Etiology and Pathogenesis
The disease is caused by various dematiaceous hyphomycetes whose natural reservoir is soil. Major genera of pathogens include Exophiala, Phialophora, Wangiella, Bipolaris, Exserohilum, Cladophialophora, and Alternaria.
Pathogen entry occurs via two main routes. The first is traumatic, where spores enter through microtraumas of the skin. The second route is inhalational, typically leading to systemic and deep tissue infections. Once inside host tissues, the fungi exhibit a characteristic morphology: they form brown yeast-like cells, pseudohyphae, and true septate hyphae. The dark pigmentation is due to melanin present in the cell walls of the pathogens.
It is important to understand the terminological distinction: if an infection is caused by non-pigmented (light-colored, 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. Two main groups of lesions are recognized:
- Subcutaneous phaeohyphomycosis. Develops predominantly after traumatic inoculation of the fungus into tissues. A painless encapsulated mass forms at the site of entry. Over time, central necrosis occurs within the lesion, ultimately leading to the formation of a true subcutaneous abscess.
- Opportunistic infections. Occur in immunocompromised individuals or those with underlying comorbidities:
- Fungal sinusitis. Most commonly develops against the background of preexisting allergic rhinitis or immunosuppression. The primary causative agents belong to the genera Bipolaris, Exserohilum, Curvularia, and Alternaria.
- Brain abscess. An extremely severe form developing via inhalational transmission. A specific neurotropically virulent agent capable of causing this pathology is Cladophialophora bantiana.
Laboratory Diagnostics and Management Principles
Confirmation of the diagnosis requires a comprehensive laboratory approach. Specimen types include skin scrapings, biopsied infected tissues, sputum, or cerebrospinal fluid (CSF) when central nervous system involvement is suspected.
- Microscopy. The specimen is treated with a 10% potassium hydroxide (KOH) solution. The key diagnostic feature is the visualization of brown, septate hyphae. This reliably differentiates phaeohyphomycosis from chromoblastomycosis, in which characteristic thick-walled, rounded sclerotic cells (Medlar bodies) are identified in tissues.
- Culture method. The sample is inoculated onto Sabouraud dextrose agar. Growth of dematiaceous fungi on nutrient media produces characteristic olive-black colonies.
Therapy requires a combined approach. In most cases, surgical excision of the lesions is necessary (especially for subcutaneous abscesses). Pharmacological treatment involves systemic antifungal medications, with amphotericin B and itraconazole being the drugs of choice.