A 28-year-old woman with a history of contact lens wear presents with corneal ulcer. KOH mount of corneal scrapings reveals branching, septate hyphae. Slit-lamp examination shows a feathery, infiltrate with minimal anterior chamber reaction. What is the drug of choice for topical antifungal therapy?

Explanation

## Fungal Corneal Ulcer — First-Line Topical Antifungal **Key Point:** Fungal keratitis requires topical antifungal therapy with **natamycin**, which achieves superior corneal penetration compared to other topical antifungals and is the only FDA-approved topical antifungal for corneal infection. ### Clinical Presentation of Fungal Keratitis The clinical triad of: - **Feathery, irregular infiltrate** (non-suppurative, minimal hypopyon) - **Branching, septate hyphae** on KOH mount (Aspergillus, Fusarium) - **Minimal anterior chamber reaction** (fungal ulcers are indolent) - **Contact lens wear** (risk factor for Acanthamoeba and fungi) ...indicates **fungal keratitis** requiring immediate topical antifungal therapy. ### Topical Antifungal Agents — Comparison | Agent | Natamycin 5% | Amphotericin B 0.15% | Fluconazole 1% | Itraconazole 1% | |-------|--------------|----------------------|-----------------|------------------| | **Mechanism** | Polyene; disrupts fungal cell membrane | Polyene; binds ergosterol | Azole; inhibits lanosterol 14α-demethylase | Azole; inhibits lanosterol 14α-demethylase | | **Corneal Penetration** | **Excellent** (only topical antifungal with good stromal penetration) | Moderate | Poor (minimal corneal penetration) | Poor (minimal corneal penetration) | | **Spectrum** | Aspergillus, Candida, Fusarium, Cryptococcus | Candida, Aspergillus, Cryptococcus | Candida, some Aspergillus | Candida, some Aspergillus | | **Dosing** | Every 1–2 hours (day 1–2), then every 2–4 hours | Every 1–2 hours | Every 1–2 hours | Every 1–2 hours | | **First-Line Status** | **YES — Gold standard** | Alternative (if natamycin unavailable) | Not recommended for keratitis | Not recommended for keratitis | | **FDA Approval** | **Only topical antifungal approved for keratitis** | Off-label | Off-label | Off-label | | **Toxicity** | Minimal; well-tolerated | Anterior chamber inflammation; toxicity | Minimal | Minimal | **High-Yield:** Natamycin is the **only topical antifungal with proven efficacy in fungal keratitis** because it is a polyene that disrupts the fungal cell membrane and achieves adequate corneal stromal penetration. Azoles (fluconazole, itraconazole) have poor corneal penetration and are reserved for systemic therapy or anterior uveitis. ### Why Natamycin 5% is First-Line 1. **Superior corneal penetration:** Polyene structure allows stromal penetration; azoles do not penetrate cornea adequately. 2. **Broad-spectrum coverage:** Effective against Aspergillus, Fusarium, Candida, and Cryptococcus. 3. **FDA-approved:** Only topical antifungal agent with FDA approval for corneal infection. 4. **Clinical efficacy:** Landmark studies (Srinivasan et al., NEJM 2013) demonstrated natamycin superiority over voriconazole in fungal keratitis. 5. **Minimal toxicity:** Well-tolerated with minimal anterior chamber inflammation. **Clinical Pearl:** Fungal keratitis is an **ophthalmologic emergency** because it progresses slowly but relentlessly to corneal scarring and blindness. Early diagnosis (KOH mount, culture) and aggressive topical natamycin therapy are critical. Systemic antifungals (fluconazole, voriconazole) are **adjunctive**, not primary therapy. **Mnemonic:** **NAT** = **N**atamycin for **A**spergillus (and other **T**opical fungal keratitis.