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 noticed colour vision defects. On examination, visual acuity is 6/36 in the affected eye. Fundoscopy reveals a swollen optic disc with blurred margins, but the swelling is more pronounced at the poles. There is no spontaneous venous pulsation. Intraocular pressure is 14 mmHg bilaterally. 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 is inflammation of the optic nerve head (papillitis) characterized by the triad of **pain on eye movement, visual loss, and optic disc swelling**. ### Clinical Presentation Analysis This patient presents with: 1. **Acute visual loss** (6/36 acuity) — typical of optic neuritis 2. **Pain on eye movement** — hallmark of optic neuritis; absent in papilloedema 3. **Colour vision defect** (dyschromatopsia) — common in optic neuritis; rare in papilloedema 4. **Optic disc swelling** with blurred margins — can occur in both conditions 5. **Normal IOP** (14 mmHg) — rules out secondary causes 6. **MRI evidence of optic nerve inflammation** — T2 hyperintensity confirms nerve inflammation ### Papilloedema vs Papillitis Comparison | Feature | Papilloedema | Papillitis (Optic Neuritis) | |---------|--------------|-----------------------------| | **Onset** | Insidious (days to weeks) | Acute (hours to 2–3 days) | | **Pain on eye movement** | Absent | **Present** ✓ | | **Visual acuity** | Normal initially; late loss | **Early, marked loss** ✓ | | **Colour vision** | Preserved until late | **Defective early** ✓ | | **Disc swelling pattern** | Bilateral, circumferential | Often polar or segmental | | **Spontaneous venous pulsation** | Absent (raised ICP) | Usually present | | **Fundus findings** | Obscured disc, no exudates initially | Disc swelling ± peripapillary inflammation | | **MRI optic nerve** | Normal or atrophic | **T2 hyperintensity** ✓ | | **Cause** | Raised ICP (brain tumour, hydrocephalus, etc.) | Demyelination (MS, NMO), infection, inflammation | **High-Yield:** The **pain on eye movement + acute visual loss + colour vision defect** triad is pathognomonic for optic neuritis. Papilloedema is painless. ### Clinical Pearl **Clinical Pearl:** Optic neuritis is the presenting feature of multiple sclerosis in 15–20% of MS patients. This young woman should undergo: - Brain and spinal cord MRI (demyelinating lesions) - Visual evoked potentials (VEP) — delayed latency - Oligoclonal bands in CSF (if MS suspected) ### Mnemonic: OPTIC NEURITIS **Mnemonic:** **OPTIC** = **O**nset acute, **P**ain on movement, **T**emporal loss (early visual loss), **I**nflammation (MRI T2 hyperintensity), **C**olour vision defect ### Management 1. High-dose IV methylprednisolone 1 g daily × 3 days (ONTT trial evidence) 2. Oral prednisolone taper 3. Investigation for demyelinating disease 4. Long-term follow-up for MS development