A 38-year-old woman from Bangalore presents with a 3-week history of progressive right eye pain, redness, and blurred vision. She is an avid contact lens wearer (daily wear, cleaned with tap water). On slit-lamp examination, there is a central corneal ulcer with an irregular, feathery border and stromal infiltration. The cornea shows a characteristic 'map-like' appearance with satellite lesions. Corneal sensation is markedly reduced. Gram stain and bacterial culture are negative. KOH mount of corneal scrapings shows trophozoites and cysts. What is the most appropriate initial management?

Explanation

## Diagnosis: Acanthamoeba Keratitis **Key Point:** The clinical triad of contact lens use (with tap water exposure), negative bacterial/fungal cultures, and KOH mount showing trophozoites and cysts is diagnostic of *Acanthamoeba* keratitis. ## Diagnostic Features of Acanthamoeba Keratitis | Feature | Acanthamoeba | Bacterial | Fungal | |---------|--------------|-----------|--------| | **Risk factor** | Contact lens + tap water | Trauma, contact lens | Organic material trauma | | **Onset** | Indolent, 2–4 weeks | Acute, 3–7 days | Subacute, 1–2 weeks | | **Pain severity** | Severe, disproportionate | Moderate | Mild to moderate | | **Corneal sensation** | Markedly reduced | Preserved/reduced | Reduced | | **Ulcer border** | Irregular, feathery, 'map-like' | Sharp, raised | Feathery with hyphae | | **Satellite lesions** | Common | Rare | Rare | | **Gram/bacterial culture** | Negative | Positive | Negative | | **KOH/fungal culture** | Negative | Negative | Positive | | **Trophozoites/cysts** | Present on KOH | Absent | Absent | **High-Yield:** Acanthamoeba is a free-living amoeba found in tap water, soil, and dust. Contact lens wearers who rinse lenses with tap water or use non-sterile saline are at highest risk. ## Why This Patient Has Acanthamoeba Keratitis 1. **Risk factor:** Daily contact lens wearer with tap water exposure (classic epidemiology) 2. **Indolent presentation:** 3-week progressive course (not acute like bacterial) 3. **Disproportionate pain:** Severe pain with relatively modest clinical findings 4. **Reduced corneal sensation:** Characteristic of Acanthamoeba (amoeba invade nerve endings) 5. **Negative cultures:** No growth on bacterial or fungal media 6. **KOH mount positive:** Trophozoites and cysts visible (diagnostic) 7. **Morphology:** 'Map-like' ulcer with feathery borders and satellite lesions (pathognomonic) ## Treatment Algorithm for Acanthamoeba Keratitis ```mermaid flowchart TD A[Acanthamoeba keratitis confirmed]:::outcome --> B[Discontinue contact lens use immediately]:::action B --> C[Topical antimicrobial therapy]:::action C --> D[Chlorhexidine 0.02% hourly]:::action C --> E[PHMB 0.02% hourly]:::action F[Systemic therapy] --> G[Oral itraconazole 200 mg BD]:::action F --> H[Oral albendazole 400 mg BD]:::action I[Adjunctive measures] --> J[Topical lubricants frequent]:::action I --> K[Avoid topical steroids unless severe inflammation]:::action I --> L[Consider penetrating keratoplasty if perforation risk]:::action D --> M[Continue 3-6 months minimum]:::action E --> M G --> M H --> M ``` **Key Point:** Chlorhexidine 0.02% and PHMB are the cornerstones of topical therapy. Both are amoebicidal and must be used hourly for weeks to months. Systemic antiamoebic agents (itraconazole, albendazole) are essential adjuncts. ## Why Each Medication Works | Agent | Mechanism | Dosing | Duration | |-------|-----------|--------|----------| | **Chlorhexidine 0.02%** | Amoebicidal; disrupts cell membrane | Hourly | 3–6 months | | **PHMB 0.02%** | Amoebicidal; alternative if chlorhexidine unavailable | Hourly | 3–6 months | | **Itraconazole** | Systemic antiamoebic; penetrates cornea | 200 mg BD | 3–6 months | | **Albendazole** | Systemic antiamoebic; alternative | 400 mg BD | 3–6 months | **Clinical Pearl:** Topical steroids are generally avoided in Acanthamoeba keratitis because they promote amoebic replication and increase risk of perforation. Use only if severe anterior uveitis threatens vision, and always with concurrent intensive amoebicidal therapy. **Warning:** Fluoroquinolones (moxifloxacin, gatifloxacin) are NOT effective against Acanthamoeba and should NOT be used as monotherapy. They may delay diagnosis by suppressing secondary bacterial infection.