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Acanthamoeba Keratitis

Evidence-based assessment and management of parasitic keratitis caused by free-living amoeba. Comprehensive guide covering etiology, pathogenesis, classification, diagnosis, and treatment protocols for optometry practice, including Singapore regulatory scope.

Ring InfiltrateCentral UlcerationPerineuralinfiltrates

Acanthamoeba Keratitis: Ring infiltrate with central ulceration, radial perineural infiltrates, and stromal infiltrates (pathognomonic appearance)

The Organism

Acanthamoeba spp. are free-living protozoa in the phylum Amoebozoa. Over 24 species have been described, classified into 20 sequence types (T1–T20) based on the 18S rRNA gene. Genotype T4 accounts for approximately 70–80% of all AK cases worldwide and is considered the primary pathogen. Genotypes T3, T6, and T11 are less frequently implicated. The two most clinically relevant species are A. castellanii (genotype T4) and A. polyphaga.

Environmental Reservoirs

Acanthamoeba is ubiquitous in nature, thriving in a wide range of environments:

  • Tap water, including domestic water supply pipelines and domestic water tanks
  • Swimming pools, hot tubs, and Jacuzzis (survives despite chlorination)
  • Natural freshwater bodies (lakes, rivers, ponds)
  • Soil and dust particles
  • Heating, ventilation and air conditioning (HVAC) systems
  • Contact lens cases and lens case biofilm — a particularly high-risk nidus
  • Bottled mineral water (in some studies)

Contact Lens-Associated Transmission

Contact lens wear is the dominant risk factor in developed countries, accounting for approximately 85–95% of cases. The contact lens case is the primary vehicle of contamination: biofilm within the case promotes cyst survival even in the presence of lens care solutions. Contaminated multipurpose solutions, use of non-sterile water for rinsing, and failure to replace lens cases are critical transmission routes. Amoebae adhere to the lens surface and are transferred directly onto the corneal epithelium.

Non-Contact Lens Cases

A minority of cases (~5–15%) occur in non-contact lens wearers. These typically involve direct corneal trauma with contaminated water or soil — agricultural injuries, water sports accidents, or exposure during natural disasters. These cases are more common in developing countries and may involve genotypes other than T4.

Organism Life Cycle

Acanthamoeba exists in two forms. The trophozoite (active form, 15–45 µm) is the feeding, replicating, and tissue-invasive form. Under adverse conditions — nutrient deprivation, desiccation, temperature extremes, chemical stress, or antimicrobial exposure — trophozoites convert to cysts (10–25 µm). Cysts possess a double-layered wall (outer ectocyst and inner endocyst) conferring resistance to chlorine, UV irradiation, desiccation, and most disinfectants. Encystation is the primary reason treatment failure and recurrence occur.

AK develops through a multi-step pathogenic cascade that exploits pre-existing epithelial compromise and evades host immune defences:

