A 28-year-old woman presents with sudden-onset blurred vision in the left eye for 2 days. She reports pain on eye movement and has a history of multiple sclerosis diagnosed 6 months ago. On examination, visual acuity is 6/36 in the left eye. Fundoscopy reveals hyperaemia of the optic disc with blurred margins, mild disc swelling, and retinal exudates around the disc. Pupils are normally reactive. Intraocular pressure is 14 mmHg bilaterally. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Optic Neuritis (Papillitis) ### Key Distinguishing Features **Key Point:** Optic neuritis presents with **acute vision loss, pain on eye movement, and disc hyperaemia in a young patient with demyelinating disease**. The clinical presentation strongly suggests **papillitis** (anterior optic neuritis): | Feature | Papillitis (Optic Neuritis) | Papilloedema | AION | |---------|---------------------------|-------------|------| | **Vision loss** | Acute, marked (6/36–6/60) | Gradual or none initially | Acute, altitudinal | | **Pain** | Yes, pain on eye movement | No pain | No pain | | **Disc appearance** | Hyperaemic, blurred margins | Blurred margins, elevated | Pale, sectoral | | **Exudates** | Often present around disc | Peripapillary | Rare | | **Pupil** | Relative afferent pupillary defect (RAPD) | Normal | May have RAPD | | **IOP** | Normal | Normal or elevated | Normal | | **Association** | MS, demyelinating disease | Intracranial hypertension | Age >50, vascular risk | ### Clinical Reasoning 1. **Age and demographics:** 28-year-old woman with known MS — optic neuritis is the presenting feature in ~20% of MS patients and occurs in ~50% over the disease course. 2. **Pain on eye movement:** Pathognomonic for inflammation of the optic nerve sheath; absent in papilloedema. 3. **Acute vision loss:** Typical of demyelinating optic neuritis; papilloedema causes gradual visual blurring. 4. **Disc hyperaemia:** Indicates inflammation of the nerve head; papilloedema shows elevation without hyperaemia initially. 5. **Normal IOP:** Rules out glaucoma. **High-Yield:** Optic neuritis is the **most common cause of optic nerve inflammation in young adults** and is strongly associated with MS, particularly in women aged 20–40 years. **Clinical Pearl:** The presence of **pain on eye movement** is the single most useful clinical sign to differentiate optic neuritis from papilloedema. This pain results from inflammation of the dura surrounding the optic nerve. ### Pathophysiology Optic neuritis is a demyelinating inflammation of the optic nerve, often the first manifestation of multiple sclerosis. The inflammatory process affects the myelin sheath, leading to: - Conduction block and vision loss - Optic nerve swelling (if anterior — papillitis) - Relative afferent pupillary defect (RAPD) ### Management Implications - **MRI brain and orbits:** To assess for demyelinating lesions and confirm MS diagnosis. - **High-dose IV methylprednisolone:** 1 g daily for 3–5 days, followed by oral taper (accelerates visual recovery). - **Neuroimaging:** To rule out structural lesions and assess MS burden. [cite:Harrison 21e Ch 378]