What is the first step in diagnosing Idiopathic Intracranial Hypertension (IIH)?

First Step in Diagnosing Idiopathic Intracranial Hypertension (IIH)

The first step in diagnosing idiopathic intracranial hypertension is urgent neuroimaging with MRI brain within 24 hours; if MRI is unavailable within this timeframe, then urgent CT brain should be performed with subsequent MRI brain if no lesion is identified. 1

Diagnostic Approach to IIH

Initial Neuroimaging

• MRI brain is the preferred initial test to exclude secondary causes of raised intracranial pressure 1
• If MRI is unavailable within 24 hours, CT brain should be performed urgently, followed by MRI when available 1
• Neuroimaging should show no evidence of hydrocephalus, mass, structural or vascular lesion, and no abnormal meningeal enhancement 1
• CT or MR venography is mandatory to exclude cerebral sinus thrombosis within 24 hours 1

Neuroimaging Findings in IIH

• Certain characteristics of raised intracranial pressure may be seen on neuroimaging, but these are not pathognomonic of IIH 1
• Common neuroimaging signs include moderate suprasellar herniation, perioptic nerve sheath distension, flattening of the globe, and transverse sinus stenosis 2
• The presence of ≥3 neuroimaging signs has a specificity of 93.5% for IIH diagnosis 2

Following Normal Neuroimaging

• After excluding structural causes with neuroimaging, a lumbar puncture should be performed to check opening pressure and ensure CSF contents are normal 1
• The lumbar puncture opening pressure should be measured in the lateral decubitus position 1
• An opening pressure ≥25 cm CSF with normal constituents supports the diagnosis of IIH 2, 3

Patient Characteristics to Consider

Typical vs. Atypical IIH

• Typical IIH: Female patients of childbearing age with BMI >30 kg/m² 1
• Atypical IIH: Patients who are not female, not of childbearing age, or have BMI <30 kg/m² - these patients require more in-depth investigation 1
• Obesity is present in approximately 87.8% of IIH patients 4

Clinical Examination Findings

• Papilledema is the hallmark finding in IIH and should be documented 1, 3
• Cranial nerve examination should be performed - typically there should be no cranial nerve involvement other than possible sixth nerve palsy/palsies 1, 3
• If other cranial nerves or pathological findings are involved, alternative diagnoses should be considered 1

Common Pitfalls and Caveats

• Where there is diagnostic uncertainty regarding papilledema, an experienced clinician should be consulted early before invasive tests are performed 1
• Pseudopapilledema must be distinguished from true papilledema to avoid unnecessary investigations 1
• Some patients (24.7% in one study) may be asymptomatic and discovered only during routine ophthalmic examination 4
• Headache presentation in IIH can be highly variable and may mimic other primary headache disorders, making clinical diagnosis challenging 1
• IIH without papilledema is a rare subtype that meets all other criteria for IIH but lacks papilledema, making diagnosis more difficult 1

Following this diagnostic algorithm will help ensure timely diagnosis of IIH while excluding other potential causes of increased intracranial pressure, ultimately protecting vision and improving outcomes for patients.