  1. Corneal surface adhesion: Trophozoites adhere to corneal epithelial cells via surface lectins. The mannose-binding protein (MBP) on the amoeba surface binds to mannose-containing glycoproteins on corneal epithelial cell surfaces. This lectin-receptor interaction is a key virulence determinant and the basis of the mannose-induced cytopathic effect (MICE). Contact lens wear upregulates corneal epithelial cell mannose-containing glycoproteins, partially explaining the strong association with lens wear.
  2. Epithelial invasion: Trophozoites disrupt epithelial tight junctions through mechanical force (pseudopod projection) and enzymatic cleavage of cell adhesion molecules. Disruption of corneal epithelium by contact lenses — through hypoxia, microtrauma, and surface desiccation — significantly facilitates invasion.
  3. Cytotoxic protease activity: Amoebae secrete a range of extracellular enzymes including serine proteases, metalloproteases (matrix metalloproteinase-like), phospholipases, and neuraminidases. These degrade corneal extracellular matrix components (collagen, fibronectin, laminin), driving stromal liquefaction and progressive tissue destruction.
  4. Neurotropism and perineural invasion: A hallmark feature of AK is the disproportionate severity of pain relative to apparent clinical signs, particularly early in the disease. This is explained by direct amoebic invasion along corneal nerve fibres (perineural invasion), causing intense neuropathic pain and accounting for the characteristic perineural infiltrate visible on slit lamp. In-vivo confocal microscopy (IVCM) can directly visualise organisms tracking along sub-basal nerve plexus fibres.
  5. Stromal invasion and liquefactive necrosis: Progressive penetration through Bowman layer into the anterior and mid-stroma causes stromal oedema, necrosis, and the development of a characteristic annular (ring) infiltrate — representing a zone of amoebic migration and host inflammatory response around a central area of necrosis.
  6. Host inflammatory response: Massive neutrophil and macrophage infiltration with upregulation of pro-inflammatory cytokines (TNF-α, IL-6, IL-8, IL-1β). Paradoxically, this robust inflammatory response contributes to tissue destruction and scarring while being partially ineffective against the organism. Macrophages can phagocytose trophozoites but not cysts.
  7. Cyst formation and treatment resistance: Upon exposure to antimicrobial agents, nutrient deprivation, or immune pressure, trophozoites encyst. The double-walled cyst is resistant to most topical antimicrobials including standard antiseptics and many anti-amoebic agents. This is the predominant mechanism of treatment failure and recurrence. Cysts remain viable within the corneal stroma for months to years.
  8. Chronic scarring and neovascularisation: Persistent inflammation and repeated cycles of active infection and partial treatment response lead to corneal vascularisation, lipid deposition, and irreversible stromal scarring.

By Clinical Stage (Dart et al. / Modified Clinical Staging)

The most widely used clinical staging system reflects disease depth and severity:

Stage 1 — Epithelial / Early

  • Superficial punctate epitheliopathy, epithelial irregularity
  • Pseudodendrites (may mimic HSV keratitis)
  • Subepithelial infiltrates, minimal stromal involvement
  • Perineural infiltrates may be present even at this stage
  • Disproportionate pain is a key early clue
  • Best prognosis if diagnosed here

Stage 2 — Anterior Stromal

  • Ring infiltrate (annular stromal infiltrate) — most characteristic sign
  • Stromal oedema and anterior chamber reaction
  • Anterior uveitis with cells and flare
  • Significant photophobia and pain

Stage 3 — Deep Stromal / Complicated

  • Deep stromal involvement, neovascularisation
  • Corneal melting or ulceration
  • Endotheliitis and keratic precipitates
  • Iris involvement (nodules, necrosis) in severe cases
  • Secondary bacterial or fungal superinfection

Stage 4 — Perforating / End-Stage

  • Corneal perforation or imminent perforation
  • Scleritis (rare but severe extension)
  • Severe anterior chamber involvement
  • Typically requires surgical intervention

By Anatomical Depth of Involvement

  • Epithelial/subepithelial: Limited to epithelium and Bowman layer; best treatment response
  • Anterior stromal: Anterior third of stroma; ring infiltrate typically present
  • Mid-stromal: Bowman to mid-stroma; moderate prognosis
  • Deep stromal / endothelial: Descemet membrane involvement; poor prognosis; keratoplasty often required
Clinical note: Unlike bacterial keratitis, AK often presents at Stage 1–2 with a delay of weeks to months from symptom onset to diagnosis. This delay — commonly due to initial misdiagnosis as herpetic or bacterial keratitis — is the primary driver of poor outcomes. A high index of suspicion in any contact lens wearer with keratitis poorly responsive to antibiotics or antivirals is essential.

