A 28-year-old woman presents with sudden onset of painless vision loss in the right eye over 6 hours. She reports eye pain on movement and mild headache for 2 days prior. On examination, visual acuity is 6/60 in the right eye, with a relative afferent pupillary defect (RAPD). Fundoscopy shows a normal optic disc. MRI brain and orbits reveals T2-hyperintense lesion in the right optic nerve. She has no prior neurological history. What is the most likely diagnosis?

Explanation

## Diagnosis: Retrobulbar Optic Neuritis ### Clinical Presentation This patient presents with the classic triad of optic neuritis: 1. **Subacute vision loss** (develops over hours to days) 2. **Periocular pain worsened by eye movement** (90% of cases) 3. **RAPD** indicating afferent pupillary defect ### Key Distinguishing Feature: Normal Fundoscopy **Key Point:** Retrobulbar optic neuritis affects the optic nerve posterior to the optic disc, hence the fundus appears normal early ("the patient sees nothing, the doctor sees nothing"). This distinguishes it from anterior optic neuritis where disc swelling is visible. ### MRI Findings **High-Yield:** T2-hyperintense lesion in the optic nerve on MRI is pathognomonic for demyelinating optic neuritis. The lesion typically shows enhancement with gadolinium. ### Association with Multiple Sclerosis **Clinical Pearl:** Optic neuritis is the presenting symptom in 15–20% of MS patients. In this young woman, the demyelinating lesion on MRI warrants further evaluation for MS (brain and spinal cord MRI, CSF oligoclonal bands). ### Pathophysiology Demyelination of optic nerve axons leads to: - Conduction block and slowed nerve conduction velocity - Relative afferent pupillary defect (RAPD) due to asymmetric optic nerve dysfunction - Recovery typically occurs over 4–12 weeks with or without corticosteroids ### Management **Key Point:** High-dose IV methylprednisolone (1 g daily × 3 days) followed by oral taper is the standard of care. The Optic Neuritis Treatment Trial (ONTT) demonstrated that IV steroids accelerate visual recovery and reduce risk of MS development over 15 years.