The role of occipital condyle and atlas anomalies on occipital cervical fusion outcomes in Chiari malformation type I with syringomyelia: a study from the Park-Reeves Syringomyelia Research Consortium

Abstract

Objective: Congenital anomalies of the atlanto-occipital articulation may be present in patients with Chiari malformation type I (CM-I). However, it is unclear how these anomalies affect the biomechanical stability of the craniovertebral junction (CVJ) and whether they are associated with an increased incidence of occipitocervical fusion (OCF) following posterior fossa decompression (PFD). The objective of this study was to determine the prevalence of condylar hypoplasia and atlas anomalies in children with CM-I and syringomyelia. The authors also investigated the predictive contribution of these anomalies to the occurrence of OCF following PFD (PFD+OCF).

Methods: The authors analyzed the prevalence of condylar hypoplasia and atlas arch anomalies for patients in the Park-Reeves Syringomyelia Research Consortium database who underwent PFD+OCF. Condylar hypoplasia was defined by an atlanto-occipital joint axis angle (AOJAA) ≥ 130°. Atlas assimilation and arch anomalies were identified on presurgical radiographic imaging. This PFD+OCF cohort was compared with a control cohort of patients who underwent PFD alone. The control group was matched to the PFD+OCF cohort according to age, sex, and duration of symptoms at a 2:1 ratio.

Results: Clinical features and radiographic atlanto-occipital joint parameters were compared between 19 patients in the PFD+OCF cohort and 38 patients in the PFD-only cohort. Demographic data were not significantly different between cohorts (p > 0.05). The mean AOJAA was significantly higher in the PFD+OCF group than in the PFD group (144° ± 12° vs 127° ± 6°, p < 0.0001). In the PFD+OCF group, atlas assimilation and atlas arch anomalies were identified in 10 (53%) and 5 (26%) patients, respectively. These anomalies were absent (n = 0) in the PFD group (p < 0.001). Multivariate regression analysis identified the following 3 CVJ radiographic variables that were predictive of OCF occurrence after PFD: AOJAA ≥ 130° (p = 0.01), clivoaxial angle < 125° (p = 0.02), and occipital condyle-C2 sagittal vertical alignment (C-C2SVA) ≥ 5 mm (p = 0.01). A predictive model based on these 3 factors accurately predicted OCF following PFD (C-statistic 0.95).

Conclusions: The authors' results indicate that the occipital condyle-atlas joint complex might affect the biomechanical integrity of the CVJ in children with CM-I and syringomyelia. They describe the role of the AOJAA metric as an independent predictive factor for occurrence of OCF following PFD. Preoperative identification of these skeletal abnormalities may be used to guide surgical planning and treatment of patients with complex CM-I and coexistent osseous pathology.

Keywords: Chiari malformation; atlas assimilation; condylar hypoplasia; congenital; occipitocervical fusion; syringomyelia.

FIG. 1.

Demonstration of radiographic indices for CVJ evaluation. A and B: Coronal T2-weighted MR (A) and CT (B) images demonstrating the AOJAA, formed by the intersection of lines drawn through the AOJs (dotted lines). C: Sagittal T1-weighted MR image demonstrating the CXA, defined as the angle formed by the intersection of the Wackenheim clivus baseline, a line drawn along the clivus and continued inferiorly into the upper cervical spinal canal, with a line drawn along the posterior surface of the axis body and odontoid process. D: Sagittal T1-weighted MR image demonstrating the pB–C2 distance, defined as the distance (red star) of a perpendicular line between the basion to inferior-posterior aspect of the C2 body and a line through the odontoid tip to the ventral dura. E: Sagittal T1-weighted MR image demonstrating C–C2SVA, defined as the distance (yellow line) from the posterior aspect of the C2–3 disc space to a plumb line dropped from the midpoint of the occiput–C1 joint (red star) to a line parallel to the C2 inferior endplate in a midsagittal MRI plane.

FIG. 2.

ROC curves for the logistic regression predictive model (C-statistic/AUC 0.96) (A) and the weight-additive regression predictive model (C-statistic/AUC 0.95) (B).