Contact Lens Wear — Primary Risk Factor

Contact lens wear accounts for 85–95% of AK cases in developed countries. Estimated incidence in contact lens wearers: 1–33 per million per year, with higher rates in countries with high soft lens penetration. Specific behaviours significantly amplify risk:

  • Rinsing lenses or lens cases with tap water
  • Showering, bathing, or swimming while wearing lenses
  • “Topping off” lens solution (adding fresh solution to old, unconsumed solution)
  • Infrequent lens case replacement (should replace every 1–3 months)
  • Failure to rub and rinse lenses before storage
  • Extended or overnight wear
  • Use of homemade saline solutions
  • Sleeping in daily disposable lenses
  • Poor hand hygiene before lens handling

Water Exposure

Any contact between contaminated water and the ocular surface represents a risk. Hot tubs and swimming pools pose the highest risk even with chlorination, as Acanthamoeba cysts are resistant to standard chlorine concentrations used in recreational water. Domestic shower water (particularly from infrequently used or poorly maintained shower heads) is an underappreciated risk source.

Corneal Surface Compromise

  • Contact lens-induced epithelial micro-trauma
  • Pre-existing corneal abrasion or epithelial defect
  • Dry eye syndrome (reduced tear film protection)
  • Previous corneal surgery (LASIK, PRK flap interface)
  • Ocular surface disease (blepharitis, recurrent erosion syndrome)

Immunosuppression

  • HIV/AIDS (CD4 count <200 cells/μL significantly increases severity)
  • Solid organ or bone marrow transplant recipients
  • Systemic corticosteroids or immunosuppressive therapy
  • Diabetes mellitus (impaired neutrophil function)

Geographic and Environmental Factors

Warm, humid climates promote Acanthamoeba proliferation in water systems. Countries in tropical and subtropical regions — including Singapore — have environmental conditions conducive to higher environmental amoeba loads. Singapore's high contact lens penetration rate combined with its tropical climate creates a relevant background risk for the local population.

Early Epithelial Signs (Stage 1)

  • Punctate epithelial erosions (PEE): Diffuse or focal, often mistaken for contact lens-induced keratopathy
  • Pseudodendrites: Branching epithelial lesions mimicking herpetic dendrites but lacking terminal bulbs; a frequent source of misdiagnosis
  • Epithelial microcysts: Small intraepithelial vesicles in some cases
  • Subepithelial haze: Anterior stromal opacity beneath early epithelial lesions

Pathognomonic and Characteristic Findings

  • Ring infiltrate (Wessley immune ring): Annular or partial arc of stromal infiltrate — the single most characteristic sign of AK; represents the leading edge of amoebic invasion combined with host immune response
  • Perineural infiltrates (radial keratoneuritis): Linear or branching infiltrates tracking along corneal nerve fibres in the anterior stroma; highly characteristic and associated with severe neuropathic pain; visible as radial white lines on slit lamp
  • Multifocal stromal infiltrates: Satellite lesions scattered in stroma around the central ring

Anterior Segment Findings

  • Central or paracentral corneal ulceration with excavated epithelial defect
  • Moderate to severe diffuse conjunctival injection and chemosis
  • Anterior uveitis: cells and flare, fibrin strands
  • Keratic precipitates (fine, stellate) on the endothelium
  • Corneal oedema and Descemet folds
  • Corneal neovascularisation (superficial and deep in chronic stages)
  • Hypopyon (in severe cases with associated anterior chamber reaction)
  • Iris nodules or sectoral iris necrosis (advanced disease)
  • Scleritis: a rare but severe manifestation indicating extension beyond the cornea

In-Vivo Confocal Microscopy (IVCM) Findings

IVCM allows real-time, non-invasive visualisation of the cornea at the cellular level and is highly valuable for AK diagnosis. Characteristic findings include:

  • Highly refractile, double-walled cysts (15–25 µm diameter) — virtually diagnostic of AK
  • Hyperreflective trophozoites (25–40 µm) with characteristic stellate morphology
  • Organisms tracking along sub-basal nerve plexus fibres
  • Reduced nerve fibre density in affected areas
  • Inflammatory cell infiltrates (hyperreflective round cells) within stroma

Key diagnostic clue: Severe ocular pain disproportionate to the degree of clinical signs on slit lamp examination is the most characteristic symptom of AK. This pain — often described as burning, stabbing, or excruciating — results from perineural invasion of corneal nerve fibres (radial keratoneuritis) and may precede visible ring infiltrate formation.

