaging (MRI) and only those patients with pBC2 ≥9 mm were included. Furthermore, retroversion of dens and retro odonotid tissue thickness was calculated in each patient. Fifteen patients underwent posterior decompression alone and one refused surgery. Follow-up was done every 3 months. Repeat MRI was done at 1 year following surgery to look for pBC2.

Results: The mean pBC2 was 11 ± 0.2 mm. Retroversion of dens was responsible for VCMC in three patients and periodontoid crown in 13. There was no correlation between the tonsillar descent, age and the pBC2. All patients improved in symptoms of cervicomedullary compression following surgery. One patient worsened 6 months after initial improvement. The pBC2 did not change, as seen on follow-up MRI done in five patients.

Conclusions: VCMC in adult patients with Chiari I malformation in the absence of BI and/or AAD is due to periodontoid tissue (crown) or retroverted dens. Though a long-term study is required, it appears that all patients with Chiari I malformation, irrespective of the VCMC, can be given a chance with posterior decompression alone. Transoral decompression with posterior fusion may be required in a small subset of patients who fail to improve or worsen following posterior decompression only.