Microbial keratitis
At a glance
Corneal infection can progress rapidly and threaten sight. Treat as an urgent ophthalmic problem.
CAVE
- Consider fungal or Acanthamoeba infection after relevant exposure, atypical signs or poor response.
Clinical presentation
- Pain, photophobia, redness, reduced vision and an infiltrate with an epithelial defect; contact-lens wear is an important risk.
Diagnosis & investigations
- Record acuity, epithelial defect and infiltrate dimensions separately, location, depth, thinning, anterior chamber reaction and IOP if safe. Test corneal sensation when herpes is possible.
- Scrape central, large, deep, atypical or treatment-resistant ulcers before antibiotics when this causes no harmful delay. Alert the laboratory to suspected bacteria, fungi or Acanthamoeba; sample the ulcer edge/base using the appropriate media. Lens/case cultures are adjuncts, not substitutes for corneal sampling.
Differential diagnosis
- Sterile infiltrate, herpetic keratitis, peripheral ulcerative keratitis and neurotrophic disease.
Treatment
- Start treatment immediately after sampling; choose the regimen according to severity and local resistance. Add cycloplegia for significant ciliary pain. Do not patch the eye or prescribe topical anaesthetic for unsupervised home use.
- Consider adjunctive topical steroid only with the corneal specialist after at least 48 hours of effective antibiotics when the organism is identified and/or infection is clearly responding. Avoid steroids in suspected fungal, Acanthamoeba or Nocardia keratitis.
| Finding | Action | Timing / detail |
|---|---|---|
| Small peripheral presumed bacterial infiltrate, no thinning or significant anterior chamber reaction | Topical fluoroquinolone, e.g. ofloxacin 0.3%, initially every 1–2 hours while awake in the cited NHS protocol | Examine again within 24 hours; contact lens wear stops immediately |
| Central, large, deep, rapidly progressive ulcer, hypopyon or stromal melt | Scrape promptly; intensive topical antibiotics, loading every 5–15 minutes then hourly; consider fortified antibiotics with the corneal service | Sight-threatening disease: include overnight dosing and admission if reliable delivery is impossible |
| Vegetative trauma, feathery margins, satellite lesions or poor antibacterial response | Request fungal microscopy/culture; specialist antifungal treatment; natamycin 5% is supported for filamentous fungal keratitis | Do not substitute empirical steroid treatment for microbiological reassessment |
| Water/contact lens exposure, disproportionate pain, perineural or ring infiltrate | Investigate Acanthamoeba with appropriate culture/PCR and, where available, confocal microscopy | Specialist biguanide-based treatment; an absent ring infiltrate does not exclude early disease |
Follow-up
- Review within 24 hours, daily initially for severe ulcers. Compare infiltrate density/size, epithelial healing, thinning, pain and chamber reaction; pain relief alone is insufficient.
- Progression or absent improvement at 24–48 hours requires adherence and microbiology review, repeat sampling or biopsy when indicated, and reconsideration of diagnosis and drug toxicity. Admit for impending perforation, scleral extension, suspected endophthalmitis or inability to sustain drops and visits.
Risk factors & context
- Contact lenses, epithelial trauma, ocular-surface disease and local immunosuppression increase risk.
- Ask specifically about overnight wear and water exposure.
Complications
- Stromal scarring, melting, perforation and intraocular spread.
- A larger infiltrate or increasing thinning requires immediate reassessment.
When to refer
- Immediate corneal specialist input for progression, melting, impending perforation or an only seeing eye.
References 5
- AAO · Bacterial Keratitis Preferred Practice Pattern ↗2024 · Diagnosis; diagnostic tests; treatment; provider and setting
- NHS GGC · Bacterial Keratitis ↗Version 7 · Initial antibiotic therapy; corneal review
- CDC · Clinical Overview of Acanthamoeba Keratitis ↗2025 · Clinical features; diagnosis; treatment
- MUTT I · Natamycin versus voriconazole for fungal keratitis ↗2013 · randomised trial · Filamentous fungal keratitis; treatment comparison
- EMA · Akantior (polihexanide) ↗EU authorisation 22 August 2024 · Indication; product information; treatment schedule
Adapt to the patient, local pathways and authorised product information.