Acanthamoeba keratitis
Protozoal corneal infection (Acanthamoeba — water exposure in contact-lens wearers)
Overview
A protozoal corneal infection, classically in contact-lens wearers exposed to water (showering/swimming in lenses, tap-water rinsing). Causes severe pain out of proportion to signs, a ring-shaped stromal infiltrate and perineural involvement (radial keratoneuritis). Hard to diagnose and treat — vision-threatening.
Recognise
- Severe pain OUT OF PROPORTION to clinical signs, photophobia, redness, reduced vision
- Ring-shaped stromal infiltrate; early radial keratoneuritis (perineural infiltrates)
- Contact-lens wear with water exposure (swimming/showering in lenses, tap-water storage)
Red flags
- Contact-lens wearer with disproportionate pain and a poor response to standard antibacterials → suspect Acanthamoeba — urgent specialist
Differentials & how to tell them apart
Investigations
Corneal scrape/confocal microscopy (cysts), culture on non-nutrient agar with E. coli overlay, PCR.
Management
Urgent specialist referral + topical anti-amoebic (PHMB ± propamidine)
- 1Urgent ophthalmology. Corneal scrape/confocal microscopy. Intensive topical biguanide (PHMB) ± diamidine for a prolonged course.Gate: Suspect it in any lens wearer with pain disproportionate to signs not responding to antibacterials — do not just escalate antibiotics
- 2Prolonged anti-amoebic therapy; corneal grafting for scarring/perforation. Prevent: no water exposure in lenses, proper hygiene.
Key points
Pain out of proportion + contact lenses + water = Acanthamoeba. It is the reason 'don't shower or swim in your lenses' is taught.
Monitor & prognosis
Prolonged review; relapse common.
Guarded; often protracted with visual loss if delayed.
Source: College of Optometrists CMG; RCOphth