A 28-year-old woman presents with sudden-onset left eye pain and blurred vision for 3 days. She reports pain on eye movements. On examination, visual acuity is 6/36 in the left eye. Fundoscopy reveals hyperaemia and swelling of the optic disc with blurred margins, and a central scotoma is detected on visual field testing. The right eye is normal. MRI brain and orbits shows T2 hyperintensity in the left optic nerve. 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 (retrobulbar inflammation), and central scotoma — all present in this case. ### Differential Diagnosis: Papilloedema vs Papillitis | Feature | Papilloedema | Papillitis (Optic Neuritis) | |---------|--------------|-----------------------------| | **Onset** | Insidious, progressive | Acute (hours to days) | | **Pain** | Absent or mild | Prominent, worse with eye movement | | **Vision loss** | Late finding, peripheral first | Early and central (scotoma) | | **Scotoma** | Arcuate or peripheral | Central scotoma | | **Bilaterality** | Bilateral (raised ICP affects both) | Unilateral | | **Disc appearance** | Blurred margins, hyperaemia, exudates, hemorrhages | Hyperaemia, swelling, may be less hemorrhage | | **Pupil** | Normal or sluggish | Relative afferent pupillary defect (RAPD) | | **MRI** | Brain shows mass/edema/hydrocephalus | Optic nerve T2 hyperintensity | | **Prognosis** | Depends on underlying cause | Good recovery, but MS risk ~50% in 15 years | **High-Yield:** The combination of **acute unilateral pain with eye movement + central scotoma + optic disc swelling = papillitis**, not papilloedema. ### Why This Is Papillitis 1. **Pain on eye movement** — pathognomonic for retrobulbar inflammation; papilloedema is painless. 2. **Central scotoma** — indicates optic nerve axon involvement; papilloedema causes peripheral or arcuate defects. 3. **Unilateral presentation** — papilloedema is bilateral (raised ICP). 4. **MRI optic nerve hyperintensity** — confirms inflammatory demyelination of the optic nerve. 5. **Acute onset** — papilloedema develops insidiously over weeks. ### Clinical Pearl **Clinical Pearl:** Optic neuritis is the presenting feature of multiple sclerosis in ~25% of MS patients. This 28-year-old woman has a 50% risk of developing MS within 15 years; brain MRI should be obtained to assess for demyelinating lesions. ### Management - High-dose IV methylprednisolone 1 g daily × 3 days (Optic Neuritis Treatment Trial evidence). - Visual evoked potentials (VEP) — delayed latency confirms demyelination. - Brain MRI to assess MS risk. - Baseline ophthalmology review and follow-up. [cite:Harrison 21e Ch 428]