Chiari Malformation and Hindbrain Descent: Characterization and New Classification Based on Mechanism and Pathogenesis, and Surgical Management.

H. Chiari described 4 types of abnormal development of the posterior fossa, which were subsequently classified as Chiari malformation types I, II, III, and IV. Many issues in neurosurgery concerning classification and surgical management are without evolving concepts. This review aims to clarify the mechanisms and pathogenesis underlying hindbrain (the brain stem and cerebellum) descent, classify them accordingly, and discuss appropriate surgical management. We propose a classification of 4 independent pathogenic mechanisms: (1) constriction in the posterior cranial fossa (PCF) due to underdevelopment of the occipital bone; (2) enlargement of hindbrain; and (3) traction caused by tethering lesions. We examine the pathogenesis of hindbrain descent from embryological perspectives and neuroradiological findings, with a particular focus on lesser-known mechanisms. Additionally, another fourth mechanism is proposed: (4) instability at the craniocervical junction. We suggest a novel classification for Chiari malformation type I based on the underlying pathogenesis, guided by morphometric (occipital bone size) and volumetric (PCF volume) analyses. Furthermore, it delves deeper into their pathogenesis by drawing on insights from developmental biology, genetic studies, and experimental research. Surgical management is tailored to the underlying mechanism, and we proposed the algorithm for decision of surgical intervention. For crowding of the PCF due to underdevelopment of the occipital bone, posterior fossa decompression is the appropriate surgical intervention. For craniocervical instability, occipitocervical fixation is recommended. We also review the recent literature on surgical outcomes associated with each treatment approach. Finally, we highlight current genetic research related to the pathogenesis of hindbrain descent.