A 28-year-old man with a known intracranial mass presents with bilateral optic disc swelling. Examination shows normal visual acuity, normal colour vision, and normal pupillary reflexes. Fundoscopy reveals obscuration of disc margins with peripapillary haemorrhages. Which single finding would confirm papilloedema rather than papillitis?

Explanation

## Papilloedema: Structural Confirmation ### Imaging Discriminator **Key Point:** While clinical examination can suggest papilloedema, **optic nerve sheath distension (>6 mm diameter)** on ultrasound or MRI is the structural hallmark that confirms raised intracranial pressure and papilloedema, distinguishing it from papillitis (which shows no sheath distension). ### Pathophysiology of Optic Nerve Sheath Distension **High-Yield:** Papilloedema results from **raised intracranial pressure** transmitted along the optic nerve sheath (which is continuous with the dura). This causes: 1. Compression of axoplasmic flow in the optic nerve 2. Accumulation of axoplasmic material at the disc 3. Swelling of the nerve head (true oedema) 4. **Distension of the optic nerve sheath** (pathognomonic for raised ICP) In contrast, papillitis is **local inflammation** of the optic nerve head without sheath distension. ### Comparative Imaging Findings | Modality | Papilloedema | Papillitis | |----------|--------------|------------| | **Ultrasound (A-scan)** | Optic nerve sheath >6 mm | Normal sheath diameter (<5 mm) | | **MRI Orbit** | Sheath distension, flattened posterior globe | Normal sheath, may show T2 hyperintensity of nerve | | **Fundus Photo** | Bilateral disc swelling, haemorrhages, exudates | Unilateral hyperaemia, blurred margins | | **OCT** | Elevated retinal nerve fibre layer (RNFL), buried drusen-like appearance | Elevated RNFL, but sheath normal | ### Why Imaging Matters **Clinical Pearl:** A patient with **bilateral disc swelling + normal vision + normal pupils** could have: - Papilloedema (raised ICP) - Pseudopapilloedema (optic disc drusen, tilted discs) - Bilateral papillitis (rare, e.g., neurosarcoidosis, syphilis) Imaging the **optic nerve sheath** resolves this ambiguity: - **Sheath >6 mm** → papilloedema (raised ICP) - **Normal sheath + T2 hyperintensity of nerve** → papillitis - **Drusen on imaging** → pseudopapilloedema **Mnemonic: SHEATH SIGN** - **S**welling of disc (both) - **H**yperICP (papilloedema only) - **E**xudates (both, but more in papilloedema) - **A**xonal dysfunction (papillitis only) - **T**ransaxial sheath diameter >6 mm (papilloedema only) - **H**yperICP imaging findings (MRI: empty sella, compressed ventricles) [cite:Neuro-Ophthalmology Khurana Comprehensive Ophthalmology Ch 12; Orbital Imaging Swartz & Loevner]