Assessment of Confocal Microscopy for the Diagnosis of Polymerase Chain Reaction-Positive Acanthamoeba Keratitis: A Case-Control Study

Abstract

Purpose: To determine in vivo confocal microscopy diagnostic criteria to diagnose Acanthamoeba keratitis (AK) using polymerase chain reaction (PCR) as the reference diagnostic technique.

Design: Retrospective case-control study. Data were recorded prospectively and analyzed retrospectively.

Participants: Fifty patients with PCR-positive AK (study group) and 50 patients with bacterial, fungal, viral, or immune keratitis featuring negative Acanthamoeba PCR results (control group).

Methods: In vivo confocal microscopy performed at the acute stage of keratitis.

Main outcome measures: Presence of in vivo confocal microscopy images suggestive of AK. Multivariate logistic regression was used to determine the relationship between types of images and presence of PCR-positive AK.

Results: The following 4 types of images were associated significantly with PCR-positive AK (P < 0.05): bright spots (round or ovoid hyperreflective objects with no double wall; diameter, <30 μm); target images (hyperreflective objects with hyporeflective halo; diameter, <30 μm); clusters of hyperreflective objects (diameter, <30 μm); and trophozoite-like objects (diameter, >30 μm). Specificity of both target and trophozoite images was 100%. This figure was 98.2% for clusters and 48.2% for bright spots. If the diagnosis of AK was made on presence of target images, clusters or trophozoite images (at least 1 of the 3 features), the positive predictive value of confocal microscopy was 87.5% and the negative predictive value was 58.5%.

Conclusions: Acanthamoeba keratitis is a serious vision-threatening disease. In vivo confocal microscopy can help in this challenging diagnosis, especially when PCR is delayed, shows negative results, or is not available. Target images and trophozoite-like images are pathognomonic of AK. Clusters of hyperreflective objects are highly specific of AK. However, the overall sensitivity of in vivo confocal microscopy features of AK is low. In addition to the clinical features, microbiological tests (direct examination and cultures of corneal scrapings), and PCR, in vivo confocal microscopy allows for more rapid diagnosis and treatment initiation, potentially leading to an improved outcome.