Current perspectives on ophthalmic mycoses

Abstract

Fungi may infect the cornea, orbit and other ocular structures. Species of Fusarium, Aspergillus, Candida, dematiaceous fungi, and Scedosporium predominate. Diagnosis is aided by recognition of typical clinical features and by direct microscopic detection of fungi in scrapes, biopsy specimens, and other samples. Culture confirms the diagnosis. Histopathological, immunohistochemical, or DNA-based tests may also be needed. Pathogenesis involves agent (invasiveness, toxigenicity) and host factors. Specific antifungal therapy is instituted as soon as the diagnosis is made. Amphotericin B by various routes is the mainstay of treatment for life-threatening and severe ophthalmic mycoses. Topical natamycin is usually the first choice for filamentous fungal keratitis, and topical amphotericin B is the first choice for yeast keratitis. Increasingly, the triazoles itraconazole and fluconazole are being evaluated as therapeutic options in ophthalmic mycoses. Medical therapy alone does not usually suffice for invasive fungal orbital infections, scleritis, and keratitis due to Fusarium spp., Lasiodiplodia theobromae, and Pythium insidiosum. Surgical debridement is essential in orbital infections, while various surgical procedures may be required for other infections not responding to medical therapy. Corticosteroids are contraindicated in most ophthalmic mycoses; therefore, other methods are being sought to control inflammatory tissue damage. Fungal infections following ophthalmic surgical procedures, in patients with AIDS, and due to use of various ocular biomaterials are unique subsets of ophthalmic mycoses. Future research needs to focus on the development of rapid, species-specific diagnostic aids, broad-spectrum fungicidal compounds that are active by various routes, and therapeutic modalities which curtail the harmful effects of fungus- and host tissue-derived factors.

FIG. 1.

Schematic representation of a cross-section of the human eyeball, depicting its parts.

FIG. 2.

Hyaline filamentous fungi S. apiospermum, Paecilomyces, and Acremonium.

FIG. 3.

Dematiaceous fungi Bipolaris, Curvularia, Exophiala, Exserohilum, Lecytophora, Phialophora, and L. theobromae.

FIG. 4.

A 5-day growth of L. theobromae on Sabouraud glucose-neopeptone agar, Emmons' modification. Growth has reached the edge of the petri dish (90 mm in diameter), indicating rapid growth. The colony is floccose and grey to brown-black. Macroscopic fruiting bodies (pycnidia) have not yet appeared.

FIG. 5.

Zygomycetes Rhizopus, Mucor, R. pusillus, A. corymbifera, A. elegans, and S. vasiformis.

FIG. 6.

Thermally dimorphic fungi P. brasiliensis, C. immitis, B. dermatitidis, and S. schenckii.

FIG. 7.

Photomicrograph showing presence of sporangia (cysts) of R. seeberi in stroma of the lacrimal sac. The cysts are of all sizes, with a sharply defined, chitinous-appearing wall. The largest sporangium reveals maturing spores (endospores). The smaller cysts represent “trophic” stages of the organism. Hematoxylin-eosin stain; magnification, ×400.

FIG. 8.

A 24-h growth of F. solani on Sabouraud glucose-neopeptone agar, Emmons' modification, that had been inoculated with corneal scrapes. Note that growth has occurred on the “C” streaks, representing the sites of inoculation. Only growth on C streaks is considered significant.

FIG. 9.

Photomicrograph showing branching, septate fungal hyphae in corneal scrapes. The cell walls and cross-walls are unstained, but the protoplasm has taken up the stain, permitting easy visualization of the fungus. Gram stain; magnification, ×400.

FIG. 10.

Surgical removal of a lacrimal sac infected with Rhinosporidium seeberi. Small, spherical structures (arrows) are seen on the surface of the sac; these represent the sporangia (cysts).

FIG. 11.

Clinical keratitis due to Fusarium solani. The necrotic slough is elevated above the surface of the cornea. Hyphate lines are seen extending into the surrounding cornea.

FIG. 12.

Clinical keratitis due to a Curvularia species. The necrotic slough exhibits a brownish pigmentation. Such pigmentation may be seen in corneal lesions due to dematiaceous filamentous fungi and should not be mistaken for an incarcerated iris.

FIG. 13.

Photomicrograph of a tissue section from a corneal button removed at the time of keratoplasty. Fungal hyphae are seen below the epithelium (E), in the superficial and middle layers of the corneal stroma. The hyphae are oriented mostly in a direction parallel to that of the corneal collagen bundles, although the hyphae in the superficial stroma are oriented vertically (arrows). GMS stain; magnification, ×100.