  • Severe, progressive ocular pain — often out of proportion to clinical findings; neuropathic in character; not relieved by conventional analgesic eye drops
  • Foreign body sensation — intense, persistent
  • Marked photophobia — often incapacitating; may precede major corneal signs
  • Excessive lacrimation
  • Redness and conjunctival irritation
  • Gradual, progressive visual blurring — correlating with increasing corneal opacity and irregular astigmatism
  • Insidious onset — typically weeks to months after initial exposure; frequently misdiagnosed as bacterial or herpetic keratitis in early stages
  • Poor response to conventional antibiotics or antivirals — a critical red flag prompting suspicion of AK
  • Symptom recurrence — apparent initial improvement followed by relapse is common due to cyst re-activation
  • Blepharospasm — in severe cases
  • Contact lens intolerance — patient may report sudden inability to tolerate previously comfortable lenses
Temporal pattern: The interval between initial exposure and symptom onset is typically 1–3 weeks (range: days to months). Given that many patients initially present to general practitioners or optometrists and are treated empirically for bacterial or herpetic keratitis, the average time from symptom onset to confirmed AK diagnosis has historically been 3–8 weeks — by which point significant stromal disease may be established.

Ocular Complications

  • Corneal scarring: Diffuse or focal stromal opacity, frequently with irregular astigmatism; may be visually significant even after infection resolution
  • Corneal perforation: In untreated or severely delayed cases; requires emergency surgical intervention (tissue adhesive, therapeutic keratoplasty)
  • Secondary bacterial or fungal superinfection: Epithelial defects provide an entry point; complicates microbiological diagnosis and management
  • Anterior uveitis with posterior synechiae: Chronic inflammation may lead to synechiae formation and pupil irregularity
  • Secondary glaucoma: From trabecular obstruction by inflammatory debris or steroid-induced elevation during treatment
  • Corneal neovascularisation: Deep stromal vessels carry poor prognosis for corneal graft survival
  • Persistent corneal neuropathic pain: May persist for months after microbiological cure; managed with neuropathic analgesics and nerve growth factor agents
  • Scleritis: Rare extension of amoebic infection into sclera; characterised by deep boring pain and scleral injection; poor prognosis
  • Permanent vision loss or blindness: In 10–15% of cases despite treatment; most common in delayed diagnosis cases
  • Recurrent erosion syndrome: From disrupted epithelial adhesion complexes

Systemic and Surgical Complications

  • Disseminated acanthamoebiasis: In immunocompromised patients; amoebae may spread to CNS, lungs, and skin — a life-threatening condition with high mortality
  • Granulomatous amoebic encephalitis (GAE): Rare but fatal CNS infection in immunosuppressed individuals
  • Corneal graft failure: High-risk for primary graft failure when keratoplasty is performed during active infection; recurrence within the graft has been reported

Immunocompromised Patients

In immunocompetent individuals, AK is largely confined to the cornea. However, in immunocompromised patients — particularly those with advanced HIV/AIDS (CD4 <200 cells/μL), solid organ transplant recipients on calcineurin inhibitors, or patients receiving high-dose systemic corticosteroids — Acanthamoeba may disseminate haematogenously to cause granulomatous amoebic encephalitis (GAE) and cutaneous acanthamoebiasis. GAE has mortality exceeding 95% if untreated. Ocular involvement in GAE is distinct from AK and may present as choroidal infiltrates or retinal lesions.

Associated Systemic Conditions Predisposing to AK

  • HIV/AIDS: Increased susceptibility; more aggressive corneal disease; systemic dissemination risk
  • Diabetes mellitus: Impaired neutrophil chemotaxis and corneal epithelial healing; delayed diagnosis and treatment response
  • Ocular cicatricial pemphigoid (OCP) and Steven-Johnson syndrome: Severe ocular surface disease with compromised epithelial barrier
  • Sjögren syndrome / severe dry eye: Reduced tear film antimicrobial proteins (lactoferrin, lysozyme, defensins)
  • Atopic keratoconjunctivitis: Chronic ocular surface inflammation with epithelial compromise
  • Systemic autoimmune disease: Patients on chronic immunosuppressive therapy

Relation to Contact Lens-Associated Conditions

AK should always be considered in the differential diagnosis of any contact lens-associated keratitis that does not respond to standard antimicrobial therapy within 48–72 hours, or where the clinical picture is unusually painful relative to the visible signs. Co-infection with other organisms (particularly Pseudomonas aeruginosa) has been reported and complicates the clinical picture.

