Successful Treatment of Acanthamoeba Keratitis Using 0.08% Polyhexamethylene Biguanide Monotherapy: A Report of a Case in Latin America.

Acanthamoeba keratitis (AK) is a rare but potentially sight-threatening corneal infection, most commonly associated with contact lens use, and frequently misdiagnosed due to its nonspecific early clinical presentation. Standard treatment regimens are prolonged and often limited by ocular surface toxicity and poor patient adherence. We report the case of a 19-year-old contact lens wearer who presented with a one-month history of ocular pain, redness, photophobia, and decreased vision in the right eye, initially treated as viral keratitis without improvement. Slit-lamp examination revealed stromal infiltrates with perineural involvement, and in vivo confocal microscopy confirmed the presence of superficial Acanthamoeba cysts without fungal elements. The patient was treated using a standardized protocol with topical polyhexamethylene biguanide (PHMB) 0.08% monotherapy, resulting in rapid clinical improvement, progressive reduction of corneal infiltrates, and complete resolution of active infection. Best-corrected visual acuity improved from 20/400 to 20/25, with no evidence of recurrence during seven months of follow-up. This case supports the potential role of PHMB 0.08% monotherapy as an effective and well-tolerated therapeutic option for AK.