A 28-year-old woman presents with acute vision loss in the left eye over 24 hours, accompanied by pain on eye movements. On examination, visual acuity is 6/60 in the affected eye with a relative afferent pupillary defect (RAPD). Fundoscopy reveals a normal-appearing optic disc. MRI brain with contrast shows enhancement of the left optic nerve. What is the most likely diagnosis?

## Clinical Diagnosis: Retrobulbar Optic Neuritis **Key Point:** Retrobulbar optic neuritis is inflammation of the optic nerve posterior to the optic disc, classically presenting with acute unilateral vision loss, pain with eye movements, and a normal fundoscopic appearance early in the disease. ### Clinical Features Supporting the Diagnosis | Feature | Retrobulbar ON | Anterior ON | AION | Compressive ON | |---------|---|---|---|---| | **Pain with eye movement** | Yes (90%) | Yes | No | No | | **Fundoscopic appearance** | Normal initially | Disc edema/hyperemia | Disc edema | Normal or atrophy | | **RAPD** | Present | Present | May be absent | May be absent | | **Acute onset** | Yes (hours–days) | Yes | Yes | Gradual | | **MRI optic nerve enhancement** | Yes (T1 + contrast) | Yes | No | Yes (mass effect) | | **Age of onset** | 20–40 years | 20–40 years | >50 years | Variable | **High-Yield:** The combination of **normal fundoscopy + pain on eye movement + RAPD + acute vision loss** is pathognomonic for retrobulbar optic neuritis. The normal disc appearance distinguishes it from anterior optic neuritis, where disc swelling is visible. ### Pathophysiology Retrobulbar optic neuritis is an inflammatory demyelinating condition affecting the optic nerve. The inflammation causes: 1. Demyelination of axons → slowed conduction → vision loss 2. Axonal swelling → pain with extraocular muscle contraction (traction on inflamed nerve) 3. Optic nerve edema (not visible on fundoscopy because the lesion is behind the disc) **Clinical Pearl:** Optic neuritis is the presenting sign of multiple sclerosis (MS) in 15–20% of MS patients. The Optic Neuritis Treatment Trial (ONTT) showed that intravenous methylprednisolone hastens visual recovery and delays MS diagnosis by ~2 years, but does not improve final visual outcome. ### Why MRI Enhancement Confirms the Diagnosis T1-weighted MRI with gadolinium shows enhancement of the inflamed optic nerve segment. This confirms demyelinating inflammation and is used to: - Confirm diagnosis - Assess for other demyelinating lesions (brain, spinal cord) suggestive of MS - Establish baseline for prognostication **Mnemonic: RAPD in ON — "Relative Afferent Pupillary Defect"** = optic nerve lesion → afferent pupil pathway blocked → pupil on affected side dilates more (appears relatively dilated when light swung to it). ### Differential Considerations **Anterior optic neuritis** would show disc hyperemia and swelling on fundoscopy — this patient's disc is normal. **AION** (arteritic or non-arteritic) typically occurs in older patients (>50 years), lacks pain with eye movements, and shows disc edema at presentation — not applicable here. **Compressive lesion** would show gradual vision loss, no pain with eye movement, and imaging would reveal a mass — the acute presentation and pain exclude this. [cite:Harrison 21e Ch 381]