Clinical Diagnostic Approach

A high index of clinical suspicion is the most important diagnostic factor, as laboratory confirmation often lags clinical presentation by days to weeks. The following clinical features should prompt immediate consideration of AK:

  • Contact lens wearer with keratitis refractory to standard topical antibiotics or antivirals
  • Severe, disproportionate ocular pain relative to clinical signs
  • History of water exposure while wearing contact lenses
  • Ring infiltrate or perineural (radial) infiltrates on slit lamp
  • Pseudodendrites without classic herpetic features (terminal bulbs, reduced corneal sensation)
  • Gradual worsening despite treatment change

Corneal Scraping — Diagnostic Workflow

Corneal scraping under slit lamp guidance is the cornerstone of laboratory diagnosis and should be performed by an ophthalmologist:

  1. Multiple scrapes from the leading edge of infiltrate using a sterile Kimura spatula or hockey-stick blade
  2. Specimens spread on glass slides for immediate microscopy and separate specimens inoculated onto culture media
  3. Scraping before initiating anti-amoebic therapy whenever possible
  4. Contact lens, lens case, and lens solution should also be submitted for culture

Laboratory Tests

Calcofluor White Staining

Fluorescent stain binding to cellulose-like cyst wall. Rapid, high sensitivity for cysts (~90%). Requires fluorescence microscope. Preferred first-line stain for fresh specimens.

Wet Mount Microscopy

Direct visualisation of motile trophozoites in saline suspension. Immediate result but requires fresh, viable specimen. Trophozoites identifiable by acanthopodia (spine-like pseudopods) and peripheral nucleus.

Giemsa / Wright-Giemsa Staining

Stains both trophozoites (20–40 µm, eccentric nucleolus) and cysts (10–25 µm, double-walled). Available in most laboratories. Trophozoites appear purple-blue.

Culture — Non-Nutrient Agar (NNA) with E. coli

Gold standard for viable organism recovery. Non-nutrient agar seeded with heat-killed E. coli or Enterobacter aerogenes as food source. Incubated at 30°C and 37°C. Plates examined daily for 4 weeks. Sensitivity ~80–90% with adequate specimen. Also allows genotyping.

PCR — Polymerase Chain Reaction

Highest sensitivity and specificity; detects Acanthamoeba DNA from small specimen volumes. Results available in 24–48 hours. Can identify specific genotype. Not affected by prior anti-amoebic treatment (unlike culture). Increasingly available as first-line test in specialist centres. Sensitivity: 93–100%; Specificity: 95–100%.

In-Vivo Confocal Microscopy (IVCM)

Non-invasive, real-time corneal imaging at the cellular level. Visualises highly refractile double-walled cysts (15–25 µm) and hyperreflective trophozoites. Also demonstrates perineural invasion and reduced sub-basal nerve density. Sensitivity 80–90%; Specificity ~80%. Valuable for monitoring treatment response without repeated scraping. May show cysts persisting at depth even after epithelial healing.

Anterior Segment OCT (AS-OCT)

Adjunctive imaging; not diagnostic for AK specifically but useful for documenting depth of stromal infiltration, corneal thickness mapping, epithelial defect extent, and monitoring treatment response. Valuable for surgical planning if keratoplasty is considered.

Immunofluorescence

Monoclonal antibodies against Acanthamoeba surface antigens; allows species identification. Available in reference laboratories.

