Objective : Achondroplasia is the most common heritable skeletal dysplasia. Cervicomedullary compression at the foramen magnum is highly associated with severe neurological complications including sudden death. Distinguishing which patients are at a high risk of neurological compromise is difficult. The aim of this study is to clarify the radiological features of the foramen magnum in achondroplasia that requires surgical decompression.
Materials and methods : We conducted a retrospective study at our department with regard to the radiological features of the foramen magnum in achondroplasia, including the sagittal diameters of the cervicomedullary junction, arachnoid space on magnetic resonance imaging (MRI), and the basion-opisthion interval on computed tomography (CT). We also measured the cross-sectional area of the cervicomedullary junction, arachnoid space on MRI, and foramen magnum on CT. Patients’ data were classified into operative and non-operative groups. From January 2012 to December 2015, nine children with achondroplasia were initially evaluated radiologically. The mean age at initial evaluation was 11.6 months, and the mean follow-up period was 33.1 months. Foramen magnum decompression was indicated for children with morphological change of the cervicomedullary junction under compression or intramedullary signal change on MRI.
Results : Seven of nine patients underwent foramen magnum decompression. The remaining two were managed conservatively. In the operative group, we found smaller sagittal diameter of the cervicomedullary junction and arachnoid space on MRI, but were not statistically significant (p=0.0556 and 0.0556, respectively). Besides, the operative group showed smaller cross-sectional area of the foramen magnum on CT, which was also insignificant statistically (p=0.0556). The morphological comparison of the foramen magnum on CT showed insufficient expansion of the posterior margin on the operative group. This anatomical feature caused ventral projection and dorsal compression to the cervicomedullary junction.
Conclusions : The smaller size of the foramen magnum in children with achondroplasia can be related to cervicomedullary signal change or compressive deformity on MRI. Further study is necessary to establish the surgical criteria for foramen magnum decompression in children with achondroplasia.