Abstract:

This video presents a microscopic analysis of a skin biopsy from the leg, initially showing an ulcerated, nodular lesion with reactive epidermal overgrowth, mimicking squamous cell carcinoma. The crust is composed of fibrin and neutrophils, indicating inflammation. A crucial diagnostic clue is the presence of neutrophil aggregates within the thickened squamous epithelium, which prompts the consideration of infectious or reactive etiologies over malignancy. The definitive diagnosis is chromoblastomycosis, confirmed by the identification of characteristic "sclerotic bodies" or "copper pennies"—perfectly round, thick-walled, brown-pigmented fungal organisms. The presenter explains that this condition often arises from traumatic inoculation of dematiaceous (melanin-producing) fungi from the environment, leading to a florid inflammatory response and reactive pseudoepitheliomatous hyperplasia. The inherent pigmentation of the fungi typically allows for direct visualization on H&E stains without the need for special fungal stains.

Understanding Chromoblastomycosis: A Microscopic Case Study of a Fungal Skin Infection

• 0:00 Initial Biopsy Presentation: The video begins with a low-power view of a skin biopsy from the leg, showing an ulcerated, pedunculated nodular lesion covered by a thick, purple-pink crust.
• 0:17 Microscopic View of the Crust: The crust is microscopically composed of bright pink fibrin and numerous dark specks, which are neutrophils, indicating active ulceration and inflammation. Incidental purple bacterial colonies are also noted, but these are generally considered part of normal skin flora overgrowing an ulcer.
• 0:52 Reactive Epidermal Hyperplasia Mimicking Carcinoma: Closer examination of the dermis reveals a very thick epidermis growing downwards, which can initially be mistaken for squamous cell carcinoma. However, it is identified as reactive pseudoepitheliomatous hyperplasia, or reactive epidermal overgrowth, in response to an inflammatory process.
• 1:49 Key Diagnostic Clue - Neutrophilic Pustules: A critical clue differentiating this reactive process from carcinoma is the presence of collections of neutrophils (pustules) trapped within the thick, glassy squamous epithelium. This observation prompts the pathologist to look for infectious or other reactive causes.
• 2:25 Discovery of Characteristic Fungal Organisms: The definitive diagnosis is made by identifying perfectly round, thick-walled, brown-colored structures, referred to as "sclerotic bodies," "medlar bodies," or "copper pennies." These are characteristic dematiaceous (melanin-producing pigmented) fungal organisms.
• 3:34 Chromoblastomycosis - Etiology and Pattern: The condition is diagnosed as chromoblastomycosis, a pattern of infection caused by various species of pigmented fungi (e.g., Exophiala). It typically results from traumatic inoculation, such as skin injury from dirty wood or environmental debris, particularly in warm, damp climates.
• 4:42 Pathological Response and Diagnostic Significance: The fungal infection leads to significant inflammation, reactive squamous proliferation, and often granuloma formation. The presence of reactive pseudoepitheliomatous hyperplasia with inflammation (neutrophils or granulomas) and pigmentation should always prompt a search for these specific pigmented fungi.
• 4:50 Chromoblastomycosis vs. Phaeohyphomycosis: The presenter differentiates chromoblastomycosis (characterized by round sclerotic bodies) from phaeohyphomycosis (characterized by pigmented hyphae). Both are skin infections caused by dematiaceous fungi that induce similar reactive epidermal changes.
• 5:55 Diagnostic Tools & Visibility: The inherent brown pigmentation of the fungi makes them readily visible on routine H&E stains, often eliminating the need for special fungal stains (e.g., PAS, GMS). However, these stains, or a Fontana Masson stain for melanin, can be useful if organisms are difficult to find or if non-pigmented fungi (like Blastomycosis) are suspected.
• 6:32 Key Morphological Features: The perfectly round shape and thick walls of the fungal organisms are highlighted as important morphological features for accurate diagnosis.