Contact Lens Equipment Evaluation

  • Culture of contact lens surfaces (remove lens with sterile forceps into transport medium)
  • Culture of lens case (swab both compartments into NNA broth)
  • Culture or PCR of lens care solution (if available)
  • Species and genotype identification for epidemiological tracing

Singapore Optometry Scope Note: Acanthamoeba keratitis is a sight-threatening corneal infection that is entirely beyond the prescribing scope of optometrists in Singapore under the Optometrists & Opticians Act (Cap. 213A). Registered optometrists in Singapore are not licensed to initiate or prescribe any therapeutic pharmacological agent, including anti-amoebic topical agents (PHMB, chlorhexidine, propamidine isethionate), topical antibiotics, antifungals, or steroids. All pharmacological treatment must be initiated and managed by an ophthalmologist or medical practitioner.

The optometrist's role in Singapore is:

  1. Early recognition — identify characteristic signs (ring infiltrate, perineural infiltrates, disproportionate pain in a contact lens wearer) and maintain a high index of suspicion in any CL wearer with keratitis not responding to antibiotics
  2. Immediate cessation of contact lens wear — instruct patient to stop all lens wear immediately; discard current lenses, cases, and solutions
  3. Preservative-free lubricating drops (unscheduled/OTC products only) — for ocular surface comfort pending specialist review
  4. Urgent same-day or next-day ophthalmology referral — do not delay for culture results or empirical treatment; document referral clearly
  5. Patient education — brief the patient on the seriousness of the condition and the need for urgent specialist care

Reference: Optometrists & Opticians Act (Cap. 213A), Singapore Statutes Online; Optometrists & Opticians Board, Ministry of Health Singapore. Optometrists & Opticians Board (OOB) Scope of Practice Guidelines. Singapore: MOH; 2022.

Mandatory Urgent Ophthalmology Referral

Acanthamoeba keratitis requires urgent specialist management. Refer same-day or within 24 hours in the following situations:

  • Any contact lens wearer with keratitis and a ring infiltrate or perineural infiltrates
  • Keratitis with severe pain disproportionate to clinical signs
  • Keratitis not responding to topical antibiotics or antivirals after 48–72 hours
  • Corneal ulceration, stromal melting, or impending perforation
  • Any suspected AK in an immunocompromised patient
  • Scleritis associated with keratitis

Pharmacological Treatment (Ophthalmologist)

All pharmacological management is initiated and supervised by an ophthalmologist. Treatment is prolonged (minimum 4–6 months) and requires close monitoring for drug toxicity and treatment response.

First-Line Topical Anti-Amoebic Agents

  • Polyhexamethylene biguanide (PHMB) 0.02%: Cationic antiseptic with biocidal activity against both trophozoites and cysts; disrupts amoebic membranes. Applied hourly (day and night) initially for the first 48–72 hours, then hourly during waking hours for 2–4 weeks, with gradual taper over months. First-line agent of choice.
  • Chlorhexidine gluconate 0.02%: Alternative first-line biguanide; comparable efficacy to PHMB; applied hourly. Used when PHMB is unavailable or as an alternative in sensitivity reactions. Both PHMB and chlorhexidine are effective against cysts.
  • Propamidine isethionate 0.1% (Brolene): Diamidine compound; effective against trophozoites but less cysticidal. Applied 4–6 times daily. Generally used in combination with a biguanide rather than as monotherapy due to limited cysticidal activity.
  • Hexamidine 0.1% (Desomedine): Diamidine; similar spectrum to propamidine; more commonly used in European centres.

Combination Therapy (Standard of Care)

Combination of a biguanide (PHMB or chlorhexidine) with a diamidine (propamidine or hexamidine) is the current standard of care. Rationale: complementary mechanisms (biguanides act on cell membranes; diamidines impair DNA synthesis and mitochondrial function) improve efficacy against both trophozoite and cyst forms and reduce the risk of resistance development.

