A 27-year-old soft contact-lens wearer presents with severe right-eye pain for three weeks, out of proportion to the findings. He was treated for herpes simplex keratitis elsewhere without improvement. On direct questioning he admits swimming in his lenses and rinsing them in tap water.
Examination Findings
Right eye: a ring-shaped stromal infiltrate (ring abscess pattern) with overlying epithelial irregularity, marked conjunctival injection, and radial perineuritis visible on close slit-lamp inspection. Corneal sensation is reduced. Vision 6/36.
Investigations
In vivo confocal microscopy shows highly reflective round double-walled structures consistent with Acanthamoeba cysts. Corneal scrapings and cultures are sent; PCR is requested where available. Herpes PCR from a previous swab was negative.
Questions to Think About
- What is the classic clinical triad that should trigger suspicion of Acanthamoeba?
- How does confocal microscopy help, and what is the specific treatment?
Diagnosis
Acanthamoeba keratitis (ring infiltrate, confocal-confirmed cysts).
Reasoning
Suspect Acanthamoeba when three things coincide: contact-lens wear with water exposure (swimming, tap water, hot tubs), severe pain out of proportion to the clinical signs, and a ring infiltrate or radial perineuritis — with a history of failed ‘herpes’ treatment as the common backstory. In vivo confocal microscopy visualises cysts in the living cornea and gives a rapid, non-invasive diagnosis while cultures incubate. Typical management: stop all lens wear permanently in the affected eye (and counsel on the fellow eye), start biguanide antiseptics (chlorhexidine or PHMB, often combined with a diamidine) intensively per local protocol, add cycloplegia and pain control, and avoid steroids early as they worsen amoebic disease. Warn that treatment lasts months, pain may temporarily worsen as cysts die, and advanced scarring may later need keratoplasty — performed only when the infection is eradicated. Diagnosis: Acanthamoeba keratitis with ring infiltrate.
Differential Diagnosis
- Acanthamoeba keratitis — confirmed: lens wear with water exposure, pain out of proportion, ring infiltrate with perineuritis, cysts on confocal.
- Herpes simplex keratitis — ruled out: no dendrite, no response to antivirals, negative PCR; misdiagnosis is common and delays treatment.
- Fungal keratitis — considered: also indolent, but the ring pattern, perineuritis and confocal cysts point to Acanthamoeba.
Management
Urgent corneal referral. Start intensive topical biguanide therapy (chlorhexidine or PHMB, often with a diamidine) per local protocol; continue for months with slow taper guided by a corneal specialist. No topical steroids in the active phase. Cycloplegia and systemic analgesia for pain. Stop contact-lens wear; discard lenses, case and solutions. Monitor for stromal melting and secondary glaucoma. Keratoplasty only after confirmed eradication if scarring limits vision.
Key Learning Points
- The Acanthamoeba triad — lens plus water exposure, pain beyond the signs, ring infiltrate/perineuritis — should be asked about actively, because the commonest error is weeks of mistreatment as herpes.
- Confocal microscopy finds cysts in vivo for a rapid diagnosis; treatment is prolonged biguanide antiseptics (not antibiotics, not steroids), and lens hygiene counselling protects the fellow eye.
Red Flags
- Pain out of proportion in a lens wearer — think Acanthamoeba before herpes; ask about water exposure directly.
- Steroids started for ‘herpes’ without a dendrite — stop and reconsider; steroids worsen amoebic keratitis.
- Deep stromal disease unresponsive to medical therapy — early surgical discussion, but grafting active infection risks recurrence.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.
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