Presentation

Contact lens wearer presenting with pain that is often out of proportion to early clinical findings, photophobia, redness, foreign-body sensation, and blurred vision.
History often includes water exposure while wearing lenses — swimming, showering, or rinsing lenses/case with tap water.
Frequently misdiagnosed early as herpes simplex keratitis because early epithelial changes can look dendritiform.

Findings

★ Signature Finding

Stromal ring-shaped infiltrate (a late finding). Mechanism: immune-mediated inflammatory response to amoebic antigens, often centred on the perineural/stromal tissue.

Radial perineural (perineuritis) infiltrates — inflammation tracking along corneal nerves — an earlier, relatively specific sign.
Early epithelial pseudodendrites can mimic HSV keratitis.
Late disease: dense ring infiltrate, stromal haze/scarring; can progress to scleritis in severe or delayed cases.

Etiology & Mechanism

Acanthamoeba is a ubiquitous free-living protozoan found in soil and water, existing in an active trophozoite form and a dormant, biocide-resistant cyst form.
Pathogenesis requires corneal trauma/epithelial defect plus exposure to the organism (commonly via contaminated water or lens solution).
Risk factors: contact lens wear with poor hygiene or water exposure; in non-lens wearers, corneal trauma followed by exposure to contaminated water/soil.

Red Flags — Do Not Miss

Critical — Do Not Miss

Pain disproportionate to signs in a contact lens wearer with water exposure history — do not anchor on a bacterial or herpetic diagnosis.
Radial perineural infiltrates or a ring infiltrate — treat empirically for Acanthamoeba rather than waiting for confirmatory culture, since delayed anti-amoebic therapy worsens outcomes.
Poor response to standard antibacterial or antiviral therapy in a red, painful eye with a contact lens history.

Investigations

In-vivo confocal microscopy to visualise cysts — rapid, non-invasive.
Corneal scraping with KOH + calcofluor white staining to visualise double-walled cysts.
Culture on non-nutrient agar overlaid with E. coli (Page's/Neff's agar).
PCR of corneal scrapings, increasingly used for confirmation.
Corneal biopsy if scraping is non-diagnostic but clinical suspicion remains high.

Management

Start empirical anti-amoebic therapy on clinical suspicion — do not wait for culture confirmation given the time-sensitivity of the disease.
Epithelial debridement to reduce organism load and improve drug penetration.
Discontinue contact lens wear.

Treatment

First line: a cationic antiseptic biguanide — PHMB 0.02% or chlorhexidine 0.02% — often combined with a diamidine (propamidine 0.1% or hexamidine), dosed intensively (e.g., hourly) initially, then tapered over weeks to months.
No medication is formally approved for this indication in most countries; treatment is off-label.
Topical corticosteroids are controversial: may reduce stromal inflammatory scarring later in the course but risk worsening amoebic proliferation if started too early.
Refractory disease: therapeutic penetrating keratoplasty deferred until medical cure achieved; adjunctive photodynamic therapy with Rose Bengal has shown encouraging results in a small pilot series.

Follow-up

Frequent early follow-up (days) to judge treatment response.
Total treatment courses are often prolonged — months, not weeks — given cyst resistance; taper slowly to reduce recurrence risk.
Monitor for scleritis and secondary glaucoma throughout treatment.

Clinical Pearl

💡 Clinical Pearl

Because early Acanthamoeba keratitis can look like HSV keratitis (pseudodendrites), persistent pain and lack of response to antivirals in a contact lens wearer should trigger a dedicated anti-amoebic workup rather than a longer antiviral trial.

Differential Diagnoses

Condition	Key Distinguishing Point
HSV keratitis	True dendrites with terminal bulbs and reduced corneal sensation; Acanthamoeba often has preserved/increased sensation early with radial perineural infiltrates.
Bacterial keratitis	More rapid onset, purulent discharge, responds to standard antibiotics.
Fungal keratitis	Feathery infiltrate margins, satellite lesions, history of vegetable-matter trauma.
Microsporidial keratoconjunctivitis	Occurs in immunocompromised hosts, diffuse punctate epitheliopathy.
Herpes zoster keratitis	Dermatomal vesicular rash, history of shingles.

Pupilla

Acanthamoeba Keratitis