Diagnostic features of Acanthamoeba keratitis via in vivo confocal microscopy

In vivo confocal microscopy (IVCM) offers a non-invasive, rapid method for diagnosing Acanthamoeba keratitis (AK) by detecting cysts or trophozoites in the initial clinic visit images. In this retrospective observational study, we reviewed HRT3 IVCM images from patients presenting to Manchester Royal Eye Hospital with clinically- suspected AK for IVCM morphological features (IVCM-MF) of both Acanthamoeba and corneal cells. Twenty-seven patients were included in the study: median age 29 years (range 16-71 years), female gender (59%; n = 16/27) and contact lens wear as the main risk factor. Median symptom duration before the initial ophthalmologist visit was 9 days (range 2 to 42 days). IVCM had a higher detection rate for AK in 85% of patients (n = 23/27), with culture positivity in only 74% (n = 20/27; 17 of whom were also IVCM-positive). Acanthamoeba IVCM-MF included: bright spots (87%, n = 20/23), double-walled cysts (56%, n = 13/23), signet-ring (22%, n = 5/23) and trophozoites (30%, n = 7/23). Bright spots and double-walled cysts coalesced in lines/clusters in 1 patient. Corneal epithelial cells had a "koilocyte" appearance in 64% (n = 14/22). Microtubules connecting adjacent keratocytes were visible in 52% (n = 12/23), particularly associated with A. polyphaga ulcers (p = 0.02). These IVCM features observed in corneal epithelial cells and keratocytes may represent potential imaging biomarkers for AK diagnosis and warrant further investigation to validate their diagnostic utility. By demonstrating IVCM's superior diagnostic performance, providing rapid and accurate diagnostics, this study advocates for its inclusion in standard diagnostic workflows for AK, paving the way for future advancements in clinical practice.