Incidence of cerebellar tonsillar ectopia in idiopathic intracranial hypertension: a mimic of the Chiari I malformation. 
IIH is a syndrome of elevated intracranial pressure without hydrocephalus, mass, or identifiable cause.  Diagnosis is made by clinical presentation, intracranial pressure measurement, and supportive imaging findings.  A subset of patients with IIH may have tonsillar ectopia, meeting the criteria for Chiari malformation type I but not responding to surgical decompression for Chiari I. The purpose of this study was to determine the incidence and morphology of cerebellar tonsillar ectopia in patients with IIH.  Forty-three patients with clinically confirmed IIH and 44 age-matched controls were included.  Two neuroradiologists with CAQs reviewed sagittal T1-weighted MRI in a blinded fashion and measured cerebellar tonsil and obex positions relative to the foramen magnum and prepontine cistern width at the level of the midpons.  Nine of 43 patients with IIH and 1/44 controls had cerebellar tonsillar ectopia of ≥5 mm.  Five of 9 of patients with IIH with ectopia of ≥5 mm also had a "peglike" tonsil configuration.  Patients with IIH had a significantly lower tonsillar position (2.1 ± 2.8 mm) than age-matched controls (0.7 ±1.9 mm, P < .05).  The obex position was significantly lower in patients with IIH versus controls (-7.9 mm [above the FM] versus -9.4 mm [above the FM], P < .05).  The prepontine width was not significantly different between the groups.  Cerebellar tonsil position in patients with IIH was significantly lower than that in age-matched controls, often times peglike, mimicking Chiari I. A significantly lower obex position suggests an inferiorly displaced brain stem and cerebellum.  When tonsillar ectopia of >5 mm is identified, imaging and clinical consideration of IIH are warranted to avoid misdiagnosis as Chiari I.