005. Do atlanto-occipital joint anomalies impact the need for occipital-cervical fusion following posterior fossa decompression for Chiari-1 malformation?

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Authors: Alexander T. Yahanda, MD

Congenital anomalies of the atlantooccipital articulation may be present in patients with Chiari-1 malformation. Yet it is unclear how these anomalies impact the biomechanical stability of the craniovertebral junction (CVJ) and whether they are associated with an increased risk for occipitocervical fusion (OCF) following posterior fossa decompression (PFD). Our objective was to determine the prevalence and clinical outcomes of condylar hypoplasia and atlas anomalies in children with Chiari-1 malformation and syringomyelia (SM).Methods: We analyzed the prevalence of condylar hypoplasia and atlas arch anomalies for the cohort of patients that underwent OCF+PFD within the Park-Reeves Syringomyelia Research Consortium (PRSRC) database. Condylar hypoplasia was defined by an atlanto-occipital joint axis (AOJA) angle >130 degrees. Atlas assimilation and arch anomalies were identified through presurgical radiographic imaging studies. A 2:1 control cohort (matched with age, sex, and duration of symptoms) that underwent posterior fossa decompression (PFD) only was similarly analyzed.
Clinical features and radiographic atlantooccipital joint parameters were compared between 19 subjects in the OCF+PFD cohort and 38 subjects in the PFD cohort. Demographic data was not significantly different between cohorts (p>0.05). Mean AOJA was significantly higher in the OCF+PFD (144 ± 12°) group compared to the PFD control group (126 ± 5°; p < 0.001). Within the OCF+PFD group, atlas assimilation and atlas arch anomalies were identified in 10 (53%) and 5 (26%) subjects, respectively. These anomalies were absent (n=0) within the PFD control group (p < 0.001 for both comparisons).
The presence of condylar hypoplasia and atlas arch anomalies impacts the biomechanical integrity of the CVJ junction. This was associated with an increased risk of instrumented fusion following decompression in patients with CM+SM. Presurgical identification of these skeletal abnormalities will guide surgical planning for and treatment of patients with complex Chiari and coexistent osseous pathology.