Abstract:Objective To investigate the pathogenesis and surgery effect of Chiari malformation Type Ⅰ (CM-Ⅰ). Methods Twenty CM-Ⅰ patients meeting inclusion criteria treated in Sir Run Run Shaw Hospital between December 2011 to December 2015 were retrospectively studied to explore the relationship between posterior cranial fossa volume(PFV) and CM-Ⅰ and the therapeutic effect of posterior fossa decompression. At the same time, 20 normal outpatients undergoing thin-section computed tomographic (CT) scans for headache and other reasons were served as controls. Results The posterior fossa volume of CM-Ⅰ patients was smaller than that of normal group[(176.2±11.5) cm³ vs (184.9±11.4)cm³, t=2.403, P=0.021]. CM-Ⅰ patients were divided into three groups according to the degree of cerebellar tonsil herniation:degree Ⅰ, Ⅱ and Ⅲ，and no differences in posterior fossa volume were found between patients of degree Ⅰ, Ⅱ and Ⅲ. [(170.1±9.9)cm³ vs (184.3±6.4)cm³ vs (174.8±12.0)cm³, F=2.232, P=0.138]. There was a significant difference in therapeutic effect of posterior fossa decompression between symptom disappearance group and symptom improvement group[(170.9±10.4)cm³ vs (181.7±9.6)cm³, t=2.306, P=0.034]. Conclusion The posterior cranial fossa stenosis may be one of the causes of cerebellar tonsil herniation and is not obviously associated with its severity in CM-Ⅰ patients. When the posterior fossa decompression is performed for CM-Ⅰ patients, the smaller the posterior fossa volume is, the better the curative effect may be.