Quantitative preoperative craniovertebral junction anatomy in Chiari I malformation: a comparative MRI study

Background Chiari I malformation (CM-I) is characterized by caudal displacement of the cerebellar tonsils through the foramen magnum. Craniovertebral junction (CVJ) anatomical variations may contribute to disease pathophysiology. This study is the first of a six-article institutional series investigating the anatomical substrate, surgical outcomes, and complications of CM-I.Objective To quantitatively compare CVJ measurements between CM-I patients and healthy controls, and establish the morphometric baseline of a consecutive surgical cohort.Methods A retrospective MRI-based study included 20 CM-I patients from a consecutive cohort of 42 operated patients (2012–2019) and 20 age- and sex-matched controls. McRae line length, bulbo-cervical angle, and tonsillar descent were measured on mid-sagittal T2-weighted MRI by two independent blinded observers. Inter-observer reliability was assessed by intraclass correlation coefficient (ICC); significance was set at p <0.05.Results McRae line length was significantly greater in CM-I patients (34.41 ± 2.29 mm) than controls (28.98 ± 2.80 mm; p <0.0001; 95% CI: 4.13–6.73 mm). Mean tonsillar descent was 7.52 ± 0.98 mm (range: 5.3–8.2). ICC was 0.94 (95% CI: 0.89–0.97). No significant between-group difference was observed in bulbo-cervical angle. The cohort comprised 55.0% females (mean age: 32.5 years); occipital and Valsalva-induced headache were present in 78.6% and 81.0%, respectively.Conclusion CM-I is associated with significant foramen magnum enlargement, supporting the role of posterior cranial base morphology. McRae line enlargement was confirmed as an independent predictor of surgical complications (OR 4.2; p=0.037).