A 28-year-old woman presents with acute loss of vision in the left eye over 24 hours. She reports pain on eye movements and colour vision appears washed out. Examination reveals a relative afferent pupillary defect (RAPD), normal fundus appearance, and visual acuity of 6/60 in the affected eye. MRI brain with contrast shows T2 hyperintense lesions in the periventricular and juxtacortical regions. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Optic Neuritis with Demyelinating Disease ### Key Clinical Features **Key Point:** Optic neuritis is inflammation of the optic nerve, classically presenting with acute monocular vision loss, pain on eye movements (98% of cases), and colour vision defects. **High-Yield:** The combination of: - Acute monocular vision loss over hours to days - Retrobulbar inflammation (normal fundus initially) - RAPD (indicates optic nerve dysfunction) - Pain with ocular movements (pathognomonic) - Colour vision loss (red desaturation) ### Demyelinating Disease Association **Clinical Pearl:** Optic neuritis is the presenting feature of multiple sclerosis (MS) in 15–20% of patients. The Optic Neuritis Treatment Trial (ONTT) showed that 50% of women and 30% of men with optic neuritis develop MS within 15 years. **Key Point:** MRI findings of periventricular and juxtacortical T2 hyperintense lesions are highly suggestive of demyelinating disease (MS). These lesions represent areas of demyelination in the white matter. ### Diagnostic Criteria | Feature | Optic Neuritis | Posterior Ischemic ON | Acute Glaucoma | |---------|---|---|---| | **Onset** | Hours–days | Sudden | Acute | | **Pain with eye movement** | Yes (98%) | No | No | | **Fundus appearance** | Normal initially | Optic disc pallor | Corneal haze, mid-dilated pupil | | **RAPD** | Present | Present | Absent | | **IOP** | Normal | Normal | Elevated (>40 mmHg) | | **MRI brain** | T2 lesions common | Normal | N/A | ### Pathophysiology 1. Demyelination of optic nerve axons → conduction block 2. Inflammation → pain and loss of function 3. Recovery typically begins within 4–6 weeks (spontaneous or steroid-accelerated) 4. White matter lesions indicate underlying CNS demyelination **Mnemonic:** **OPTIC** = **O**ptic nerve inflammation, **P**ain with movements, **T**emporal evolution (acute), **I**nflammatory (demyelinating), **C**olour vision loss ### Management Approach ```mermaid flowchart TD A[Acute monocular vision loss + pain]:::outcome --> B{RAPD + normal fundus?}:::decision B -->|Yes| C[Suspect optic neuritis]:::action C --> D[MRI brain + orbits]:::action D --> E{Demyelinating lesions?}:::decision E -->|Yes| F[High-dose IV methylprednisolone]:::action E -->|No| G[Oral corticosteroids]:::action F --> H[Neurology referral for MS workup]:::action G --> I[Follow-up imaging in 3 months]:::action ``` **Key Point:** High-dose IV methylprednisolone (1 g daily × 3 days) accelerates visual recovery and may delay MS onset, though long-term visual outcome is similar to oral steroids alone. [cite:Harrison 21e Ch 448]