  • Standard regimen: PHMB 0.02% + Propamidine 0.1%, alternating hourly initially
  • Chlorhexidine 0.02% can replace PHMB in the combination
  • Minimum treatment duration: 4–6 months; some authorities recommend up to 12 months to ensure cyst eradication

Emerging and Adjunctive Agents

  • Miltefosine (oral and topical): Alkylphosphocholine; initially developed for leishmaniasis; shows in vitro and emerging clinical activity against Acanthamoeba including cysts; used in treatment-resistant cases. Availability is limited; obtainable via compassionate use in many countries.
  • Voriconazole 1% (topical): Triazole antifungal with anti-amoebic properties; used as adjunctive agent in refractory cases or when fungal co-infection is suspected.
  • Topical corticosteroids: Controversial; use is generally deferred until active infection is controlled (typically after minimum 3–4 weeks of anti-amoebic therapy). May be introduced cautiously to reduce inflammation-mediated stromal damage in later stages. Risk: reactivation of encysted organisms if used prematurely.
  • Corneal crosslinking (CXL): Emerging adjunctive therapy; UV-A / riboflavin crosslinking has shown in vitro and clinical anti-amoebic properties and may reinforce a weakened cornea; limited evidence currently.
  • Oral itraconazole 200–400 mg daily: Used in some protocols as adjunctive systemic agent; good corneal penetration.

Supportive Pharmacological Care (Ophthalmologist)

  • Topical cycloplegics: Atropine 1% or cyclopentolate 1% for ciliary spasm and pain relief; reduces posterior synechiae risk in uveitis
  • Topical antibiotics: Cover for secondary bacterial superinfection if epithelial defect present
  • Preservative-free lubricants: Frequent application to protect corneal surface during prolonged topical treatment (which itself has epithelial toxicity)
  • Oral analgesics: NSAIDs or neuropathic agents (pregabalin, amitriptyline) for perineural pain; topical analgesics avoided due to corneal toxicity with prolonged use
  • IOP-lowering agents: If secondary glaucoma develops

Surgical Interventions (Ophthalmologist)

  • Corneal debridement: Superficial epithelial debridement of infected tissue to improve drug penetration and remove a reservoir of organisms; often performed at initial presentation in specialist setting
  • Therapeutic penetrating keratoplasty (PK): Indicated for impending or established corneal perforation during active infection; carries significant risk of graft failure and recurrence in the graft. Elective PK for visual rehabilitation is deferred until complete microbiological cure (typically >12 months disease-free)
  • Deep anterior lamellar keratoplasty (DALK): Preferred over PK for visual rehabilitation in post-infective scarring when endothelium is intact; preserves host endothelium and reduces rejection risk
  • Phototherapeutic keratectomy (PTK): Excimer laser ablation for superficial or anterior stromal scarring following infection resolution; limited depth of treatment
  • Tissue adhesive (cyanoacrylate glue): Temporary measure for small corneal perforations or descemetocoele pending definitive surgical management
  • Amniotic membrane transplantation: As adjunct to promote epithelial healing and reduce inflammation in persistent epithelial defects

Contact Lens Management (All Practitioners)

  • Immediate permanent discontinuation of all contact lens wear until infection is fully resolved and ophthalmologist confirms it is safe to consider resumption
  • Discard all current contact lenses, lens cases, and lens care solutions — all may be contaminated
  • Submit lenses, case, and solution for culture and PCR if possible (diagnostic value)
  • Patient education: detailed counselling on correct lens hygiene practices before any future lens wear is considered
  • Future lens wear (if ever resumed): only with confirmed-sterile solutions, strict no-water-contact protocols, and regular monitoring

Prognosis for AK is critically dependent on the stage at diagnosis and time to initiation of appropriate therapy. Early-stage disease (epithelial/anterior stromal) treated promptly carries an excellent prognosis; advanced disease with deep stromal involvement or delayed diagnosis has significantly worse visual outcomes.

