A 55-year-old female presented to the ophthalmologists with a painful left eye.On examination, the acuity of the left eye was 6/60.The conjunctiva was injected. A dark brown, ovoid plaque was present over an ulcerated corneal surface.The clinical impression was of iris melanoma prolapsing through a perforated cornea.The patient gave a history of a foreign body (nature uncertain) entering the left eye 3 months previous.An enucleation was performed. The cornea exhibited axial and paraxial ulceration. The anterior chamber contained a hypopyon. The chamber angle was secondarily closed, due to irido-corneal adhesion via peripheral anterior synaechiae. An acute iritis, cyclitis and anterior vitritis were also present. No melanoma was identified.
Higher power examination revealed brown coloured, branching, septate hypha, lining the ulcer base and ramifying through the stroma (fig 1-arrowed), without Descemet’s infiltration. Some of the hypha were swollen, especially at the surface.
The fungi were positive for PAS (fig 2) and for Grocott stains (not shown).
Fig 3 shows that the fungi were positive for a Masson Fontana indicating the presence of fungal melanin.
1.What is the differential diagnosis of pigmented fungi in tissue sections? In this case, which differential is most likely?
2. What is thought to be the function of fungal melanin?
3. What are the main predisposing factors for fungal keratitis?
1. Phaeohyphomycosis, chromoblastomycosis and eumycetomas.
The technical name for pigmented fungi are Dematiacetes, or Dematiaceous fungi.
If pigmented hypha are seen, the fungus belongs to the phaehyphomycosis group (pigmented filametous fungi).
If the fungus is composed of pigmented brown spherical cells, it belongs to the chromoblastomycosis group.
Mycetomas often make grains and granules and include Aspergillus, Actinomyces Nocardia and Streptomyces.
The pigment in the first 2 groups of fungi is melanin.
In this case, the fungus belongs to the Phaeohyphomycosis group, according to the above criteria.
2. Fungal melanin is thought to provide ‘armour’ for this infective agent. It has antioxidant therapy and is thought to increase virulence. Experimental models have shown increased survival of infected vertebrates after neutralising fungal melanin.
Infection and Immunity 1995; 63(12): p4944
Current opinions in Infectious diseases 2003; 16(2): p91
3. Trauma, especially in healthy young patients, especially those engaged in outdoor or agricultural work, where there is a high chance of animal or plant material contaminating the cornea.
Humidity, rainfall and wind also influence the occurrence of filamentous fungal keratitis and determine seasonal variation in frequency.
Other predisposing factors include: immune suppression, steroid and antibacterial eye drops, ‘allergic’ conjunctivitis, hydrophilic contact lenses, dry eye, defective eyelid closure.