MRI Features of Idiopathic Intracranial Hypertension
MRI of the head and orbits is the optimal imaging modality for evaluating IIH, with specific findings including empty sella, posterior globe flattening, optic nerve sheath distension, optic nerve tortuosity, intraocular protrusion of the optic nerve head, and transverse sinus stenosis. 1
Primary MRI Findings with Diagnostic Value
Orbital and Optic Nerve Findings
- Posterior globe flattening demonstrates 56% sensitivity and 100% specificity for IIH diagnosis, making it highly specific when present 1
- Intraocular protrusion of the optic nerve head shows 40% sensitivity and 100% specificity 1
- Horizontal tortuosity of the optic nerve has 68% sensitivity and 83% specificity 1
- Enlarged optic nerve sheath (perioptic subarachnoid space distension) averages 4.3 mm in IIH patients versus 3.2 mm in controls 1
Sellar and Pituitary Findings
- Empty sella or partially empty sella is present in approximately 65.9% of IIH patients 1
- Decreased pituitary height with mean size of 3.63 mm in IIH versus 5.05 mm in controls 1
Venous Sinus Findings
- Transverse sinus stenosis (bilateral or involving the dominant sinus) is a supportive finding 1
- MR venography (MRV) is useful for demonstrating narrowing of the distal transverse sinuses and excluding cerebral venous sinus thrombosis 1
Additional MRI Findings
- Optic nerve sheath enhancement with IV contrast has been reported 1
- Slit-like or normal-sized ventricles (absence of hydrocephalus is part of diagnostic criteria) 1
- Widening of the foramen ovale (average 30.03 mm² in IIH vs 24.21 mm² in controls, with 50% sensitivity and 81% specificity at cut-off of 30 mm²) 2
- Enlarged Meckel caves may be present 3
- Cerebellar tonsillar descent can occasionally occur 3
- Cephaloceles are rare findings 3
Critical Imaging Considerations
Diagnostic Approach
MRI brain with and without contrast is superior to CT due to higher soft tissue contrast resolution, particularly for evaluating intracranial and intraorbital structures. 1 The imaging protocol should include dedicated orbital sequences to optimally assess the optic nerve findings 1
Important Caveats
- Individual MRI findings are common in the general population and not pathognomonic for IIH - approximately 49% of patients undergoing brain MRI for any indication have at least one sign of intracranial hypertension, but only 1.7% have papilledema 3
- The presence of multiple findings increases diagnostic probability - papilledema prevalence increases from 2.8% with one MRI sign to 40% with four or more signs 3
- MRI findings are not prognostic - these features do not predict visual outcome, visual worsening, headache improvement, or freedom from headache 4
- Absence of MRI findings does not exclude IIH - the diagnosis ultimately requires elevated opening pressure on lumbar puncture with appropriate clinical context 1, 5
Role in Differential Diagnosis
MRI must demonstrate no evidence of hydrocephalus, mass lesion, structural or vascular lesion, and no abnormal meningeal enhancement to support the diagnosis of IIH 1, 5 MRV is mandatory within 24 hours to exclude cerebral venous sinus thrombosis, particularly in non-obese prepubertal children who are at higher risk 1, 5