Visual Outcomes

  • Early diagnosis and treatment (Stage 1–2): 70–80% of patients achieve final visual acuity ≥20/40; minimal residual scarring
  • Delayed diagnosis (>4 weeks from symptom onset to treatment): Significant vision loss in 30–40%; corneal transplantation required in 10–20%
  • Average treatment duration: 6–12 months of topical anti-amoebic therapy; premature discontinuation is the most common cause of relapse
  • Recurrence: 10–30% due to cyst persistence; most within 3 months of treatment discontinuation
  • Corneal transplantation outcomes: High-risk for primary graft failure if performed during active infection; much better outcomes (>80% graft survival at 5 years) when performed after confirmed microbiological cure
  • Chronic pain: Neuropathic corneal pain may persist for months after infection resolution, even with good visual outcomes

Prognostic Factors

Favourable:

  • Early diagnosis (Stage 1, epithelial only)
  • Rapid initiation of combination anti-amoebic therapy
  • Genotype T4 (most susceptible to standard therapy)
  • Immunocompetent patient
  • Adherence to prolonged treatment regimen
  • No prior empirical steroid use

Poor prognosis:

  • Delayed diagnosis (>1 month)
  • Inappropriate empirical treatment (antibiotics/antivirals) masking progression
  • Prior topical corticosteroid use (promotes cyst formation)
  • Deep stromal or endothelial involvement
  • Scleritis
  • Immunosuppression
  • Treatment non-adherence
Singapore context: The subtropical climate and high contact lens penetration in Singapore make AK a relevant clinical concern for local practitioners. High awareness among optometrists to recognise and promptly refer suspected cases is a critical determinant of patient outcomes.

AK is frequently misdiagnosed, with a median delay from symptom onset to correct diagnosis of 3–8 weeks in retrospective series. The most important differential diagnoses to consider are:

1. Herpetic Keratitis (HSV / VZV)

  • Similarity: Pseudodendrites in AK can mimic HSV dendrites
  • Distinction: HSV dendrites have terminal bulbs; AK pseudodendrites do not. HSV has prior history of recurrence; responds to aciclovir. Reduced corneal sensation in HSV (not typically AK early). Ring infiltrate absent in HSV. HSV PCR positive.
  • Risk of misdiagnosis: Very high — topical antiviral treatment may partially suppress AK symptoms and delay diagnosis

2. Bacterial Keratitis

  • Similarity: Also contact lens-associated; causes painful red eye with infiltrate
  • Distinction: Bacterial keratitis has rapid progression (24–48 hours), suppurative infiltrate with mucopurulent discharge, responds to topical antibiotics. Ring infiltrate and perineural infiltrates are absent. Positive bacterial culture/stain.
  • Risk: AK is commonly misdiagnosed as bacterial keratitis and treated empirically with antibiotics, causing delayed diagnosis

3. Fungal Keratitis

  • Similarity: Indolent course, ring infiltrate possible, poor antibiotic response
  • Distinction: History of vegetation or soil trauma (filamentous fungi); immunosuppression (Candida). Feathery border to infiltrate; hypopyon common. KOH preparation positive. Does not cause perineural infiltrates.

4. Contact Lens-Associated Infiltrative Keratitis (CLAIK / CLPU)

  • Similarity: Contact lens wearer; corneal infiltrates
  • Distinction: Peripheral infiltrates, no ring pattern. Resolves with lens discontinuation and lubricants ± short-course steroids. Minimal pain. No perineural infiltrates. Negative cultures.

5. Thygeson Superficial Punctate Keratitis

  • Similarity: Punctate epithelial lesions; may mimic Stage 1 AK
  • Distinction: Bilateral (AK almost always unilateral); no contact lens history required; coarse grey-white elevated epithelial lesions; responds to topical steroids or aciclovir. No stromal involvement.

6. Corneal Dystrophy / Ectasia

  • Distinction: Bilateral; non-inflammatory; no contact lens water exposure history; no pain or acute red eye; no organisms on microscopy.

Diagnostic Comparison Table

FeatureAKHSVBacterialFungal
Pain severitySevere, disproportionateMild–moderateModerate–severeMild–moderate
Ring infiltrateCharacteristicAbsentAbsentPossible
Perineural infiltratesCharacteristicAbsentAbsentAbsent
Contact lens history95% CL-relatedNot requiredCommonTrauma/soil
Response to antibioticsNoneNoneGoodNone
Progression speedWeeks–monthsDays–weeksHours–daysDays–weeks
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