A 24-year-old contact lens wearer has a severely painful red right eye for ten days, worsening despite antibiotic drops. The pain is extreme — she cannot sleep — yet the eye does not look as bad as she feels. She swims in her monthly lenses and rinses the storage case under the tap.
Examination Findings
Visual acuity: right 6/36, left 6/6. Right cornea: hazy with radial perineuritis (infiltrates tracking along corneal nerves) and an early ring-shaped stromal infiltrate; epithelium irregular. Pain is severe — she rates it far beyond what the eye looks like. She wears monthly contact lenses, swims in them, and rinses the case with tap water. No hypopyon yet.
Investigations
Slit-lamp examination documents the ring infiltrate and perineuritis. Corneal scrapings or biopsy for Acanthamoeba (culture, PCR or confocal microscopy where available) confirm the diagnosis — this organism is missed by routine bacterial stains. Contact lens hygiene history is taken in detail. In vivo confocal microscopy, where available, can visualise the cysts directly.
Questions to Think About
- What single clinical clue most strongly suggests Acanthamoeba here?
- Why have the antibiotic drops failed?
- Which everyday habit caused this infection?
Diagnosis
Acanthamoeba keratitis in a contact lens wearer: severe pain out of proportion to the signs, with perineuritis and an early ring infiltrate.
Reasoning
Reasoning: Severe pain out of proportion to signs, radial perineuritis and a ring infiltrate in a contact lens wearer with tap-water exposure and swimming in lenses is classic Acanthamoeba keratitis. Bacterial keratitis would show a denser infiltrate matching the pain and would usually respond to antibiotics; herpes lacks the ring pattern. Diagnosis: Acanthamoeba keratitis, right eye. Management: typical management includes urgent specialist referral, anti-amoebic therapy started on clinical suspicion per local protocol, pain control, and permanent cessation of the offending lens habits. She is told the tap water and swimming caused it and that treatment takes months.
Differential Diagnosis
- Bacterial keratitis — ruled out: the pain is far worse than the clinical signs suggest, and the ring infiltrate with perineuritis is atypical for bacteria.
- Herpes simplex keratitis — ruled out: no dendritic pattern; the ring infiltrate and radial perineuritis point to amoebic disease.
- Fungal keratitis — ruled out: no vegetative trauma, no feathery margins or satellite lesions; the contact lens and tap-water exposure history fits Acanthamoeba.
Management
Typical management includes urgent referral to a corneal specialist, anti-amoebic therapy (such as biguanide-based regimens per local protocol) started on clinical suspicion without waiting for culture, and aggressive pain control. Contact lens wear is stopped entirely. She is told the infection comes from water exposure with lenses, that treatment is prolonged (months), and that early intensive treatment gives the best chance of saving the cornea.
Key Learning Points
- Pain OUT OF PROPORTION to the signs is the classic clue — when the patient suffers more than the eye shows, think Acanthamoeba.
- Ring infiltrate plus radial perineuritis in a contact lens wearer is Acanthamoeba until proven otherwise.
- Tap water is the enemy: never rinse lenses or cases with tap water, never swim or shower in lenses.
- Routine bacterial cultures miss it — ask the lab specifically for Acanthamoeba testing.
- Early diagnosis saves the cornea; late diagnosis often ends in transplant.
Red Flags
- Excruciating pain with modest signs in a lens wearer — suspect Acanthamoeba; do not just change antibiotics.
- Ring infiltrate in a contact lens wearer — urgent specialist referral; this organism blinds slowly and surely.
- Tap-water lens hygiene — stop it today in every lens wearer you counsel.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.
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