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. On examination, the left pupil is relatively afferent, and fundoscopy reveals hyperaemia of the optic disc with blurred margins. Visual acuity is 6/60 in the affected eye. MRI brain shows demyelinating lesions. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Acute Optic Neuritis (Papillitis) ### Key Distinguishing Features **Key Point:** Acute optic neuritis (papillitis) presents with acute unilateral vision loss, pain on eye movement, and disc hyperaemia in the context of demyelinating disease. ### Clinical Presentation Analysis This patient exhibits the classic triad of optic neuritis: 1. **Acute unilateral vision loss** (24 hours onset, 6/60 acuity) 2. **Pain on eye movement** (retrobulbar inflammation) 3. **Relative afferent pupillary defect (RAPD)** — indicates optic nerve dysfunction ### Papillitis vs Papilloedema: Key Differences | Feature | Papillitis (Optic Neuritis) | Papilloedema | |---------|---------------------------|---------------| | **Onset** | Acute (hours to days) | Insidious (days to weeks) | | **Vision loss** | Marked and early (6/60 or worse) | Late finding; initially preserved | | **Pain** | Present (pain on eye movement) | Absent | | **RAPD** | Present (unilateral) | Absent (bilateral if bilateral papilloedema) | | **Disc appearance** | Hyperaemia, blurred margins | Blurred margins, hyperaemia, but more diffuse | | **Aetiology** | Demyelination (MS), infection, inflammation | Raised ICP | | **Systemic signs** | Neurological (MS plaques on MRI) | Headache, vomiting, focal neurological signs | **High-Yield:** The presence of **pain on eye movement** is pathognomonic for optic neuritis and absent in papilloedema. ### MRI Findings The presence of **demyelinating lesions on MRI brain** strongly supports optic neuritis in the context of multiple sclerosis. Optic neuritis is the presenting feature in 15–20% of MS patients and occurs in 50% of MS patients over their lifetime. **Clinical Pearl:** In a young woman with acute unilateral vision loss, pain on eye movement, RAPD, and demyelinating lesions on MRI, optic neuritis is the diagnosis until proven otherwise. ### Why Papilloedema Is Ruled Out - Papilloedema is **bilateral** in most cases (raised ICP affects both optic nerves) - Vision loss is a **late feature** in papilloedema; early vision is preserved - **Pain on eye movement is absent** in papilloedema - No systemic signs of raised ICP (headache, vomiting, focal deficits) mentioned - Papilloedema would not explain the RAPD or the demyelinating lesions [cite:Harrison 21e Ch 428]