Acanthamoeba Keratitis With Herpetic Endotheliitis: A Rare Co-infection and Diagnostic Challenge.

Acanthamoeba keratitis (AK) is a rare but potentially vision-threatening corneal infection that most commonly affects contact lens (CL) wearers and is frequently associated with exposure to water. The clinical presentation may mimic other infectious keratitides, particularly herpetic keratitis, thereby delaying diagnosis and appropriate treatment. We report a case of AK complicated by herpes simplex virus (HSV) endotheliitis, highlighting the diagnostic and therapeutic challenges of this uncommon co-infection. A 29-year-old woman presented with acute keratitis of the right eye after swimming while wearing CLs. Slit-lamp examination revealed a corneal epithelial defect with a stromal ring infiltrate suggestive of AK. Initial microbiological investigations were negative, but in vivo confocal microscopy demonstrated multiple hyperreflective cystic structures within the anterior stroma, consistent with Acanthamoeba cysts. Intensive anti-amoebic therapy with topical polyhexamethylene biguanide and chlorhexidine was initiated. Twelve days after treatment onset, the patient developed corneal edema and keratic precipitates with worsening vision, raising suspicion of viral endotheliitis. Polymerase chain reaction analysis of the aqueous humor was positive for HSV DNA, supporting the diagnosis of herpetic endotheliitis. Oral valaciclovir and topical corticosteroids were introduced while anti-amoebic therapy was continued. The patient showed gradual clinical improvement, with recovery of best-corrected visual acuity to 1.0 (20/20 Snellen) and resolution of corneal edema. At the six-month follow-up, no Acanthamoeba cysts were identified on confocal microscopy, and no recurrence was observed. This case shows the diagnostic complexity of AK, particularly when the clinical course is atypical or when co-infection occurs. It also highlights the importance of advanced diagnostic techniques, such as confocal microscopy and PCR, in guiding clinical management. Recognition of possible viral co-infection during the course of AK is essential, as appropriate adjustment of therapy can lead to favorable visual outcomes despite the potentially conflicting treatment strategies required for these two conditions.