Mycotic keratitis: epidemiology, diagnosis and management

Abstract

Mycotic keratitis (an infection of the cornea) is an important ocular infection, especially in young male outdoor workers. There are two frequent presentations: keratitis due to filamentous fungi (Fusarium, Aspergillus, phaeohyphomycetes and Scedosporium apiospermum are frequent causes) and keratitis due to yeast-like fungi (Candida albicans and other Candida species). In the former, trauma is usually the sole predisposing factor, although previous use of corticosteroids and contact lens wear are gaining importance as risk factors; in the latter, there is usually some systemic or local (ocular) defect. The clinical presentation and clinical features may suggest a diagnosis of mycotic keratitis; increasingly, in vivo (non-invasive) imaging techniques (confocal microscopy and anterior segment optical coherence tomography) are also being used for diagnosis. However, microbiological investigations, particularly direct microscopic examination and culture of corneal scrape or biopsy material, still form the cornerstone of diagnosis. In recent years, the PCR has gained prominence as a diagnostic aid for mycotic keratitis, being used to complement microbiological methods; more importantly, this molecular method permits rapid specific identification of the aetiological agent. Although various antifungal compounds have been used for therapy, management of this condition (particularly if deep lesions occur) continues to be problematic; topical natamycin and, increasingly, voriconazole (given by various routes) are key therapeutic agents. Therapeutic surgery, such as therapeutic penetrating keratoplasty, is needed when medical therapy fails. Increased awareness of the importance of this condition is likely to spur future research initiatives.

Pathogenesis of microbial keratitis

“…They may account for more than 50% of all culture-positive microbial keratitis especially in tropical and sub-tropical countries [129,196,213]. A strong geograph ical correlation has been reported to exist between the occurrences of different types of keratomycosis.…”

“…Filamentous fungi such as Fusarium, Aspergillus, Phaeohyphomycetes, Curvularia, Paecilomyces and Scedosporium apiospermum are most commonly associated with keratitis caused by filamentous fungi [196]. However Candida albicans and other Candida species are most common keratitis-causing yeast-like fungi.…”

“…However Candida albicans and other Candida species are most common keratitis-causing yeast-like fungi. Former is reported to be due to ocular trauma which is usually the most important predisposing factor in healthy young males engaged in agricultural or other outdoor activities [53,129,196]. These fungi do not invade the intact cornea and penetration only occurs once the epithelium is abraded.…”

“…These fungi do not invade the intact cornea and penetration only occurs once the epithelium is abraded. Traumatizing agents of animal origin or vegetative matter, soil or dust particle either directly implant fungal conidia on abraded corneal epithelium for fungal invasion [53,129,196]. The use of corticosteroids, ocular surgery, ocular surface disease and contact lens wear has now also been increasingly identified as a significant risk factor.…”

“…In case of filamentous fungal keratitis, clinical manifestations include sudden onset of pain along with photophobia, discharge with reduced vision and opacity on the surface of the cornea suggestive of an ulcer [7]. It may involve any part of the cornea and show firm, sometimes dry elevated slough, hyphate lines extending into the normal cornea beyond the edge of the ulcers, multifocal granular or feathery grey-white satellite stromal infiltrates, immune ring, Descemet's fold and mild iritis [163,178,194,196]. Although each fungal keratitis exhibits these basic features, they may vary depending on the etiological agent.…”