Phaeohyphomycosis

Etiology and pathogenesis

The disease is caused by various dematiaceous hyphomycetes, whose natural reservoir is soil. The main genera of pathogens include Exophiala, Phialophora, Wangiella, Bipolaris, Exserohilum, Cladophialophora, as well as Alternaria.

The pathogen enters the body through two main routes. The first is traumatic, when spores enter via microtraumas of the skin. The second route is inhalational, which typically leads to the development of systemic and deep infections. Once inside the host tissues, the fungi exhibit a characteristic morphology: they form brown yeast-like cells, pseudohyphae, and true septate hyphae. The dark color is due to the presence of melanin in the cell walls of the pathogens.

It is important to understand the terminological distinction: if the infection is caused by non-pigmented (light, hyaline) fungi such as Aspergillus, Fusarium, or Penicillium, the condition is classified as hyalohyphomycosis.

Clinical manifestations

The clinical presentation largely depends on the route of pathogen entry and the patient's immune status. There are two main groups of infections:

1. Subcutaneous phaeohyphomycosis. Develops predominantly after traumatic inoculation of the fungus into tissues. A painless encapsulated mass forms at the site of entry. Over time, necrosis occurs in the center of the lesion, ultimately leading to the formation of a true subcutaneous abscess.
2. Opportunistic infections. Occur in individuals with weakened immunity or underlying conditions:
3. Fungal sinusitis. Most commonly develops against the background of pre-existing allergic rhinitis or immunosuppression. The main causative agents are representatives of the genera Bipolaris, Exserohilum, Curvularia, and Alternaria.
4. Brain abscess. An extremely severe form that develops via inhalational transmission. A specific neurotropic pathogen capable of causing this pathology is Cladophialophora bantiana.

Laboratory diagnostics and treatment principles

Confirming the diagnosis requires a comprehensive laboratory approach. Test materials include skin scrapings, biopsies of affected tissues, sputum, or cerebrospinal fluid (CSF) when central nervous system involvement is suspected.

• Microscopy. The sample is treated with a 10% potassium hydroxide (KOH) solution. The key diagnostic feature is the detection of brown septate hyphae. This reliably distinguishes phaeohyphomycosis from chromoblastomycosis, in which specific rounded muriform cells (Medlar bodies) are identified in the tissues.
• Culture method (inoculation). The material is plated on Sabouraud dextrose agar. When dematiaceous fungi grow on the nutrient medium, they form characteristic olive-black colonies.

Treatment of the disease requires a combined approach. In most cases, surgical excision of the lesions is necessary (especially for subcutaneous abscesses). Medical treatment involves prescribing systemic antifungal drugs, with amphotericin B and itraconazole being the drugs of choice.

Class and exam questions

What else to study on this topic

• Dematiaceous hyphomycetes: morphological properties and classification
• Opportunistic mycoses of the central nervous system
• Differential diagnosis of deep mycoses
• Hyalohyphomycosis: clinical significance and main causative agents
• Mechanism of action of amphotericin B and itraconazole

Other topics in the section "Subcutaneous Mycoses (Sporotrichosis, Chromoblastomycosis, Mycetoma)"

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