Can MRI detect idiopathic intracranial hypertension (IIH) in a young, overweight woman presenting with pulsatile tinnitus, migraines, and exercise-induced headaches?

MRI Can Detect Supportive Signs of IIH But Cannot Definitively Diagnose It Alone

MRI is essential for excluding secondary causes of raised intracranial pressure and can reveal highly specific imaging features that support the diagnosis of IIH, but the definitive diagnosis still requires lumbar puncture with elevated opening pressure (≥25 cm H₂O) in the appropriate clinical context. 1

Why MRI is Performed First

• MRI brain must be obtained urgently within 24 hours as the first step in evaluating suspected IIH to exclude secondary causes such as mass lesions, hydrocephalus, structural abnormalities, vascular lesions, or abnormal meningeal enhancement 1
• CT or MR venography is mandatory within 24 hours to exclude cerebral sinus thrombosis, which can mimic IIH 1
• MRI provides superior soft tissue contrast compared to CT and is particularly valuable for detecting subtle signs of elevated intracranial pressure 1

Specific MRI Findings That Support IIH Diagnosis

Your patient's presentation (young, overweight woman with pulsatile tinnitus, migraines, and exercise-induced headaches) fits the typical IIH profile 1, and MRI can reveal several highly specific findings:

High Specificity Signs (Nearly 100% Specific)

• Posterior globe flattening: 57-66% sensitivity but 97-100% specificity 1, 2
• Optic nerve disc protrusion (intraocular protrusion): 30-40% sensitivity but 100% specificity 1, 2
• Combination of any 3 of 4 key MRI features: nearly 100% specific while maintaining 64% sensitivity 3

Moderately Specific Signs

• Perioptic subarachnoid space distension with enlarged optic nerve sheath: mean diameter 4.3 mm in IIH versus 3.2 mm in controls, with 68% sensitivity when associated with horizontal optic nerve tortuosity 1, 2
• Transverse venous sinus stenosis: 78-80% sensitivity but specificity is undetermined 1, 2, 3
• Empty or partially empty sella: 44-80% sensitivity but only 64% specificity, making it less reliable 1, 4

Critical Limitations of MRI

• A normal MRI does not exclude IIH, as many patients with confirmed IIH have completely normal CT or MRI scans 1
• MRI findings alone cannot establish the diagnosis—lumbar puncture with opening pressure ≥25 cm H₂O measured in lateral decubitus position with legs extended is mandatory to confirm IIH 1
• MRI features are not prognostic of visual or headache outcomes, so their presence or absence doesn't predict disease severity 4

Diagnostic Algorithm for Your Patient

1. Obtain urgent MRI brain with MR venography within 24 hours to exclude secondary causes and cerebral sinus thrombosis 1

2. Assess for the combination of MRI signs: If 3 or more of the following are present (posterior globe flattening, optic nerve disc protrusion, enlarged optic nerve sheath with tortuosity, transverse sinus stenosis), this is highly specific for intracranial hypertension 2, 3

3. Perform lumbar puncture with opening pressure measurement in lateral decubitus position with legs extended after normal neuroimaging to confirm elevated CSF pressure ≥25 cm H₂O 1

4. Document papilledema on fundoscopic examination, which is the hallmark finding, though IIH without papilledema is a rare subtype 1, 5

Common Pitfalls to Avoid

• Don't rely solely on MRI findings: Even with multiple supportive MRI features, lumbar puncture is mandatory for diagnosis 1
• Don't use CT as a substitute when MRI is available: CT is significantly less sensitive for detecting secondary signs of elevated intracranial pressure 1
• Don't assume normal imaging excludes IIH: Proceed to lumbar puncture if clinical suspicion remains high despite normal imaging 1
• Ensure proper LP technique: Improper positioning (not lateral decubitus with legs extended) or measuring before pressure stabilizes can lead to false readings 1