Fungal keratitis

Overview

Uncommon but sight-threatening corneal stromal infection - more insidious onset than bacterial keratitis. Delayed diagnosis is the single most important avoidable cause of poor outcomes.

Risk Factors

Key risk factors
Agricultural/plant trauma (filamentous fungi)
Contact lens wear
Immunocompromised state
Prolonged topical corticosteroid use
Diabetes mellitus
Fusarium / Aspergillus - tropical/agricultural
Candida - immunosuppression/developed countries

Presentation

•Corneal infiltrate with feathery/filamentous borders - hallmark; irregular 'fluffy' edges vs sharper bacterial margins
•Satellite lesions - small peripheral infiltrates separate from main ulcer (peripheral hyphal invasion)
•Hypopyon - may be mobile ('shifts with head position')
•Endothelial plaque - yellowish posterior corneal deposit; indicates deeper invasion
•Dry, raised, rough ulcer surface - vs wet/necrotic appearance of Pseudomonas
•Eye pain, photophobia, blurred vision, redness - symptoms evolve over days to weeks
🚨
Worsening keratitis despite topical antibiotics ± steroids = suspect fungal keratitis immediately. Stop steroids and refer urgently to ophthalmology.

Investigations

🏆 Gold standard

•corneal scraping for microscopy and culture - from base and advancing edges; calcofluor white staining identifies fungal hyphae; Sabouraud's agar culture for species ID and sensitivity

🥇 First-line

•slit-lamp biomicroscopy - characterises infiltrate morphology (feathery borders, satellite lesions, hypopyon)
•corneal confocal microscopy - non-invasive; directly visualises hyphae in stroma; rapid real-time diagnosis

🥈 Second-line

•PCR on corneal scraping or aqueous humour - high sensitivity, useful when culture negative (prior antifungal treatment)
•Blood glucose / HbA1c - screen for diabetes

Differential Diagnosis

•Acanthamoeba keratitis - contact lens wear, ring-shaped infiltrate, severe pain disproportionate to signs
•Herpes simplex keratitis - dendritic ulcer, reduced corneal sensation
•Sterile marginal keratitis - peripheral, associated with blepharitis, not infective
🎯
Classic triad: agricultural/plant trauma → failure to improve on topical antibiotics → feathery infiltrate with satellite lesions = filamentous fungal keratitis until proven otherwise.

Management

•Urgent ophthalmology referral in all cases; treatment courses 6-12 weeks
Antifungal treatment by organism
FeatureFilamentous (Fusarium/Aspergillus)Candida
First-line topicalNatamycin 5% eye drops - hourly initially; only licensed topical antifungalVoriconazole 1% or amphotericin B 0.15% eye drops (Candida resistant to natamycin)
Second-line topicalVoriconazole 1% eye drops (compounded) - excellent stromal penetrationVoriconazole 1% eye drops
Systemic (add if deep/severe)Oral voriconazole or oral itraconazole - for deep stromal/scleral involvement or topical failureOral voriconazole
Surgical (third-line)Therapeutic penetrating keratoplasty - perforation, endophthalmitis risk, maximal medical therapy failureTherapeutic penetrating keratoplasty
•Adjunct: cyclopentolate 1% cycloplegic drops - relieve ciliary spasm, prevent posterior synechiae
•Intrastromal/intracameral voriconazole injection - specialist centres for deep stromal/anterior chamber involvement
⚠️
Topical corticosteroids are absolutely contraindicated in suspected or confirmed fungal keratitis - they suppress local immunity and cause rapid deepening of the ulcer. Stop immediately if already prescribed.

Complications

•Corneal perforation - ocular emergency requiring urgent surgery
•Endophthalmitis - intraocular spread; very high risk of irreversible vision loss
•Corneal scarring/opacification, secondary glaucoma (anterior synechiae), cataract