A 28-year-old woman presents with sudden-onset blurred vision in the left eye and pain on eye movements. Fundoscopy reveals hyperaemia and blurring of the optic disc margins with peripapillary flame haemorrhages. Visual evoked potentials show delayed latency. What is the most common cause of optic disc inflammation in this clinical presentation?

## Diagnosis: Demyelinating Optic Neuritis **Key Point:** Demyelinating optic neuritis is the most common cause of acute optic disc inflammation presenting with pain on eye movements, visual loss, and delayed VEP latency. ### Clinical Features of Optic Neuritis - Sudden unilateral vision loss (hours to days) - Pain on ocular movements (95% of cases) — pathognomonic - Relative afferent pupillary defect (RAPD) — always present - Optic disc hyperaemia and blurring (especially at margins) - Peripapillary flame haemorrhages may be present - Scotoma on visual field testing (typically central) ### Distinguishing Papillitis from Papilloedema | Feature | Papillitis (Optic Neuritis) | Papilloedema | |---------|---------------------------|---------------| | **Onset** | Acute (hours–days) | Insidious (weeks–months) | | **Pain on eye movement** | Yes (95%) | No | | **Visual acuity** | Markedly reduced early | Normal until late | | **RAPD** | Present | Absent | | **Disc appearance** | Hyperaemic, blurred margins | Blurred margins, swollen | | **Haemorrhages** | Peripapillary flame | Circumferential, splinter | | **Cause** | Demyelination (MS), infection, inflammation | Raised ICP | **High-Yield:** Optic neuritis is the most common cause of papillitis in young adults and is associated with multiple sclerosis in 50% of cases (especially in women aged 20–40 years). ### VEP Findings in Optic Neuritis - **Delayed latency** (>120 ms) — indicates demyelination - Reduced amplitude - Persistent abnormality even after clinical recovery **Clinical Pearl:** Pain on eye movements is virtually pathognomonic for optic neuritis and helps distinguish it from papilloedema, where eye movements are painless. **Mnemonic: PAPILLITIS** — **P**ain on movements, **A**cute onset, **P**upil defect (RAPD), **I**nflammation (demyelinating), **L**oss of vision, **L**ateral geniculate lesion involvement, **I**ncreased latency on VEP, **T**ypically unilateral, **I**ntermediate optic nerve, **S**welling of disc margins.