Monocular vision loss and scotomas

Overview

•Lesion anterior to chiasm (optic nerve/retina) → monocular visual loss only

•Lesion at or behind chiasm → binocular homonymous field defects (same side of visual field in both eyes)

•Defect is always contralateral to the lesion

Optic radiation lesions - lobe vs field defect
Lesion locationFibres affectedVisual field defect
Parietal lobeSuperior fibres of optic radiationContralateral homonymous inferior quadrantanopia ('pie on the floor')
Temporal lobe (Meyer's loop)Inferior fibres of optic radiationContralateral homonymous superior quadrantanopia ('pie in the sky')

Lesion location	Fibres affected	Visual field defect
Parietal lobe	Superior fibres of optic radiation	Contralateral homonymous inferior quadrantanopia ('pie on the floor')
Temporal lobe (Meyer's loop)	Inferior fibres of optic radiation	Contralateral homonymous superior quadrantanopia ('pie in the sky')

🎯
Left parietal lobe lesion → disrupts superior fibres of right optic radiation → right homonymous inferior quadrantanopia. Left temporal lobe lesion → Meyer's loop → right homonymous superior quadrantanopia.

Presentation

Key distinguishing features by cause
ConditionKey featuresFundoscopy
CRAOSudden painless complete monocular loss; RAPD present; vision ~counting fingersPale retina, arterial attenuation, cherry red spot at fovea
CRVOSudden painless visual loss'Stormy sunset' - flame haemorrhages all 4 quadrants, dilated tortuous veins, disc swelling
Optic neuritisPainful visual loss; pain on eye movement; RAPD; red desaturationNormal (retrobulbar) or swollen disc (papillitis)
Arteritic AION (GCA)Sudden painless altitudinal loss on waking; age >60; headache, jaw claudication, ESR/CRP raisedSwollen pale optic disc
Retinal detachmentFloaters and photopsia → spreading shadow/curtain; peripheral to centralDetached retina; assess macula status
Vitreous haemorrhageSudden 'cobwebs', 'red haze'; painlessObscured/lost fundal reflex

Condition	Key features	Fundoscopy
CRAO	Sudden painless complete monocular loss; RAPD present; vision ~counting fingers	Pale retina, arterial attenuation, cherry red spot at fovea
CRVO	Sudden painless visual loss	'Stormy sunset' - flame haemorrhages all 4 quadrants, dilated tortuous veins, disc swelling
Optic neuritis	Painful visual loss; pain on eye movement; RAPD; red desaturation	Normal (retrobulbar) or swollen disc (papillitis)
Arteritic AION (GCA)	Sudden painless altitudinal loss on waking; age >60; headache, jaw claudication, ESR/CRP raised	Swollen pale optic disc
Retinal detachment	Floaters and photopsia → spreading shadow/curtain; peripheral to central	Detached retina; assess macula status
Vitreous haemorrhage	Sudden 'cobwebs', 'red haze'; painless	Obscured/lost fundal reflex

💡
RAPD (swinging flashlight test) = pre-chiasmal lesion (optic nerve or extensive retinal disease) on the affected side.

Investigations

🥇 First-line

•Visual acuity, colour vision (Ishihara), visual field testing, fundoscopy, swinging flashlight test (RAPD)

•Urgent same-day: ESR and CRP in all patients >60 with acute visual loss - exclude GCA

•Cardiovascular workup in CRAO/amaurosis fugax: ECG (AF), carotid Doppler (stenosis), FBC, fasting glucose, lipids

•MRI brain and orbits (gadolinium): optic neuritis, demyelination, posterior visual pathway lesion

🏆 Gold standard

•Fluorescein angiography + OCT - retinal perfusion and macular structure (CRAO, CRVO, macular pathology)

•Temporal artery biopsy: confirms GCA; valid up to 2 weeks after starting steroids - do NOT delay treatment

•B-scan ultrasound: if fundal view obscured (vitreous haemorrhage) to assess retinal integrity

Management

•CRVO: No acute intervention to restore vein; anti-VEGF injections (ranibizumab or aflibercept) first-line for macular oedema; laser photocoagulation for neovascularisation

•Optic neuritis: IV methylprednisolone 1 g daily for 3 days - speeds recovery but does not alter final visual outcome (ONTT); avoid oral steroids alone (increased relapse risk); urgent neurology referral for MS investigation

•Retinal detachment: Surgical emergency - urgent reattachment (pneumatic retinopexy, scleral buckle, or vitrectomy); macula-off detachment = more urgent (permanent central vision loss if delayed)

🚨
GCA causing visual loss - start high-dose prednisolone immediately on clinical suspicion. Do NOT wait for biopsy results. Without treatment, the fellow eye is at risk of irreversible blindness within 24-48 hours.

Prognosis

•CRAO: Poor prognosis - most retain only counting fingers or worse; critical window likely within 4-6 hours

•Optic neuritis: Good prognosis - most regain near-normal acuity within 6 months; subtle colour/contrast deficits may persist; MS develops in up to 50% over 15 years

•Retinal detachment: Excellent if repaired before macula detaches; poor if macular detachment occurs

•GCA visual loss: Typically permanent - prevention through urgent steroid initiation is paramount

Scotoma Patterns and Localisation

•Central scotoma - loss at fixation; macula or optic nerve pathology

•Arcuate scotoma - follows nerve fibre layer; hallmark of glaucoma

•Altitudinal defect - upper or lower half loss in one eye; ischaemia of optic nerve (AION) or large branch retinal artery occlusion

•Amaurosis fugax - transient complete monocular loss (seconds to minutes); emboli to retinal circulation; TIA equivalent