MRI findings differentiating tonsillar herniation caused by idiopathic intracranial hypertension from Chiari I malformation.

Abstract

Some patients with idiopathic intracranial hypertension (IIH) have cerebellar tonsillar herniation ≥ 5mm mimicking Chiari malformation I (CMI), which can result in misdiagnosis and unjustified treatment. Our purpose was to identify IIH patients with tonsillar herniation ≥ 5mm (IIHTH) and compare with CMI patients to assess imaging findings that could distinguish the two conditions. Ninety-eight patients with IIH, 81 patients with CMI, and 99 controls were retrospectively assessed. Two neuroradiologists blindly reviewed MR images. IIHTH patients were compared with CMI patients and controls regarding the extent of tonsillar herniation (ETH), bilateral transverse sinus stenosis (BTSS), hypophysis-sella ratio (HSR), and bilateral tortuosity of optic nerve (BTON). 13/98 (13.2%) IIH patients had tonsillar herniation ≥ 5mm (IIHTH) and were significantly younger and had higher BMI compared with CMI patients and controls. ETH was significantly less in the IIHTH than CMI (6.5 ± 2.4mm vs. 10.9 ± 4.4mm; p < 0.001). BTSS and HSR < 0.5 were more common in IIHTH than CMI (p < 0.001 and p = 0.003, respectively). No differences were seen between CMI and controls. BTON was significantly more common in IIHTH compared to control (p = 0.01) but not to the CMI (p = 0.36). Sensitivity and specificity to differentiate IIHTH from CMI were 69.2% and 96.1% for BTSS and 69.2% and 75.3% for HSR < 0.5. The presence of BTSS and/or HSR < 0.5 in patients with ETH ≥ 5mm should suggest further evaluation to exclude IIH before considering CMI surgery.