A contact lens wearer presents with a painful, rapidly progressive corneal ulcer with a 'ring infiltrate'. Confocal microscopy shows hyperreflective round bodies with double-walled cysts at 4-5 μm. What organism is most likely, and what is the first-line treatment?

• A Candida albicans; treated with topical natamycin
• B Pseudomonas aeruginosa; treated with topical ciprofloxacin
• C Herpes simplex virus; treated with topical acyclovir
• D Acanthamoeba species; treated with topical polyhexamethylene biguanide (PHMB) + chlorhexidine ✓

Explanation

Acanthamoeba keratitis is strongly associated with contact lens wear (especially with non-sterile water/tap water rinsing). The ring infiltrate (immune ring of Wessley) and confocal microscopy findings of double-walled cysts (4-5 μm) are pathognomonic. First-line treatment is dual anti-amoebic therapy: topical polyhexamethylene biguanide (PHMB) 0.02% combined with chlorhexidine 0.02%, administered hourly for the first 48 hours. Prognosis is poor if treatment is delayed, as cysts are resistant to many antimicrobials.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.