Natural history of craniocervical alignment in Chiari patients and the impact of posterior fossa decompression

OBJECTIVE: Chiari malformation (CM) involves a broad disease spectrum, where rare complex CM cases can be associated with craniocervical junction (CVJ) instability and require occipitocervical fusion. However, the natural progression of CVJ alignment in the general CM type I and 1.5 populations treated with posterior fossa decompression (PFD) remains insufficiently characterized. The authors aimed to compare CVJ alignment changes in patients who underwent PFD versus patients with CM who did not undergo surgery. METHODS: The authors conducted a retrospective cohort study at their institution of all patients diagnosed with CM I and 1.5 from 2000 to early 2023. Demographic, clinical, and surgical data were collected, along with preoperative and postoperative MRI measurements, including tonsillar herniation, brainstem descent, clivoaxial angle (CXA), and condylar-C2 sagittal vertical alignment (C-C2SVA). RESULTS: A total of 241 patients were included, with 201 undergoing PFD and 40 managed conservatively (controls). No significant differences were observed between groups in age at diagnosis, sex, or genetic diagnoses. In the PFD group, 55% underwent duraplasty and 45% underwent bone-only decompression. Baseline craniocervical alignment measurements showed a lower CXA in the PFD group (144.4° ± 13.4°) compared to controls (148.5° ± 14.2°) (p = 0.04) but no difference in C-C2SVA. Changes over time showed a small but significant decrease in CXA at < 1 year after surgery in the PFD group (-2.7°) compared to controls (-2.0°) (p = 0.008), but no differences were noted at 1-2 years. No differences in C-C2SVA were observed over time in either group. CONCLUSIONS: PFD does not significantly impact craniocervical alignment or increase the risk of occipitocervical fusion in CM I and 1.5 patients. These findings support PFD as a safe first-line treatment, even in complex CM cases, and provide important information for patient education regarding the risks of surgery.