Morphometric assessment of the posterior cranial fossa and its contents in patients with chiari malformation type I and type 0

BACKGROUND: Chiari Malformation Type I and Type 0 are congenital malformations diagnosed by MRI findings of at least 5 mm and less than 3 mm of cerebellar ectopy below the foramen magnum respectively. In this study we aimed that to comparatively analyze the morphometry of the clivus, tentorium, and posterior cranial fossa in patients with Chiari malformation Types I and 0 versus healthy subjects, and to assess the potential contribution of these measurements to diagnostic accuracy. METHODS: In this study mid-sagittal MRI scans of 326 individuals obtained between 2018-2025 and including patients with Chiari Type I malformation (n = 111), Chiari Type 0 malformation (n = 27), and age- and sex-matched healthy controls (n = 188), were retrospectively analyzed. Multiple cranial base, posterior cranial fossa, cerebellar, and intracranial morphometric parameters and area-based ratios were measured on mid-sagittal T1-weighted images. RESULTS: We observed that, compared with Chiari Type 0 patients and controls, Chiari Type I patients exhibited a significantly shorter clivus, reduced posterior cranial fossa area, and increased cerebellar area, whereas foramen magnum diameter and total brain area were comparable among groups. Ratio-based analyses demonstrated significantly higher cerebellum-to-posterior fossa and cerebellum-to-brain area ratios in Chiari Type I, indicating increased relative cerebellar occupancy despite preserved global cranial proportions. CONCLUSION: Chiari Type I malformation is characterized by specific cranial base remodeling. These morphometric alterations are absent in Chiari Type 0, which remains anatomically comparable to healthy controls. These findings suggest that CM-I is associated with distinct morphoanatomical features, whereas CM-0 does not demonstrate a consistent structural substrate and may represent a functionally defined condition.