A contact lens wearer presents with severe pain, photophobia, and a ring-shaped corneal infiltrate. Corneal scraping stained with calcofluor white reveals double-walled cysts with polygonal morphology. Which organism is responsible and what is the first-line treatment?

• A Fusarium — treat with natamycin 5% eye drops
• B Acanthamoeba — treat with polyhexamethylene biguanide (PHMB) and propamidine isethionate ✓
• C Pseudomonas aeruginosa — treat with topical fluoroquinolone
• D Herpes simplex virus — treat with topical acyclovir

Explanation

The ring infiltrate (immune ring of Wessely) and the history of contact lens wear with calcofluor white showing double-walled polygonal cysts (Acanthamoeba trophozoites and cysts) are pathognomonic of Acanthamoeba keratitis. First-line treatment is a combination of biguanides (PHMB or chlorhexidine 0.02%) and diamidines (propamidine isethionate/Brolene) for prolonged duration (6–12 months minimum) as cysts are notoriously resistant. Natamycin targets fungi, not protozoa. Pseudomonal ulcers have a rapidly progressive course with hypopyon but not ring infiltrate or the cyst morphology. HSV keratitis produces dendritic ulcers, not ring infiltrates.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.