A 28-year-old woman presents with sudden onset of painless vision loss in the right eye over 6 hours. She reports eye pain on lateral gaze and mild headache. On examination, visual acuity is 6/60 in the right eye with a relative afferent pupillary defect (RAPD). Fundoscopy shows a normal-appearing optic disc. MRI brain with contrast reveals T2/FLAIR hyperintense lesions in the periventricular and infratentorial regions. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Diagnosis **Key Point:** This patient has acute optic neuritis (ON) with clinical and radiological features suggestive of demyelinating disease (likely multiple sclerosis). ### Diagnostic Criteria Met | Feature | Finding | Significance | |---------|---------|---------------| | **Vision loss** | Sudden, painless, unilateral | Typical of retrobulbar ON | | **Pain on eye movement** | Present (lateral gaze) | Retrobulbar inflammation | | **RAPD** | Present | Confirms optic nerve involvement | | **Fundoscopy** | Normal disc appearance | Retrobulbar location (inflammation behind disc) | | **MRI findings** | Periventricular + infratentorial lesions | Demyelinating pattern (Dawson's fingers, Barkhof criteria) | ### Pathophysiology of Optic Neuritis 1. **Inflammation** of the optic nerve (usually retrobulbar segment) 2. **Demyelination** of axons → conduction block 3. **Relative afferent pupillary defect** develops due to asymmetric optic nerve damage 4. **Normal fundoscopy** in retrobulbar disease (inflammation anterior to optic disc head) **High-Yield:** Optic neuritis is the **presenting symptom in 15–20% of MS patients** and occurs at some point in 50% of MS patients over their lifetime [cite:Harrison 21e Ch 380]. ### Association with Multiple Sclerosis **Clinical Pearl:** The presence of **brain MRI abnormalities** (especially periventricular and infratentorial lesions meeting Barkhof criteria) in a patient with acute ON significantly increases the risk of developing clinically definite MS within 15 years. **Mnemonic: ONION** — **O**ptic Neuritis, **N**euroimaging abnormalities, **I**nflammatory markers elevated, **O**ther demyelinating features, **N**eurological follow-up needed. ### Management Approach ```mermaid flowchart TD A[Acute Optic Neuritis]:::outcome --> B{MRI brain findings?}:::decision B -->|Normal| C[Low MS risk]:::outcome B -->|Demyelinating lesions| D[High MS risk]:::outcome D --> E[IV Methylprednisolone 1g daily × 3 days]:::action E --> F[Oral prednisolone taper]:::action F --> G[DMT consideration if MS diagnosed]:::action C --> H[Observation + supportive care]:::action ``` **Key Point:** IV methylprednisolone accelerates visual recovery but does not improve final visual outcome; however, it delays MS progression in patients with brain MRI lesions.