Optic neuritis

Overview

•Painful monocular vision loss - subacute onset over hours to days, ranging from mild blur to near-total loss

•Pain on eye movement - traction on inflamed optic nerve sheath; highly specific feature

•Red desaturation (dyschromatopsia) - red objects appear washed out; colour vision often first deficit to appear

•Central scotoma - most common visual field defect; papillomacular bundle involvement

•Relative afferent pupillary defect (RAPD) - Marcus Gunn pupil on swinging light test; paradoxical dilation when light swung to affected eye

•Uhthoff's phenomenon - temporary worsening with raised body temperature (exercise, hot bath); does NOT represent a new relapse

•Fundoscopy: retrobulbar neuritis (most common) - normal disc acutely; papillitis (anterior) - swollen disc; chronic - disc pallor

💡
Classic teaching: in retrobulbar neuritis 'the patient sees nothing, and neither does the doctor' - normal fundoscopy does NOT exclude optic neuritis.

Investigations

🥇 First-line

•MRI brain and orbits with gadolinium - investigation of choice; gadolinium enhancement confirms active inflammation; T2/FLAIR white matter lesions stratify MS risk

•Visual acuity (Snellen), colour vision (Ishihara plates), swinging light test, visual field testing (automated perimetry), fundoscopy

🥈 Second-line

•AQP4 antibodies (NMOSD), MOG-IgG (MOGAD), FBC, CRP, ESR, ANA, ANCA, ACE, syphilis serology - for atypical/bilateral/recurrent cases

•Visual evoked potentials (VEPs) - prolonged P100 latency due to demyelination

Differential diagnosis

Optic neuritis vs anterior ischaemic optic neuropathy (AION)
FeatureOptic neuritisAION (GCA)
Age20-40 years>50 years (typically >70)
PainPain on eye movementHeadache, jaw claudication, scalp tenderness
Vision lossSubacute, often recoversSudden, often permanent
FundoscopyNormal disc (retrobulbar) or swollenSwollen, pale optic disc
AssociationMultiple sclerosisGiant cell arteritis
Urgent treatmentIV methylprednisoloneHigh-dose corticosteroids immediately

Feature	Optic neuritis	AION (GCA)
Age	20-40 years	>50 years (typically >70)
Pain	Pain on eye movement	Headache, jaw claudication, scalp tenderness
Vision loss	Subacute, often recovers	Sudden, often permanent
Fundoscopy	Normal disc (retrobulbar) or swollen	Swollen, pale optic disc
Association	Multiple sclerosis	Giant cell arteritis
Urgent treatment	IV methylprednisolone	High-dose corticosteroids immediately

🚨
AION due to giant cell arteritis requires immediate high-dose corticosteroids to prevent bilateral blindness - do not miss this diagnosis.

Management

•Urgent ophthalmology referral - all patients; subsequent neurology input for MS risk stratification

•First-line (acute): methylprednisolone 1 g IV once daily for 3 days - speeds visual recovery but does NOT improve final visual acuity at 6-12 months; reserved for poor vision in fellow eye, bilateral involvement, or occupational necessity

•Mild unilateral disease: supportive monitoring - most recover spontaneously within 4-12 weeks

•MS risk: neurological referral if MRI shows demyelinating lesions; early disease-modifying therapy (e.g. interferon-beta, natalizumab) reduces risk of second demyelinating event

•Uhthoff's phenomenon: advise avoidance of hot environments; reassure it is reversible and not a new relapse

⚠️
Oral prednisolone alone is contraindicated in optic neuritis - the Optic Neuritis Treatment Trial (ONTT) showed it increases recurrence rate compared to IV methylprednisolone or placebo.

Prognosis and MS risk

•Most recover visual acuity to 6/9 or better within 4-12 weeks; subtle colour and contrast deficits may persist

•~50% of patients with MS experience optic neuritis; ~50% of first-episode optic neuritis develop MS within 15 years

•Normal MRI at presentation: ~25% MS risk at 15 years; ≥2 T2 lesions: ~72% MS risk at 15 years