Concurrent Chiari malformation type I and hydrocephalus: Integrating mechanistic and pathophysiological insights toward a unified management paradigm.

Chiari malformation type I (CM-I) and hydrocephalus often occur together, but their connection can involve a variety of different and overlapping pathophysiological mechanisms. These include posterior fossa hypoplasia, cerebrospinal fluid (CSF) flow obstruction at the foramen magnum, venous outflow restriction, craniospinal pressure dissociation, and genetically mediated brain overgrowth. Such mechanisms often converge in complex developmental disorders, most notably syndromic craniosynostoses where premature suture fusion restricts posterior fossa expansion and perturbs venous and CSF dynamics, driving hindbrain herniation and ventriculomegaly. Concurrent CM-I and hydrocephalus may be best categorized into one of three mechanistic patterns: 1) hydrocephalus with secondary Chiari-like tonsillar descent (acquired CM-I); 2) CM-I with secondary hydrocephalus; or 3) simultaneous/complex presentations driven by shared developmental anomalies. Recognizing these distinctions is crucial for guiding treatment selection, as clinical management often relies on identifying the primary pathological driver, with CSF diversion (ventriculoperitoneal shunt [VPS] or endoscopic third ventriculostomy [ETV]) used for hydrocephalus-driven cases, posterior fossa decompression (PFD) for CM-I-driven cases, and multidisciplinary staged interventions reserved for complex or syndromic cases. Future directions include genetic stratification, advanced CSF dynamics imaging, computational biomechanical modeling, and the integration of multimodal data to individualize intervention timing and techniques. A mechanism-based framework, rather than a purely anatomical classification, may enhance diagnostic accuracy and improve surgical outcomes in CM-I with hydrocephalus.