Protozoan Keratitis

Introduction & Etiology - Tiny Eye Terrors

• Sight-threatening corneal infection caused by protozoa.
• Predominant pathogen: Acanthamoeba species (ubiquitous free-living amoeba).
• Key Risk Factors:
  • Contact lens (CL) wear: improper hygiene, overnight use, tap water rinsing.
  • Corneal trauma.
  • Exposure to contaminated water (e.g., swimming, hot tubs).
• Two forms: active trophozoites & highly resistant cysts.

⭐ Severe ocular pain, often disproportionate to clinical findings, is a hallmark of Acanthamoeba keratitis (AK).

Risk Factors & Pathogenesis - Entry Points & Ploys

• Contact Lens Wear: Major risk.
  • Improper hygiene, overnight use.
  • Contaminated solutions (e.g., tap water rinse).
• Corneal Trauma: Especially with soil/vegetation.
• Water Exposure: Swimming, hot tubs with contaminated water.
• Pathogenesis:
  • Acanthamoeba entry via epithelial microtrauma.
  • Trophozoites adhere (mannose glycoproteins).
  • Release enzymes (proteases) → stromal invasion, inflammation.
  • Cyst formation → resistance, recurrence.
  • Radial keratoneuritis (nerve invasion) → severe pain.

⭐ Contact lens wear, particularly soft lenses with poor hygiene (e.g., tap water rinsing), is the predominant risk factor in developed countries.

Clinical Features - Painful Eye Alarms

• Severe, excruciating eye pain: Classically out of proportion to visible signs. ⚠️
• Intense photophobia, marked ↓ vision, redness (circumciliary), tearing.
• Persistent foreign body sensation.
• Key Signs:
  • Early: Epithelial irregularity, punctate erosions, pseudodendrites (mimics HSV).
  • Radial keratoneuritis: Infiltration along corneal nerves; early & highly specific.
  • Stromal infiltrates: Patchy, nummular, diffuse; may progress to abscess.
  • Ring infiltrate: Characteristic (late, ~50% cases), due to Ag-Ab complexes.
  • Anterior uveitis, hypopyon, ↑IOP, scleritis.

⭐ Pain out of proportion to signs is a critical red flag for Acanthamoeba keratitis, especially in contact lens wearers.

Diagnosis - Microbe Spotting Mission

• Corneal Scraping: Key diagnostic step.
  • Obtain multiple deep scrapings from ulcer edge & base.
• Microscopy & Stains:
  • Calcofluor white (CFW): For Acanthamoeba cysts (apple-green fluorescence).
  • Giemsa: Cysts (purple), trophozoites (blue).
  • Gram stain: Usually negative (rules out bacteria).
  • KOH wet mount: Visualizes cysts.
• Culture:
  • Non-nutrient agar (NNA) + E. coli overlay.
  • Characteristic "snail tracks".
• Confocal Microscopy: In vivo visualization of cysts & trophozoites.
• PCR: Highly sensitive for Acanthamoeba DNA.

⭐ Calcofluor white (CFW) stain is considered the most sensitive method for detecting Acanthamoeba cysts, which appear with apple-green fluorescence under UV light.

Management - Eviction Strategy

• Goal: Eradicate cysts & trophozoites, control inflammation, promote healing.
• Topical Antiamoebics (First-line):
  • Polyhexamethylene biguanide (PHMB) 0.02% - 0.08%
  • Chlorhexidine 0.02% - 0.2%
  • Propamidine isethionate (Brolene) 0.1%
  • Diamidines: Hexamidine
  • Often used in combination (e.g., PHMB + Brolene).
• Treatment Regimen:
  • Intensive phase: Hourly, day & night for 48-72 hrs.
  • Tapering phase: Gradually reduce frequency over weeks to months (typically 3-6 months, sometimes longer).
• Adjunctive Therapy:
  • Cycloplegics: For pain relief, prevent synechiae.
  • Topical steroids: ⚠️ Controversial; consider late in course for severe inflammation, ONLY under antiamoebic cover.
  • Oral NSAIDs: Pain management.
• Surgical Intervention (Resistant Cases):
  • Debridement
  • Amniotic membrane transplantation
  • Therapeutic penetrating keratoplasty (TPK) - for perforation or non-resolving infection.

⭐ Dual therapy with a biguanide (PHMB or chlorhexidine) and a diamidine (propamidine or hexamidine) is often preferred for initial treatment due to synergistic effects and to combat resistance.

• Pain Management: Crucial; often severe.
• Follow-up: Close monitoring for response and complications (e.g., glaucoma, cataract).

High‑Yield Points - ⚡ Biggest Takeaways

• Acanthamoeba keratitis: protozoan infection linked to contact lens wear and contaminated water.
• Hallmark: severe eye pain, strikingly out of proportion to clinical signs.
• Key signs: radial keratoneuritis (early), ring infiltrate (late, pathognomonic).
• Diagnosis: corneal scraping (microscopy, culture on NNA + E. coli), confocal microscopy.
• Treatment: prolonged topical biguanides (e.g., PHMB) and diamidines (e.g., propamidine).
• Corticosteroids: generally avoided in active infection; may worsen the condition.