A 35-year-old woman presents with sudden vision loss in the left eye and pain on eye movements. Fundoscopy reveals hyperaemia of the optic disc with blurred margins, and visual evoked potentials show delayed latency. Which single feature best distinguishes papillitis from papilloedema?

Explanation

## Distinguishing Papillitis from Papilloedema ### Key Clinical Discriminator **Key Point:** Papillitis is characterised by unilateral optic nerve inflammation with an **afferent pupillary defect (APD)** and **decreased visual acuity**, whereas papilloedema is bilateral, preserves vision, and has normal pupillary function. ### Comparative Table | Feature | Papillitis | Papilloedema | |---------|-----------|---------------| | **Laterality** | Unilateral | Bilateral | | **Visual Acuity** | Decreased (often marked) | Preserved (early–mid stage) | | **Afferent Pupillary Defect** | Present | Absent | | **Colour Vision** | Impaired (red desaturation) | Normal | | **Pain on Eye Movement** | Yes (characteristic) | No | | **Aetiology** | Demyelination, infection, inflammation | Raised intracranial pressure | | **VEP Latency** | Delayed | Normal | ### Why This Matters **High-Yield:** The presence of an **APD with unilateral vision loss** is the single most reliable discriminator. Papillitis is a **demyelinating optic neuritis** (often first presentation of MS in young patients), whereas papilloedema is a sign of **raised ICP** from a space-occupying lesion, hydrocephalus, or idiopathic intracranial hypertension. **Clinical Pearl:** Papillitis causes **pain with eye movements** (retrobulbar inflammation) and **colour vision defects** (red desaturation), neither of which occurs in papilloedema. The APD reflects optic nerve axonal dysfunction; papilloedema does not impair axonal conduction until very late (malignant hypertensive papilloedema). ### Mnemonic: PAPILLITIS vs PAPILLOEDEMA **PAPILLITIS** = **P**ain, **A**cuity loss, **P**upil defect, **I**nflammation (optic neuritis), **L**ateral (unilateral), **L**ate colour loss, **I**ncreased VEP latency, **T**ransient visual obscuration (rare), **I**nfection/demyelination, **S**wollen disc **PAPILLOEDEMA** = **P**reserved vision, **A**symmetric (bilateral), **P**upil normal, **I**ncreased ICP, **L**ateral (both eyes), **L**ate visual loss, **O**ptic nerve sheath distension, **E**dema (true oedema), **D**isc elevation, **E**xudates (hard exudates, cotton wool spots), **M**alignant HTN, **A**cute angle closure [cite:Neuro-Ophthalmology Harrison 21e Ch 28]