Idiopathic intracranial hypertension and Chiari I malformation: Overlapping clinical and imaging features

Idiopathic intracranial hypertension (IIH) and Chiari I malformation (CM-I) are traditionally considered distinct entities, yet accumulating evidence reveals substantial overlap in their clinical presentations and imaging features. Patients with IIH may develop acquired tonsillar herniation mimicking CM-I, while patients with CM-I may harbor unrecognized intracranial hypertension. Failure to identify the primary condition risks unnecessary posterior fossa decompression in patients whose tonsillar descent is pressure-driven, or untreated intracranial hypertension in patients mislabeled as having congenital CM-I. Thus, despite their overlapping presentations, identifying distinct traits in each condition is critical in delivering effective treatment. In this review, we synthesize the neuroimaging features of each condition, identifying their distinct features and their overlap, present illustrative cases spanning the IIH-Chiari diagnostic spectrum, and propose a framework for differentiating the primary pathology to guide management.