Optic Neuritis

Subacute monocular vision loss over hours-days, painful especially on eye movements, ↓ colour vision (red desaturation), RAPD, central scotoma, ± optic disc swelling (papillitis — 30%) or normal disc (retrobulbar — 70%). Differentials: AION, ischaemic optic neuropathy, infiltrative (sarcoid, lymphoma), compressive, infectious (syphilis, Lyme, TB, HIV, herpes). Workup: • Ophthalmology + neurology. • Visual acuity, colour vision (Ishihara), visual field, fundoscopy, OCT. • MRI brain + orbits (with gadolinium): identifies MS plaques, optic nerve enhancement. • Bloods: FBC, ESR, B12, syphilis, HIV, Lyme, ANA, ACE, anti-MOG, anti-AQP4 (NMOSD). • LP if MRI atypical — oligoclonal bands. • ECG before steroid.