Fungal Keratitis Not Improving on Hourly Natamycin and Voriconazole
A 46-year-old rice farmer presents with a 6-day history of a painful right eye after a rice husk struck the cornea. Best-corrected acuity is counting fingers at 1 m. Slit lamp shows a 4.5 mm central feathery infiltrate with satellite lesions, an epithelial defect, and a 1.2 mm hypopyon. Confocal microscopy shows branching hyphae; fungal culture is pending. He was started on natamycin 5% suspension every hour while awake, alternating with compounded voriconazole 1% every hour, by a local clinic. After 72 hours the infiltrate is unchanged, the epithelial defect has enlarged to 5.5 mm, and he reports intense burning. The pharmacy's natamycin bottle lists benzalkonium chloride as preservative. He has no diabetes and no immunosuppression, and he cannot afford hospitalization. Intraocular pressure is 14 mmHg and the fundus view is hazy.
1. How should the lack of response at 72 hours be interpreted, and what specific changes to the topical antifungal regimen are justified? Explain the pharmacologic reasoning.
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Interpretation
Fungal keratitis often improves slowly, so 72 hours is early. However, an unchanged infiltrate plus an enlarging epithelial defect and severe burning is not simple slow response; it suggests drug toxicity, poor adherence, inadequate penetration, or progression. The key is to separate these before escalating.
Reasoning by mechanism
- Natamycin 5% is first-line for filamentous fungi but penetrates stroma poorly. Its vehicle contains benzalkonium chloride; hourly BAK exposure on a 5.5 mm epithelial defect can cause punctate epitheliopathy, burning, and delayed re-epithelialization. The suspension also settles, so if the patient does not shake it, each dose may be subtherapeutic.
- Voriconazole 1% has much better corneal penetration and broad mold coverage, but hourly alternating with a preserved suspension doubles toxic exposure.
- 'While awake' dosing leaves a 7–8 hour overnight gap, which is inadequate for a severe central infiltrate.
Justified changes
- Confirm adherence and shaking technique; observe instillation.
- Switch natamycin to a preservative-free compounded suspension if available, or reduce BAK exposure by using voriconazole 1% as the main hourly agent and natamycin every 2 hours.
- Extend natamycin to every hour including overnight for 48 hours if tolerated, because frequency drives concentration-dependent killing.
- Consider intrastromal voriconazole 50 µg/0.1 mL or oral voriconazole 200 mg twice daily for severe stromal disease, after liver and interaction review.
- Perform corneal scraping/debridement for culture and to reduce organism load.
Do not simply add another topical antifungal without addressing penetration and toxicity.
2. Should topical corticosteroid be added at this visit? Justify with timing, risks, and the conditions that would change your answer.
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No, not at this visit. The infiltrate is not shrinking and the epithelial defect is enlarging. Corticosteroids suppress neutrophil and macrophage control of fungal replication and inhibit fibroblast-mediated repair, increasing the risk of perforation. They also mask worsening pain and inflammation.
The pharmacologic rationale for steroid use is to limit immune-ring inflammation, synechiae, and scarring, but the preconditions are: (1) the organism is identified, (2) an effective antifungal has been delivered for at least 48–72 hours, and (3) the infiltrate and hypopyon are clearly decreasing. None is met here.
Conditions that would change the answer:
- Culture confirms a filamentous fungus and the infiltrate/hypopyon is regressing on optimized therapy; then a low-potency steroid such as fluorometholone 0.1% or loteprednol 0.5% could be started with close follow-up.
- A severe immune ring is causing progressive stromal melting despite adequate antifungal coverage; steroid may be considered with infectious disease input.
- Bacterial coinfection must be treated first.
If steroid is ever started, use the lowest effective potency and frequency, monitor daily initially, and continue antifungal cover.
3. If culture later identifies Fusarium solani versus Candida albicans, how would the regimen change and why?
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Fusarium solani (filamentous mold)
Natamycin remains the drug of choice because it binds ergosterol and is particularly active against filamentous fungi, especially Fusarium. Continue or optimize natamycin 5% hourly; add voriconazole 1% topically and consider intrastromal or oral voriconazole for deep stromal involvement. Fusarium is often resistant to amphotericin B, so substituting amphotericin would be a mistake.
Candida albicans (yeast)
Yeasts respond better to topical amphotericin B 0.15% or voriconazole 1%; natamycin has weaker activity against Candida. For severe keratitis, add oral fluconazole or voriconazole depending on susceptibility and renal/hepatic status. Amphotericin B is more toxic to the epithelium, so preservative-free formulation and less frequent dosing are preferred.
Reasoning
Drug selection follows organism morphology and susceptibility, not just 'broad antifungal' coverage. Penetration also matters: natamycin stays superficial, so deep Candida keratitis needs a penetrating agent such as voriconazole or amphotericin. Repeat scraping if the response remains poor, because mixed infection and resistance are possible.