Abstract
Purpose: :
In Vivo Confocal Microscopy, IVCM, of the cornea is a known useful tool for investigating atypical forms of keratitis such as acantamoeba keratitis (AK) and fungal keratitis (FK). A recent blinded study, however, showed limited accurracy of IVCM in detecting acantamoeba cysts. Cyst recognition is usually based on reflections from the cyst wall, which generate a ring-like structure arround the organism. However, there is no clinical consensus in the area. We studied the IVCM images from 6 patients with AK, 3 patients with FK and cultured acanthamoebas in both trophozoite and cystic forms.
Methods: :
9 patients with atypical keratitis were included. 3 patients were diagnosed with FK and 6 with AK. AK cases were verified with PCR. Patients were examined with the HRT-III RCM confocal microscope with an attached Rostock joystick (prototype). Acantamoeba sp. were cultured from two different clinical samples on agar plates with E. coli (feeder layer) and examined with light microscopy and IVCM. On one plate there were Acantamoeba cysts only and the other plate contained both Acantamoeba cysts and trophozoites. Confocal images from patients and agar plates were examined and the appearence of cysts and hyphae were noted.
Results: :
In all patients (3/3) with FK, IVCM revealed the presence of highly reflective branching structures, some more than 400 μm long and 1-10 μm wide. In 1 of 6 cases (17 %) of AK IVCM images showed a "classical" acantamoeba cyst with a surrounding ring structure. In 5/6 cases (83 %) 10-20 μm uniformly round and highly reflective cysts were seen. IVCM of agarplates showed acantamoeba cysts of 5-15 μm in size without a visible ring structure. Trophozoites were 50 μm and irregular.
Conclusions: :
IVCM has been used for years to provide an immediate diagnosis in cases of atypical keratitis. However, there is no consensus on how to recognize the organisms involved. We here show that histologically confirmed acantamoeba cysts in culture as well as acantamoeba cysts in vivo often lack a visible ring structure. This makes the organism harder to differentiate from inflammatory cells. In comparison we noticed fungal hyphae in all FK patients. This finding indicates that fungal keratitis can be diagnosed with higher sensitivity and specificity by IVCM than acantamoeba keratitis.
Keywords: cornea: clinical science • fungal disease • Acanthamoeba