Acanthamoeba Keratitis

Acanthamoeba keratitis is a painful, sight threatening infection of the cornea by a free living protozoan (trophozoite and cyst stages), found in soil, tap water, swimming pools, hot tubs, and contact lens cases.

About 85 percent of cases are in contact lens wearers (soft lenses, tap water rinsing, swimming, showering, and poor lens hygiene); non-wearers are affected after trauma with contaminated water or soil.

Cysts are resistant to many disinfectants and to freezing, and this makes the disease prolonged, with a risk of relapse.

Often misdiagnosed as herpetic or bacterial keratitis, and delay of diagnosis beyond a few weeks is the main determinant of a poor outcome.

• Pain out of proportion to the clinical signs (from radial keratoneuritis), photophobia, tearing, redness, and reduced vision; unilateral in most.
• Early: irregular epithelium, pseudodendrites, punctate keratopathy, and perineural infiltrates (radial keratoneuritis).
• Later: patchy stromal infiltrate, ring shaped stromal infiltrate (immune ring), epithelial defect, stromal oedema, hypopyon, and scleritis.
• Complications: corneal melt, perforation, secondary glaucoma, cataract, and uveitis.

• Corneal scraping (or biopsy): calcofluor white or Giemsa staining for cysts and trophozoites; culture on non-nutrient agar with an Escherichia coli overlay (growth within days); PCR for Acanthamoeba DNA is rapid and sensitive.

• In vivo confocal microscopy shows double walled cysts (hyperreflective round structures 10 to 25 micrometres) and is useful for early diagnosis.

• Culture of lenses, case, and solution.

Consider the diagnosis in any contact lens wearer with a painful keratitis not responding to antibacterial therapy.

• Stop lens wear and discard lenses and cases.
• Topical amoebicidal therapy (a cystocidal biguanide with or without a diamidine):
  • Polyhexamethylene biguanide (PHMB) 0.02 percent (0.06 percent used in some protocols for severe disease) or chlorhexidine 0.02 percent: 1 drop hourly day and night for 48 hours, then hourly during the day for 1 week, then every 2 hours, gradually tapering over weeks to months according to response (usually total 3 to 6 months or longer).
  • Add propamidine isethionate 0.1 percent or hexamidine 0.1 percent (diamidine) with the same schedule in severe or non-responding disease.
  • Drug toxicity (epitheliopathy, punctate keratopathy) occurs with long treatment and can mimic relapse; taper frequency rather than abruptly stopping.
• Oral analgesia for severe pain: paracetamol 1 g every 6 hours, an NSAID such as naproxen 500 mg twice daily, gabapentin, or opioids if required.
• Oral antimicrobials as adjuncts in deep or scleral disease: oral voriconazole 200 mg twice daily (some activity), and oral miltefosine 50 mg three times daily for refractory disease (off label, limited evidence).
• Corticosteroids: use only after 2 or more weeks of effective amoebicidal therapy and clinical improvement, for severe inflammation, scleritis, or immune ring, with prednisolone acetate 1 percent once or twice daily and slow taper; steroids early can worsen the disease.
• Cycloplegia: atropine 1 percent once or twice daily.
• Corneal debridement of epithelium and corneal cross linking are adjunctive options in a few centres.
• Keratoplasty: optical keratoplasty is delayed until the infection is quiescent for several months; therapeutic penetrating keratoplasty for perforation or uncontrolled infection has high recurrence and failure rates.
• Follow up: daily at the start, then weekly. Recurrence is common after stopping therapy, so continue for at least 1 to 2 months beyond clinical resolution and monitor.
• Prevention: avoid exposure of lenses to water, daily disposable lenses, and strict cleaning.