A 32-year-old man presents with progressive blurring of vision in the left eye over 4 days, accompanied by pain on eye movements. He denies any systemic symptoms or prior neurological illness. Visual acuity is 6/36 in the left eye with a positive RAPD. Fundoscopy shows mild optic disc swelling with blurred margins. Formal visual field testing reveals a central scotoma. What is the most appropriate first-line investigation to confirm the diagnosis and assess for demyelinating disease?

Explanation

## Investigation Strategy in Optic Neuritis **Key Point:** MRI brain and orbits with gadolinium contrast is the gold-standard first-line investigation to confirm optic neuritis and screen for demyelinating disease (MS). ### Role of MRI in Optic Neuritis | Investigation | Purpose | Diagnostic Yield | |---|---|---| | **MRI orbits (T2/FLAIR)** | Visualize optic nerve inflammation, demyelination | Confirms ON; shows T2 hyperintensity in nerve sheath | | **MRI brain** | Detect asymptomatic demyelinating lesions | 50–70% of ON patients have brain lesions; predicts MS risk | | **Gadolinium enhancement** | Identifies active inflammation (BBB breakdown) | Acute lesions enhance; chronic lesions do not | | **OCT** | Measures retinal nerve fiber layer (RNFL) thickness | Useful for **follow-up** (RNFL thinning at 3–6 months), not acute diagnosis | | **Visual field** | Characterizes functional deficit | Confirms central scotoma but does not diagnose etiology | **High-Yield:** The **Optic Neuritis Treatment Trial (ONTT)** demonstrated that patients with **≥3 brain MRI lesions** at presentation have a **50% risk of MS diagnosis within 15 years** [cite:Harrison 21e Ch 380]. ### Clinical Features Supporting Optic Neuritis 1. **Subacute vision loss** (hours to days) 2. **Pain on eye movement** (retrobulbar inflammation) 3. **RAPD** (asymmetric optic nerve damage) 4. **Central scotoma** on visual field (typical pattern) 5. **Disc swelling** (anterior ON) or normal disc (retrobulbar ON) ### Why MRI is First-Line ```mermaid flowchart TD A[Suspected Optic Neuritis]:::outcome --> B[MRI Brain + Orbits with Gd]:::action B --> C{Demyelinating lesions present?}:::decision C -->|Yes| D[High MS risk]:::outcome C -->|No| D2[Low MS risk]:::outcome D --> E[IV Methylprednisolone + DMT counseling]:::action D2 --> F[IV Methylprednisolone + observation]:::action E --> G[Neurology referral]:::action F --> G ``` **Clinical Pearl:** MRI findings **stratify MS risk** and guide treatment intensity. Patients with brain lesions benefit from early disease-modifying therapy (DMT) initiation to prevent conversion to clinically definite MS. ### Why Other Options Are Secondary **OCT** — Excellent for monitoring RNFL thinning over weeks to months (correlates with axonal loss), but does not diagnose acute ON or detect demyelinating lesions. Used as **adjunct** after MRI confirms diagnosis. **FFA** — May show optic disc hyperfluorescence and late staining in ON, but is **non-specific** and does not detect demyelinating lesions; rarely used in modern practice. **Visual field testing** — Confirms functional deficit (central scotoma is classic), but is **descriptive**, not diagnostic; does not identify etiology or MS risk. **Mnemonic: MRI First** — **M**ultiple sclerosis risk assessment, **R**etrobulbar inflammation visualization, **I**nflammatory lesion detection.