A 28-year-old woman presents with acute loss of vision in the left eye over 24 hours. She reports pain on eye movement and has a history of multiple sclerosis diagnosed 2 years ago. On examination, visual acuity is 6/60 in the left eye with a relative afferent pupillary defect (RAPD). Fundoscopy shows hyperaemia of the optic disc with blurred margins, mild disc swelling, and peripapillary flame-shaped haemorrhages. Visual evoked potentials (VEP) show delayed latency. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Optic Neuritis (Papillitis) ### Key Distinguishing Features **Key Point:** Optic neuritis presents with acute unilateral vision loss, pain on eye movement, RAPD, and delayed VEP — a demyelinating process affecting the optic nerve head. ### Comparison: Papilloedema vs Papillitis | Feature | Papilloedema | Papillitis (Optic Neuritis) | |---------|--------------|-----------------------------| | **Onset** | Gradual (days to weeks) | Acute (hours to 24 hrs) | | **Vision loss** | Often late/asymptomatic initially | Early, marked (6/60 or worse) | | **Pain** | Absent | Present (pain on eye movement) | | **RAPD** | Absent (bilateral process) | Present (unilateral) | | **Disc appearance** | Hyperaemia, blurred margins, obscured vessels | Hyperaemia, blurred margins, flame haemorrhages | | **Associated findings** | Headache, transient visual obscurations, CN VI palsy | Demyelinating disease history (MS) | | **VEP** | Normal latency | Delayed latency | | **Aetiology** | Raised ICP (tumour, hydrocephalus, etc.) | Demyelination (MS, NMO, ADEM) | **High-Yield:** The triad of **acute vision loss + pain on eye movement + RAPD** is pathognomonic for optic neuritis. ### Pathophysiology 1. Demyelination of optic nerve axons → conduction block 2. Inflammation → disc hyperaemia and swelling 3. Unilateral involvement → RAPD present 4. Delayed neural transmission → prolonged VEP latency **Clinical Pearl:** Optic neuritis is the presenting feature of MS in 15–20% of cases. Any young adult (especially women) with acute optic neuritis should undergo brain MRI to screen for demyelinating lesions. **Mnemonic: OPTIC NEURITIS = Onset acute, Pain on movement, Temporal demyelination, IRAP (RAPD present), Conduction delay (VEP), Neuro-inflammatory (MS link)** ### Why This Patient Has Papillitis, Not Papilloedema - **Acute onset** (24 hours) rules out papilloedema, which develops over days to weeks - **Severe vision loss** (6/60) is typical of optic neuritis; papilloedema is often asymptomatic until late - **Pain on eye movement** is a hallmark of optic neuritis; papilloedema is painless - **RAPD** indicates unilateral optic nerve dysfunction; papilloedema is bilateral - **Delayed VEP** confirms demyelination; papilloedema does not alter VEP latency - **MS history** makes demyelinating optic neuritis the expected diagnosis [cite:Harrison 21e Ch 379]