Chiari malformation type I is a common developmental anomaly of the craniovertebral junction, most notably characterized by downward herniation of the cerebellar tonsils through the foramen magnum. Traditionally, Chiari malformation type I etiology has been attributed to congenital hypoplasia of the osseous posterior cranial fossa, resulting in a mismatch between volume of intracranial contents and available cranial space. However, with advanced imaging technologies and molecular genetic research, this classical understanding has been increasingly challenged. In recent years, a range of alternative pathogenic theories have emerged, including brain tissue overgrowth, atlantoaxial instability, tethered cord syndrome, idiopathic intracranial hypertension, and dysfunction of the myodural bridge complex; these theories offer novel perspectives from different anatomical and physiological dimensions, complementing or contesting the traditional view. Notably, some of these hypotheses have led to the development of innovative treatment strategies. Nevertheless, these emerging theories remain controversial and lack comprehensive validation. This review gives a systematic overview on the existing etiological hypotheses of Chiari malformation type I, elucidating its multifactorial pathogenesis and highlighting the distinct clinical phenotypes associated with each theory, aiming to provide a theoretical foundation for developing personalized treatment approaches in clinical management of Chiari malformation type I.

Chiari malformation type I is a common developmental anomaly of the craniovertebral junction, most notably characterized by downward herniation of the cerebellar tonsils through the foramen magnum. Traditionally, Chiari malformation type I etiology has been attributed to congenital hypoplasia of the osseous posterior cranial fossa, resulting in a mismatch between volume of intracranial contents and available cranial space. However, with advanced imaging technologies and molecular genetic research, this classical understanding has been increasingly challenged. In recent years, a range of alternative pathogenic theories have emerged, including brain tissue overgrowth, atlantoaxial instability, tethered cord syndrome, idiopathic intracranial hypertension, and dysfunction of the myodural bridge complex; these theories offer novel perspectives from different anatomical and physiological dimensions, complementing or contesting the traditional view. Notably, some of these hypotheses have led to the development of innovative treatment strategies. Nevertheless, these emerging theories remain controversial and lack comprehensive validation. This review gives a systematic overview on the existing etiological hypotheses of Chiari malformation type I, elucidating its multifactorial pathogenesis and highlighting the distinct clinical phenotypes associated with each theory, aiming to provide a theoretical foundation for developing personalized treatment approaches in clinical management of Chiari malformation type I.