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Acanthamoeba keratitis

URGENT — Cornea

Symptoms

Severe ocular pain — characteristically out of proportion to clinical findings (hallmark).

Foreign body sensation, redness, and photophobia.

Chronic course: symptoms often persist for several weeks before correct diagnosis.

Clinical Findings

Early (epithelial)

  • Epitheliitis with pseudodendrites and epithelial ridges.
  • Whorls and diffuse subepithelial microcysts.

Intermediate

  • Radial keratoneuritis: Perineural inflammation along corneal nerves (earliest pathognomonic finding).
  • Subepithelial infiltrates.

Late (3–8 weeks)

  • Ring-shaped stromal infiltrate (Wessely ring): Pathognomonic in advanced cases.
  • Corneal vascularization and potential perforation.

Anterior segment

  • Eyelid swelling and circumcorneal conjunctival injection.
  • Anterior chamber cell and flare with minimal discharge.

Etiology

Causative agent

  • Free-living protozoan (Acanthamoeba spp.) found in tap water, soil, and swimming pools.
  • Exists as trophozoites and highly resistant cysts (resistant to most topical agents).

Risk factors

  • Contact lens wear (dominant factor): Especially with tap-water rinsing or swimming in lenses.
  • Non-contact lens cases: Ocular trauma involving soil or water exposure.

Red Flags

Contact lens wearer + severe pain out of proportion to exam findings.

Ring infiltrate, pseudodendrites, or perineuritis in a non-responsive keratitis.

Misdiagnosis as HSV: Steroid use without amoebicidal coverage accelerates damage.

Diagnosis delay >2–3 weeks significantly worsens visual prognosis.

Investigations

Smears and staining

  • Corneal scrapings for Gram, Giemsa, PAS, and Calcofluor white stains.

Culture

  • Non-nutrient agar with E. coli overlay.
  • Culture of contact lenses and storage cases.

Imaging and biopsy

  • Confocal microscopy: May show double-walled cysts in vivo.
  • Corneal biopsy: Indicated if clinical suspicion is high but cultures are negative.

Treatment

Topical biguanides (first-line)

  • Polyhexamethylene biguanide (PHMB) 0.02% q1h.
  • Chlorhexidine 0.02% q1h.

Topical diamidines

  • Propamidine isethionate 0.1% drops q1h.
  • Dibromopropamidine isethionate 0.15% ointment.

Systemic therapy

  • Oral Antifungals: Itraconazole (400mg load, then 100-200mg daily), Voriconazole, or Ketoconazole.
  • Oral Miltefosine: 50mg BID/TID for 4 weeks (Monitor BMP, LFTs, and pregnancy status).

Adjunctive & surgical

  • Avoid topical steroids until confirmed amoebicidal kill.
  • Penetrating keratoplasty (PKP) for medical failure or perforation.

Clinical Pearl

Severe pain + ring infiltrate + CL wearer exposed to water = Acanthamoeba. Often misdiagnosed as HSV due to pseudodendrites; radial keratoneuritis is the earliest pathognomonic finding.

Differential Diagnosis

  • Herpes simplex keratouveitis (most common misdiagnosis)
  • Bacterial keratitis
  • Fungal keratitis
  • Contact lens-related sterile infiltrate

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