In vivo confocal microscopy ineffective for Fusarium and Aspergillus differentiation
It’s very hard to tell the difference between Fusarium and Aspergillus species with in vivo confocal microscopy (IVCM) of fungal keratitis, researchers say.
However, IVCM remains an effective tool to detect fungal filaments, according to Jaya Devi Chidambaram, London School of Hygiene and Tropical Medicine, London, and colleagues. They published their findings in the British Journal of Ophthalmology.
The incidence of fungal keratitis is increasing throughout the world, with filamentous fungi to blame for more than 60% of the resultant ulcers in warm, humid climates such as India, they reported.
The outcomes vary between Fusarium and Aspergillus, the two primary species of these fungi. Compared with Fusarium, which is more vulnerable to natamycin, Aspergillus is associated with slower re-epthelialisation, increased risk of perforation, and worse visual acuity at 3 months after presentation.
The two fungi also have different shapes. For example, in one previous study, donor corneas infected in vitro with A. fumigatus and Fusarium solani, Fusarium filaments were reported to have a hyphal branching angle of 90° in IVCM images from patients and from the infected donor cornea. By contrast, in A. fumigatus, the branching angle in the infected donor cornea was measured as 45°.
In Aspergillus, but not Fusarium, the apical filament can directly bifurcate instead of generating side branches, a shape known as dichotomous branching. Fusarium can develop spores in the infected tissue.
Chidambaram and colleagues examined 106 patients culture positive for Fusarium or Aspergillus keratitis to see whether they could distinguish between them using IVCM.
They excluded 8 patients from the IVCM analysis (5 Fusarium, 2 Aspergillus flavus, 1 A. fumigatus) due to the absence of any measurable branching hyphae in the IVCM images. In the remaining 98 participants, 68 were culture-positive for Fusarium, 24 for A. flavus, 4 for A. fumigatus, and 2 for Aspergillus terreus.
They applanated the HRT3 laser scanning confocal microscope with Rostock Corneal Module (Heidelberg Engineering) onto corneas anaesthetized with 0.5% proparacaine eye drops. They manually focused and recorded a series of volume scans at the centres and margins of the ulcers.
Chidambaram, who was masked to the microbiological diagnosis, personally measured all the branching hyphae present in each IVCM image within each section of each volume scan. The researchers assessed the presence or absence of dichotomous branching in all sections.