Acanthamoeba

• Acanthamoeba keratitis is a rare form of microbial keratitis associated with poor visual outcomes.
• Acanthamoeba are free-living protozoa commonly isolated from soil, pond, sea, tap water and chlorinated water.
• In developed countries, the vast majority of patients with acanthamoeba keratitis are contact lens wearers.
• Risk factors for acanthamoeba keratitis include:

  • Contact lens wear: especially associated with swimming with contact lenses and poor hygiene
  • Corneal trauma
• Symptoms are variable but may be severe, including reduced visual acuity, pain, redness, foreign body sensation, photophobia, epiphora
• Characteristically, pain is severe and disproportionate to relatively mild clinical findings.
• Typical clinical findings on examination may include:

  • Lids and lashes: mild swelling and erythema
  • Conjunctiva: diffuse injection
  • Cornea: classically a ring-shaped infiltrate with radial perineural infiltrates
  • Pseudodendrites (similar to HZO)
  • Reduced corneal sensation
• The diagnosis of acanthamoeba keratitis is difficult and can often be delayed. Early cases may mimic HSV keratitis.
• Relevant investigations may include:

  • Corneal scrape: samples for PCR, culture (non-nutrient agar with E. coli overlay) and histology
  • Contact lenses, cases, and solutions should be sent for culture
  • Confocal microscopy (if available): direct visualisation of acanthamoeba cysts
  • A corneal biopsy may be considered in culture-negative cases
• Management:

  • Contact lens use should be discontinued immediately. 
  • Topical antiamoebic agents - PHMB or chlorhexidine
  • Debridement
  • Oral NSAIDs for pain
  • Topical steroids
  • Penetrating keratoplasty (full-thickness corneal transplant): considered in cases with severe corneal scarring or extensive necrosis. Recurrence may occur in grafted tissue.
• Complications of acanthamoeba keratitis include:

  • Extension of disease to involve the sclera
  • Recurrent infection
  • Corneal perforation