What investigations should be done for a patient suspected of having idiopathic intracranial hypertension (IIH)?

Investigations for Suspected Idiopathic Intracranial Hypertension

All patients with suspected IIH require urgent MRI brain within 24 hours (or CT brain if MRI unavailable, followed by MRI), CT or MR venography within 24 hours, and lumbar puncture with opening pressure measurement after normal imaging is confirmed. 1, 2

Immediate Neuroimaging (Within 24 Hours)

Primary Imaging

• MRI brain is the mandatory first-line investigation to exclude secondary causes including hydrocephalus, mass lesions, structural abnormalities, vascular lesions, and abnormal meningeal enhancement 1, 2
• If MRI is unavailable within 24 hours, perform urgent CT brain followed by MRI when available 1, 2
• MRI provides superior soft tissue contrast and is more sensitive for detecting subtle signs of elevated intracranial pressure compared to CT 2

Mandatory Venous Imaging

• CT or MR venography must be performed within 24 hours to exclude cerebral venous sinus thrombosis, which is a critical secondary cause that mimics IIH 1, 2
• This is particularly important in non-obese prepubertal children where venous thrombosis is more common 2

Supportive Neuroimaging Findings

While not diagnostic alone, these MRI findings support IIH diagnosis when present 2, 3:

• Posterior globe flattening (56% sensitivity, 100% specificity) 2
• Intraocular protrusion of optic nerve (40% sensitivity, 100% specificity) 2
• Horizontal tortuosity of optic nerve (68% sensitivity, 83% specificity) 2
• Enlarged optic nerve sheath (mean 4.3 mm in IIH vs 3.2 mm in controls) 2
• Empty sella turcica 2, 3
• Transverse sinus stenosis 3

Recent evidence suggests that ≥3 neuroimaging signs have 59.5% sensitivity and 93.5% specificity for IIH diagnosis 3

Lumbar Puncture

Timing and Technique

• Perform lumbar puncture only after neuroimaging confirms no mass lesion, hydrocephalus, or structural abnormality 1, 2
• All patients with papilledema require LP to measure opening pressure and confirm normal CSF composition 1

Critical Measurement Requirements

• Patient must be in lateral decubitus position with legs extended, relaxed, and breathing normally 2
• Wait for pressure to stabilize before recording 2
• Opening pressure ≥25 cm H₂O (≥250 mm H₂O) is diagnostic 2

Common Pitfall

• Improper positioning (sitting, legs flexed, patient straining) causes falsely elevated readings and misdiagnosis 2
• If opening pressure is borderline (20-24 cm H₂O), arrange close follow-up with repeat LP at 2 weeks, as pressure fluctuates 2

Clinical Examination

Ophthalmologic Assessment

• Document presence and grade of papilledema - this is the hallmark finding 1, 2
• Perform formal visual field testing 1
• Consider optical coherence tomography (OCT) to measure peripapillary retinal nerve fiber layer thickness as supplementary objective evidence 4

Neurological Examination

• Record complete cranial nerve examination 1
• Only sixth nerve palsy should be present in typical IIH - involvement of other cranial nerves suggests alternative diagnosis 1, 3
• If other cranial nerves are affected, reconsider the diagnosis 1

Essential Baseline Measurements

• Measure blood pressure to exclude malignant hypertension (diastolic >120 mmHg) as a cause of papilledema 1, 5
• Document body mass index (BMI) and recent weight changes 6

Patient Phenotype Considerations

Typical IIH (Standard Workup)

• Female, childbearing age, BMI >30 kg/m² 1, 2
• Proceed with standard investigation protocol above 1

Atypical IIH (Extended Workup Required)

• Male, outside childbearing age, or BMI <30 kg/m² 1, 2
• Requires more extensive investigation to exclude secondary causes including: 7
  • Medication review (vitamin A, retinoids, tetracyclines, steroids)
  • Screening for anemia, renal disease, endocrine disorders
  • Consider infectious etiologies
• Even typical phenotypes should be screened for secondary causes given 40% of secondary IIH patients are obese/overweight 7

CSF Analysis

• CSF composition must be normal (normal cell count, protein, glucose) 2
• Abnormal CSF suggests alternative diagnosis such as meningitis or carcinomatosis 1

Algorithm Summary

1. Urgent MRI brain (or CT if MRI unavailable) + CT/MR venography within 24 hours 1, 2
2. If imaging shows no mass/hydrocephalus/thrombosis → Lumbar puncture with opening pressure (proper technique mandatory) 1, 2
3. Document papilledema and perform cranial nerve examination 1
4. Measure blood pressure and BMI 1, 5
5. If atypical phenotype → screen for secondary causes 7